psithurismaccountantΠολεοδομικά Έργα

29 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

496 εμφανίσεις



Modern motor vehicle has attained a high standard of performance and reliability
resulting in an appreciably increased road speeds and distances covered. This long distances
and high speed running have greatly increased the necessity of maintenance and se
cing of
the vehicles. By proper servicing the vehicles become more comfortable, useful and easier to
drive. Servicing or overhauling in general is the process of restoring the vehicle to its original
high state of performance.

Different parts of an aut
omobile usually fail or get out of adjustment due to gradual use. A
vehicle can be maintained in a completely trouble free state by knowing which parts are subject
to deterioration and the factor controlling the rate of degree of deterioration as well as b
adjusting or replacing the parts before complete failure of the vehicle. Maintenance is the
process of keeping a vehicle in good running condition.

There are three important stage in the life of a vehicle, namely, 1) initial running in period,
2) normal

life period, 3) post

overhauled period. By proper servicing and maintenance, the total
life of the vehicle can be increased.




The various signs and symptoms which
give an indication of t
he fact that the engine
requires overhauling are as follows:


Excessive carbon deposits causing knocking and overhauling.


General loss of engine compression.


Knocking and overhauling during hill climbing and acceleration.


Excessive fuel and lubrication oil


Rough running.


Abnormal engine noise.

Generally a properly maintained new vehicle will show a progressively slight increase in power
output after covering 3000 to 7000 km (initial running in period). During this period, the surfaces
of piston
rings, cylinders and bearings having become well bended
in and polished. After this
period, if the engine is not decarbonized, there will be a slight fall in the performance. When
climbing a hill or accelerating, there will be a tendency to knock on full t
hrottle. After long periods
of service, the engine will show signs of overheating on steep hills and boiling of cooling water
These symptoms indicate formation of carbon deposits in the combustion chambers, piston
crown and valves. This carbon deposits ac
t as non conductor of heat, thereby raising the
working temperature.

The loss of compression when the engine is hot is an important sign showing the
necessity of overhauling. The compression pressure can be checked by using compression

Apart from
the compression knock such as detonation and pre
ignition, engine sometimes
develop a regular tapping knock. This may be due to excessive clearance between the valve
stem and tappet or rocker arm and valve stem. This type of knock is called mechanical knoc
which may also be produced at high speeds due to “crank shaft” thumping. Crank shaft
thumping is a metallic heavy and dull knock which increases
the frequency with increase of
engine speed. Crankshaft thumping may originate due to (a) excessive main bea
ring clearance,
(b) excessive crankshaft axial play, (c) out of round or ovalized journal, (d) crankshaft out of
balance, (e) insufficient lubrication and (f) loose flywheel.

Excessive lubricating oil and fuel consumption clearly indicate
that the engine
overhauling. Apart from this, there is an important fact that a worn out and badly carbonized
engine works less efficiently from the heat utilization of the thermodynamic point of view. An
engine which is thermodynamically inefficient will not giv
e so much useful energy from the fuel.

Crank case dilution is another sign of overhaul necessity; a leaky piston tends to promote
crank case dilution by the escaping of fuel past the piston rings.

It is very difficult to give exact information about the

road life or length of service before a
motor vehicle engine requires overhauling. So much it depends on the way in which the vehicle
has been maintained and operated.


It is the
operation of removing carbon deposits from
cylinders, valves, piston ring grooves,
combustion chambers in cylinders head and piston crown. The carbon deposits have a low
coefficient of heat transmission which affects the cooling of the head and pistons resulting in
faulty spark plug functioning.

certain zones of the combustion chamber, temperature will be
more due to the deposition of the carbon and cause reduction in the volume of the combustion
chamber. The compression is therefore increased resulting to pre
ignition or auto ignition to

abnormal engine noises and thus increase engine stresses. There are three methods
for engine decarbonising.

Scraping Method:

In this method carbon deposits are removed by scrapers and suitable wire
brushes. For this purpose drain the water from the cyl
inder block by means of drain cock.
Disconnect all external fittings from the cylinder head and remove it. After removing the cylinder
head gasket, plug the water passages with clean cotton. Scrape the carbon deposits from the
piston crown, cylinder wall a
nd head by means of a scraper. Remove valves from the head and
scrape the carbon deposits from the valve stem
. Clean the valve seat and face with a wire
brush. Now clean and adjust the spark plugs. Clean all the carbon deposits from the combustion
by using kerosene and compressed air.

Oxygen Decarbonising:

It is the process of removing carbon deposits from the cylinder wall
and piston crown by using an oxygen flame without removing cylinder head. This method is
preferably used in non
detachable type cylinder head engines. The oxygen flame is applied to
combustion space by inserting a flexible delivery jet through the spark plug hole. The flame
will burn away all carbon deposits.

Chemical Decarbonising:

To prevent carbon formation and loosen the carbon deposits, a
number of chemical preparations in the

form of powder, tablet and liquids are available. These
preparations, when placed in the cylinder will loosen the carbon deposits. The liquid
decarbonizer dissolve

the carbon accumulation in the combustion chamber as well as on the
piston and rings. The
solution used is usually caustic soda or it is mixed with water. They are
poured in to the cylinder or sometimes mixed with the fuel.


The engine compression test measures ability of the cylinder to hold compression. An
engine compr
ession test is done in the following manner provided that the battery has sufficient

Run the engine until normal operating temperature is reached. Stop the engine, clean the dirt
around the spark plug holes and remove the plugs
. Remove the air clea
ner and hold the throttle
valve wide open. Insert the compression gauge firmly into the spark plug hole of the first cylinder
and crank the engine with the starter motor. The pointer of the gauge will gradually rise and
when it shows maximum reading, stop
cranking after noting the reading. Atleast four
compressions are necessary to get the maximum reading. Repeat this test for all cylinders. The
cylinder should read less than 1892 KPa for Diesel Engine and about 800 KPa for Petrol

Compression pres
sure should be uniform in all cylinders and greater than the minimum
given by the manufacturers. When the compression is low in a particular cylinder, the condition
of piston, rings, cylinder walls and valves may be suspected. In order to point out the exa
defect, spray
a little engine oil into the combustion chamber though the spark plug hole. Crank
the engine to distribute the oil around the cylinder wall and repeat

the compression test. If the
pressure increases, the low compression is due top the leakage past the piston rings because oil
will seal the rings temporarily so that they can hold the compression pressure better. The trouble
in this case is caused by worn

out piston rings, piston or cylinder wall. If squirting of oil does not
increase the compression pressure, the leakage is probably past the valves. This could be
caused by broken valve springs, incorrect valve adjustment, sticking valves, worn or burnt va
and valve seats. Low compression reading between two adjustment cylinders is probably
caused by the blown head gasket between the cylinders.


The inlet manifold vacuum, varying with different operating conditions as well as with
ferent engine defects, is measured by vacuum gauge. Different
engine defects are indicated
by the manner in which the vacuum varies from the normal value. The Gauge is connected to
the inlet manifold by removing the adopter screw. If no adopter is provided

it will be necessary to
drill and
tap it for this purpose. Now the engine is started and operated at a specified idling
speed which is measured by a
. When the engine is
idling at operating temperature
the gauge reading is taken.


For an engine

in good condition, the reading should be 420mm to 520mm of Mercury.


If the reading varies between 370mm to 500mm of Mercury the Air fuel mixture is too rich
or too lean.


A reading of 475mm
dropping to 375mm indicates a leaky valve.


A low steady reading of

375mm indicates late ignition timing and a reading of 300mm
indicates late value timing. A reading of 25mm above normal value indicates early ignition


Irregular low reading of 250mm to 375mm may be caused by leaks through the carburetor
gaskets, i
nlet manifold or other vacuum connections. A sharp drop of 250mm of less may
be due to blown head gasket. A normal reading of the gauge differs from place to place
according to altitude.


The cylinder leakage test is a way of findin
g smallest of cylinder leaks. Air applied to the
cylinder at control volume and pressure measures the percentage of air leakage. It is normal for
an engine to lead a small amount of air passed the pisto
n ri
ngs into the crankcase. However,
any leak through
an inlet valve, an exhaust value,

head gasket, head or
block and excessive
leakage passed the piston rings indicate trouble.

To perform a cylinder leakage test, run the engine until normal operating temperature is
reached. Cleaned the area around the spa
rk plug and remove them. Remove the crankcase oil
filler cap and the radiator filler cap. Set the cylinder leakage

tester by adjusting the air supply to
the right pressure at the tester input air connection. Adjust the pressure regulator on the tester to
ive a zero leakage reading on the gauge. Select the proper adopter and install it in the spark
plug hole of first cylinder. Rotate the engine until timing mark aligns with the T.D.C mark.
Connect the tester hose to the adopter and hole the percentage of le
akage on the tester gauge.
Listen for escaping of air

through the carburetor, the exhaust pipe and the crankcase filler cap.
Check for air bubbles in the radiator. Disconnect the tester hose from the adopter and rotate the
engine until the piston of next c
ylinder according to the firing orders comes to T.D.C. Install the
tester to the spark plug hole of this cylinder. Continue this procedure until all cylinders have been
tested. Record reading from each cyl
inders so that they may be analyzed

to determine t
condition of engine. Gauge reading should be about the same for all cylinders. Reading of 20%
leakage or more indicates excess leakage and loss in compression. Air escaping through the

indicates a leaking inlet valve and air escaping thru the

exhaust pipe indicates a leaky
exhaust valve.

High leakage between two adjacent cylinders can be caused by leaky head gasket or a
crack in the
cylinder head. Air escaping thru the radiator indicates that the compression is
leaking into the water jacket.
Air leaking thru the crank case filler cap indicates a crank case
lead, the result of worn piston rings or cylinders.


In an engine, excessive oil consumption is caused due to following reason:


of oil in the combustion chamber: It is indicated by smoky exhaust in blu
ish color. Oil
may enter in the combustion chamber due to the following reason:


orn out cylinder,

piston and rings.


Excessive clearance between inlet valve stem and guide or defecti
ve valve guide seal.


Excessive oil pump pressure.

2. L
: leakage of oil occurs due to worn out seals at front and rear main bearing,
defective crank case packing, Timing cover packing and valve cover packing.

Passing out in the form of mist or vapor
thru the crank case ventilation system: high speed and
high operating temperature make the oil thinner and oil vapor will be formed. This oil vapor will
loose through crank case ventilation system.




Mount the engine in the s
tand and drain the lubricating oil.


Remove clutch release fork and release bearing.


Remove clutch from the fly wheel.


Remove starting motor, clutch housing and fly wheel


Disconnect the vacuum line from the distributor.


Disconnect all H.T leads from spark p
lugs and remove the distributor.


Remove fuel pump, carburetor, generator, spark plugs.


Remove water pump


Remove manifolds.


Remove valve cover


Remove rocker arm assembly


Remove all push rods and tappets.


Loosen all cylinder head balls in the correct sequenc
es and remove cylinder head and


Compress valve springs and remove valve locks, spring cap oil seal, springs and the


Remove crank shaft using a puller


Remove timing gear cover, crank shaft gear and crank shaft.


Remove oil pan and oil pump.


Check connecting rods and pistons for cylinder number identification if there is no mark,
make new marks on them.


Remove connecting rod lock nuts , oil dipper and the cap


Push the piston and the connecting rod assembly upwards and remove them from the top


Remove piston rings by using ring expander and remove the connecting rod from piston


Remove main bearing balls and remove the bearing caps.


Lift the crank shaft out of its seating and place it in a tray


bearing shelf from the block .wash all parts i
n kerosene and blow them in with
compressed air.


After cleaning cylinder head may be checked visually.

Some times there may be
warpage, cracks and surface scoring. The warpage can be checked by means of a straight
metal rule placed against machined surfaces in a number of directions. To detect small cracks
wet the head with kerosene by means of a clean clo
th and strike lightly. The cracks will be
clearly visible. The cylinder may be inspected for ridge formation at the top. If ridge if found, it
must be removed before taking the piston and connecting rod assembly from the top otherwise
the rings are liable
to break. The ridge is removed by a means of a ridge reamer. Before starting
reaming, keep the piston at B.D.C. then stuff the cylinder with a clean cloth and cover other
cylinders, valve ports, etc, so that the metal cuttings do not fall on them. Then pla
ce the reamer
at the top and rotate to cut away the ridge.

The ovality and taper of cylinders can be checked by means of a dial gauge or inside
micrometer. The diameter is measured both at the top and at the bottom of cylinders in two
directions, i.e. in
the longitudinal direction of the cylinder block and in the perpendicular direction
to it. In each case, several measurements are taken to get the maximum reading. The difference
in reading at the top in a perpendicular direction gives the ovality there an
d the difference in the
top and bottom readings in the same direction gives taper in that direction. The permissible
ovality and taper are 0.01mm and 0.25mm respectively. Cylinders with less than 0.01mm ovality
need not be rebored. They can be made good by

bending. However, if the ovality exceeds
0.01mm, reboring or liner replacement should be carried out.

The clearance between the piston and cylinder wall should be checked

with a feeler
gauge by inserting the piston upside down. Generally a clea
rance of 0.003 times the diameter of
piston is provided. If the clearance is exceeded, check the clearance again with a new piston. If
no variation is noted, it indicates that the cylinder requires reboring.

Pistons are available

in various sizes above th
e standard size, increasing the diameter in
steps of 0.254mm. by measuring the diameter of cylinders and finding the maximum worn out
diameter, it is possible to as certain whether they require 1

oversize, 2

oversize piston, etc.
After reboring, the cy
linder wall should be polished by honing machine.

Piston rings are to be checked for weak tension, scratches and wear. Sometimes all that
required is to free up the rings in the ring grooves by cleaning the carbon deposit. The tension of
ring can be check
ed by placing the worn ring vertically over a new ring and pressing together
while observing whether the gap of the worn ring closes more than the new ring. New rings are
to be installed depending upon the condition of cylinder wall. If the wall has some t
aper, but not
enough for reboring, special rings (eg: simplex ring) should be used. These rings have greater
tension and more flexibility which enable them to expand and contract as they move up and
down in the cylinder. The gap between the ring ends, when

fitted in the cylinder
, should also be
checked. To check the gap, the
ring is placed in the cylinder slightly above the lowest
point of
bottom ring travel then the piston in the inverted position is placed on the ring moved down till it
comes to the posit
ion of the lowest ring travel. By doing so, the ring become square with cylinder
wall. The end gap is then measured with a feeler gauge.


Out of the various components in an engine which require att
ention, the most important
ones are the cylinder and piston. They are subjected to wear because of very high temperature
of burning gases. Hard carbon particles present in the gases are also responsible for wearing of
the cylinders. Pistons operates at ver
y high speed along the cylinder wall with their rings will also
cause wearing of cylinders. In the case of petrol engines, the washing action of petrol entering in
to the combustion chamber can also cause cylinder wear. The other factors which affect the
ylinder wear are; the mixture strength, the nature of ignition timing, the crankcase ventilation,
the material of cylinder, broken rings, bent in the connecting rod, dust particles in the air etc.

In addition to friction, the cylinder wear

differs from one cylinder to other because of the
nature of cooling in different cylinders. It has been found that the outer cylinders are better
cooled than the middle ones. Moreover, the amount of lubricating oil which reaches the cylinders
and its comp
onents is also a factor controlling the cylinder wear.

Engine cylinders do not wear uniformly along their surface. They wear more at the top
than at the bottom. The reason is, when the piston is at T.D.C. at the start of the power stroke,
the explosion pre
ssure is maximum. Therefore, the compression rings apply the maximum
pressure against the cylinder wall. It can be found a ridge of metal unaffected by the travel of the
piston above the top ring when it is at T.D.C.

Cylinders tend to wear in an oval shape
. This kind of wear is due to the side thrust of the
piston against the cylinder wall. More wear occurs along the cylinder surface perpendicular to
the piston pin axis.


A portable boring bar is used when a large number of cylinders are to be rebored. The
tool head carries a tungsten carbide cutter which is accurately centered with reference to the
cylinder by means of four cats paws. The cutter is set to the correct over
size diameter by means
of a micrometer and rotated at the correct cutting speed and feed the tool head is driven by an
electric motor. It is possible for the cutter to rotate at 2 or 3 different speeds and it can finish the
work within 10 to 15 minutes aft
er the centering operation.

The cylinder head and studs are to be removed for fixing the machine. The machine is
fixed by means of clamping device which is located in the near by cylinder. The machined
surface ensures that the cutter is moving perpendicula
r to the cylinder wall. A trip switch is also
provided to cut off the electric supply when the cutter has reached its lowest position. The cutter
is raised to the top by using a hand wheel to give the second cut.


Honing is the process of finishing rebored cylinder. Honing stones of rectangular cross section
are used on the driving spindle for this purpose. The stones can be adjusted for different radii
depending upon the cylinder bore. During the operation, the hon
ing head is rotated by an
electric motor and moved up and down in the cylinders. The action of honing stone is to remove
metal at a slow rate in the form of powder. The maximum thickness of metal that can be
removed by honing is 0.04mm.


he objective of cylinder lapping is to polish the surface which is finished by honing. It
removes fine scratches by means of abrasive powder mixed with oil. An old piston smeared with
a mixture of powder and oil is used for its purpose. The piston is split

in to two halves and
rotated in the cylinder. It is moved up and down in the cylinder similar to the honing operation. A
powerful spring is placed in between the piston halves. The lapping is a very slow process and
no metal is actually removed, but it gi
ves a very smooth surface.


Hydraulic press methods
A hydraulic press with a capacity of about 6000 kg to 8000 kg is
used for this purpose. The pressure is applied by the ram of the press. It has a pressure gauge
and other controls. The pressure gauge shows the fit or liner in the cylinder block. A low read
indicates loose fit and a high reading indicates tight fit. Before indicating, the liner and cylinder
block should be smeared with oil.

Screw press method

A common screw press used in garages for refitting and removing cylinder
liners is shown
in fig (1) and fig (2) respectively. It has a block with stepped cones for centering the liners of
different diameters. A square threaded screw with a T
yoke at the bottom is used to insert the
liner. The yoke is hinged the screw bar. The n
ut of the screw is rotated by ratchet arrangement
and a handle. The downward thrust of the nut is transmitted to the ball bearing so that moving
down the stepped cone along with the cylinder liner.

The removal of liner is done with the fittings as shown in

the fig (2). A draw head is
connected to the screw bar at the bottom, and a conical guide is provided at the top. A sleeve of
sufficient diameter is placed over the cylinder block as a support of the liner. When the handle is
rotated the liner is pulled u
p alone with the draw head.

Freezing method
This is the quickest method for inserting the cylinder liner. The liner is
placed refrigerator in which liquid oxygen or liquid carbon dioxide is used as the cooling agent.
The liner is contracted very sligh
tly along its diameter. In this condition, it is removed by the
operator with the aid of special teeth insulated gloves. The liner in this condition is inserted in the
cylinder block. When it expands, the required tolerance fit will be obtained.


Valves and seats:

he valve face may be checked for distortion, burning, pits, roughness,
pocketing etc. Lapping of the valve to the seat will remove very slight pits. To remove deep pits
or roughness a valve grinder may be us
ed for regrinding valves. Only a minimum of metal
should be ground from the face otherwise the thickness of margin will be reduced. The machine
consists of a grinding wheel operated by an electric motor. The valve is held in the chuck which
is also rotated

slowly by the same motor. Provision is made to set the valve chuck at any desired
angle. After starting the machine, a light cut is taken first the machine is stopped and the valve
face is inspected. If the cut was taken only on one side of the face, it m
eans that the valve has
bent its stem and it should be discarded. If the stem has no bent, restart the machine and reface
the valve taking light cuts till there are no more pits on the face.

The faults like roughness, pitting on the valve seat may be remov
ed by grinding. In the
case of integral valve seat which is generally cast iron, this may be done with a valve seat cutter
whereas in the case of insert type valve seat which is made up of hard metal valve seat grinding
is done with special grinders using
abrasive stones. To use the valve seat cutter, the cutter of
proper angle must be selected, which is fitted in the valve guide through a centering pilot. The
cutter is rotated by means of a sliding handle provided at the top.

If the valve seat becomes wide
r due to a number of grindings it can be narrowed down to
the desired width by grinding with wheels of lower angle at the top and wheels of higher angle at
the bottom as shown in the figure.


Clean the valve and the
valve seats. Smear the stem which lubricating oil to reduce
friction while rotating the valve. Before placing the valve in the original valve guide apply a small
quantity of grinding paste on the face. Lap the valve and valve seats using a slight pressure
means of sucker place don the valve head. Rotate the valve through quarter of a turn in each
direction. Now lift the valve and add more paste if required and repeat the process. Clean the
valve seats and face and apply pencil or chalk mark on the valve
face. Place the valve in the
guide and turn with a slight pressure. In case, some marks remain, continue lapping till all marks

Valve guide:
Valve guide must be inspected and serviced, if required before proceeding with
any work on valve and va
lve seat. The guides may be cleaned by a wire brush fitted in an
electric drill. After cleaning the valve and guide, the stem clearance in the guide is checked with
a dial gauge which is installed at any suitable place on the cylinder head near the valve.
The dial
gauge is so installed that its stem touches the valve stem which has been raised a little by
turning the cam shaft. The valve is moved to and fro in the guide and the dial gauge reading
noted to register the maximum difference which is the stem cl
earance. A worn out guide can be
driven by hammering with a proper drift or with a special tool called guide remover. However, for
fitting a new guide, hammering should not be resorted to, instead it should be installed by using
a special guide installer.

Valve spring:

Valve springs are tested for proper tension and squareness. Spring tension is
checked the in a special spring tester. The pressure required to compress the spring to the
proper length is measured by this test.

For testing the squarene
ss of the spring, t should be placed on a surface plate with the closed
coil downwards, along the scale f a trisquare as shown in the figure. It is then rotated to see if
the top coil moves away from the edge of scale more than 2 mm out of square or if it
does not
have the proper tension at the specified length, it should be replaced.

Cam shaft:
Cam shaft in modern engine should normally run for the entire life of the engine, and
therefore, used not be replaced. However, it can be checked for bent by placi
ng it on v

and using a dial gauge. In many case a bend cam shaft can be straightened in a hydraulic press.
The wear of the cams should also be checked. If the lift of each cam is less than the prescribed
limit, replace the cam shaft. The side clear
ance may be measured and if found excessive, the
thrust plate or the camshaft itself is to be replaced.


In the case of low power and high fuel consumption are noted the valve timing should be
checked. To obtain correct valve timing, suitable

marks will be made on the gears or chain and
sprockets. If the cam shaft is driven by gears, two marks are made on to adjacent teeth on a
gear and the similar mark is made on one tooth of the other gear. The timing will be correct
when the marks are align
ed together. If the am shaft is driven by the chain and sprockets,
suitable marks will be made on the teeth of sprockets and chain links. In case the marks do not
line up correctly, remove one gear and turn the other gear so that the marks line up when the

removed gear is fitted. For chain drive, remove the chain and turn both sprockets to the correct


The amount of clearance between valve stem and rocker arm or tappet has an influence
both on the running a
nd the output of an engine. This clearance allows for expansion of the
valve stem as the engine becomes hot. Too much clearance causes in correct valve timing and
valve opens late and closes earlier. This will reduce the amount of mixture taken in to the
ylinder and reduce the quantity of exhaust gasses rejected out of the cylinder. As result a
power output will be reduced and the engine will get over heated. If sufficient clearance is not
given the valve will not properly when the engine attains its norma
l working temperature. The
exhaust valve has more clearance than that having the inlet valve because the exhaust valve
exposed to the exhaust gasses is subjected to greater heat, while the inlet valve is cooled by the
incoming air fuel mixture.

The valve
tappet clearance should be adjusted when the valve is completely closed. To
adjust the clearance the method recommended is rotate the engine until the corresponding valve
of corresponding cylinder is in full lift. Corresponding cylinders are 1
6, 2
5, 3
and 1
4, 2
3 in 6
and 4 cylinder engines respectively. In the case of side valve engines, the tappet has an
adjusting bolt and lock nut for the purpose adjustment. Tappet wrenches and feeler gauge are
used for the adjustment. After holding the tappet, the
lock nut is loosened and the adjusting bolt
is turned until the recommended clearance is obtained. After setting the clearance crank the
engine for a few seconds and then check all clearances again.

In the case of overhead valve engine to adjust the cleara
nce, the lock nut on the screw at
the push rod end of rocker arm is unscrewed first. Now tighten or slacken the screw with screw
driver to decrease or increase the clearance. Now maintaining pressure on the screw driver,
after checking the clearance tighte
n the lock nut fully. The sequence of adjustment is as follows.


Valve sticking

in most engines valve that are sticking will be indicated by a regular or
occasional valve noise when the engine is working. Valve checking

Valve burning

Poor exhaust valve seating is one of the main reason of valve burning. The
other reason of valve heating is clogged water circulation around the valve seat, engine over
loading, lean air
fuel mixture etc.

Valve breakage

if there is
excessive valve tappet clearance the tappet will strike the valve
like a hammer. If the valve seat is eccentric to stem, the valve will be subjected to sidewise
movement every time it seats. This will cause fatigue and breakage.

Necking of valve stem

This defect may be caused due to leakage of exhaust gases at the
valve seat. These high speed and high temperature gases are corrosive and tend to erode the
valve stem. This erosion and corrosion effects combine to cause necking of valve stem.

Valve face


Valve face wearing takes place due to excessive tappet clearance or dirt
on the valve face or seat. Excessive tappet clearance causes heavy impact seating which
promote face and seat wear.

Valve Deposits

excessive rich mixture, improper combu
stion, defective ignition system and
loss of compression are the cause of carbon and oxide deposits on the valve.


It is very important to clean the piston thoroughly so as to remove all traces of carbon or
other deposits. When the piston

rings already removed, the piston is cleaned on both inner and
outer sides. Piston crown and inner side can be cleaned with a carbon scraper, where as ring
grooves are cleaned with a ring grooves cleaner. Any blocked holes in the oil ring groove may
be cl
eaned with a drill bit of correct size.

After cleaning thoroughly, the piston is inspected for pitting, cracks, scuffing, collapsed skirt
etc. A badly pitted, cracked or damaged piston should be replaced. Scuffing of piston occurs
when a newly ins
talled piston is loaded heavily without sufficient lubrication due to which it
expands and takes up the small clearance between itself and the cylinder wall. A heavily scuffed
piston always is replaced.

For checking whether the piston skirts is collapsed o
r not measures the diameter of the
skirt at its top and bottom ends. If the bottom diameter is less than the top, it means that the skirt
is collapsed. This can be remedied by skirt expanders or by shot blasting.

The wear of ring grooves is checked with a

new ring and feeler gauge. New piston ring is
placed with its outer side anywhere in the ring groove and clearance between the ring and the
side of the ring groove is measured with the feeler gauge. If the clearance is more than 0.15
mm, the piston needs



Piston scuffing or scoring:

this occurs due to excessive heat due to which the piston expands
and becomes tight in the cylinder. As a result, the lubricant is squeezed out from the cylinder
wall causing metal to metal
contact. The main reasons for piston scuffing are,


Insufficient lubrication.


Engine over heating due to overloading.




Inefficient cooling system.


Too tight a piston pin either the piston bosses the connecting rod bush.


Piston burning:

the m
ain reason for this trouble is detonation or pre
ignition. The burning due
to detonation is generally at a point farthest from the spark plug where the hot end gases rapidly
release their energy. In the case of pre
ignition, the burning is usually near the

centre of the
piston crown.

3. Damaged ring land: reasons


Detonation or pre


Ring is not compressed properly while installing.


Attempt is made to take out the piston without removing cylinder ridge.


Leakage of water in to the cylinder.

4. Damage
d piston bosses and circlip grooves:


Bent connecting rod.


Tapered crank pins.


To much end play of the crankshaft.


Loose installation of the circlips.



Electro deposition method: This method consists of coating the surface of piston by
electroplating. The process is carried out by specialist electroplaters and suitable only
when a large number of pistons are available for reconditioning. After electroplat
ing they
are to be machined to the required size.


Metal spraying: In this method molten metal is deposited on the piston surface. Finely
divided molten metal is sprayed with the help of compressed air. A special type of
spraying gun is used for this purpos


Shot peening method: In this method, the principle adopted is to expand the outer surface
of the piston by hammering inside. A controlled blast of air and lead shots is used for this
purpose. Only solid skirt pistons are subjected to this process.


The connecting rod is mainly inspected for mis
alignment and wear on bearings. Apart
from this, the oil holes should be checked in order to ensure that they are not blocked the studs
and nuts on the big end should also be checked for fitness

An easy method of checking alignment of the connecting rod is to remove the oil pan and
crank the engine while observing the big end. If alignment is correct, it should remain centered
on the crank pin. If on rotating the crankshaft, the big end moves

to and fro on the crank pin. It
must be due to misalignment. Special fixtures (connecting rod alignes) for checking the
alignment of connecting rods are also available.

Defects of connecting rod: Usual defects found with connecting rod are sideways
g, twisting and bent in its own plane. These defects effect the smooth working of the
engine. Due to sideways bending the piston pin becomes inclined to the axis of the crank pin.
This causes increased wear in the bearing surfaces and in extreme case will
result in loss of
power. Another defect is the bent in its own plane. This is not so serious if not to greater degree.
In such cases the axes of piston pin and crankpin remain parallel. Once too much, it would
reduce the compression ratio of the particular




Check for sideways bent

To check sideways bending, first of all the bearings are to be removed from each end of
the connecting rod. Pair of mandrels (cylindrical rods) are then inserted through the ends.
mandrel at the big end is clamped in position with a connecting rod cap and bolts. The
connecting rod is then placed on a pair of v

blocks. It is held in vertical position and the heights
x and y as shown in the figure (a) are measured. If the differe
nce in height is more than 0.125
mm for a mandrel length of 175 mm to 200 mm, the connecting rod has sideways bent. The
sideways bent can be straightened in a press.

For checking the twist, the same arrangement can be used by placing it horizontal
ly as
shown in the fig(2). A dial gauge is used to measure the heights at each end of the small end
mandrel. The difference heights gives the amount of twist.

Straightness of connecting rod:

After checking the connecting rod the defective portions are
ked with a chalk. It is straightened by applying forces in the opposite direction. Special hand
press tools are available for this purpose.

Figure 1 shows a form of straightner

of two point and central screw press type. The
connecting rod is held between the fixed and movable jaws. The convex bent side of the rod
should face the screw. Force is applied by rotating the handle till the rod straightens.

A twisted connecting rod can

also be straightened. For this the big end is clamped with
mandrel in a rigid vice. A steel bar with slotted end is used to apply the twist in the opposite
direction to remove the twist.


The usual defects found with crankshaft are scoring of j
ournal, cracks on surfaces, journal
wear and and bending. Wearing of crankshaft main bearings ang journals occurs due to lack of
lubrication, scratches occurs due to dirt in the oil, poor seating of the bearings, out of round of
bearing or journals. Connec
ting rod bearing and crank pin wear also occur due to the above

The bearing surfaces of the crankshafts are finished to extreme smoothness. The ovality
and taper of the crank journals can be checked by taking measurements with an outsid
micrometer at different place as shown in the figure.

AB= ovality at the left side

A1B1= ovality at the right side

A1= taper in the vertical direction

B1= taper in the horizontal direction

If ovality

or taper is noted, the journals should be reground to the nearest under size
diameter and under size bearings should be fitted. Main and connecting rod bearings are
available in standard size and under sizes. Under sizes of 0.015 mm to 0.5 mm are used whe
the journal is slightly worn, but not out of round. If the wear is extensive, under size bearings up
to 1.52 mm are also available. If the journals are worn beyond the maximum diameter, they can
be brought to the standard size diameter by electro

tion or metal spraying methods.

The end play (axial play) of the crankshaft should be about 0.025 mm. It can be measured
by taking one of the main bearing side clearances. If it is excessive, the bearing will knock at
various speeds. This knock so
metimes can be noted when engaging or releasing the clutch. The
main and connecting rod bearing clearance should be between 0.025 mm to 0.1 mm.

Crankshaft alignment can be checked with a dial gauge by supporting it on v
blocks. It is
rotated in th
e v

blocks and the dial gauge, which is on the journal, will show any mis
A bent crankshaft can be straightened in a heavy press. While checking the alignment, the
crankshaft should be supported on wooden blocks at each journal, so that it may
not sag. While
servicing the crankshaft, it should be thoroughly cleaned in a suitable solvent. Oil passages are
cleaned by a rifle type wire brush. The bearing surface are to be re
oiled after they have been
cleaned to prevent corrosion.



With inside and outside micrometers

The internal diameter of the connecting rod bearing is
measured with an inside micrometer. The connecting rod cap should be tightened with specified
torque before the measurement. The diameter

should be taken at different points to get the
maximum value. The outside diameter of the crank pin is found at number of points by means of
an outside micrometer. Half the difference of internal diameter of big end bearing and the
outside diameter of the

crank pin gives the bearing clearance.


With feeler gauge:

Dip a feeler gauge of about 6mm wide and of any small thickness, say
0.05mm thick, in lubricating oil, place it between the bearing and the crank pin and tighten the
cap nuts with specified

torque. Rotate the big end about the pin. If any additional drag is felt than
before, it means that the thickness of the feeler gauge is more than the bearing clearance. Then
another feeler gauge of smaller thickness may be tried. If no additional drag is

felt, the thickness
of gauge is less than the bearing clearance, and this necessitates another trial with a slightly
thicker gauge. This method is continued till the two consecutive sizes of feeler gauge indicate no
drag and slightly drag respectively. Th
e mean of the two thickness can be taken as the bearing


With plastic gauge:

It is a special gauge manufactured by Perfect circle corporation

of U.S.A. to measure the
bearing clearance. It consists of a fine thread of plastic material which changes shape under
pressure, and a graduated gauge to indicate the bearing clearance. The gauge is available in
different ranges. The gauge is placed on th
e bearing cap in the cross direction which is then
fitted to the big end with specified torque. The nuts are then loosed and cap removed. The size
of the plastic thread will be changed. The graduated gauge is then placed along it to read the
bearing cleara
nce. Two precautions must be observed while using plastic gauge. a) The thread
is soluble in lubricating oil. Therefore, the bearing must be thoroughly cleaned before using the
plastic gauge. b) The crank pin and bearing should not be given relative rotary

motion which will
distort the plastic thread.

Adjustment of bearings:

In olden days, white metal built in bearings were used for the
connecting rod big end, main bearing and camshaft bearing. The white metal in the molten form
is cast along its surface a
nd machined according to the crank pin dimensions. The bearings are
later adjusted when wear occurs, the adjustment of this type of bearings are done in the
following manner.

The surface of the crank pin are thoroughly cleaned and the crank journal is sme
with some marking material. The big end is then bolted without placing old shims and rocked to
and fro. Upon removal of the cap and bearing, it will be seen that there are marked spots
corresponding to the high area. The excess metals are scrapped off

by using bearing scrapers.
The cap is tightened again and the process is repeated. When the proper adjustment is
obtained, the bearing surface will be evenly covered with marking material. Now the process is
completed and at this stage connecting rod will

fall down by its own weight from 15° position
from the vertical at the top. This is the indication for the correct fit.

In modern engines, the tendency is to use shell type white metal lined bearing. These are
of non
adjustable type bearings which cannot

be remetalled for recondition purpose and do not
use any shims as in the case of white metal built

in bearings. When the bearings are worn,
replacement is the only remedy.



The old white metal is removed from the surface of the bearing by using bearing
scrapes and the surface is cleaned thoroughly. A coating of graphite or asbestos paste is
applied to the
part that does

not require metal deposition. A simple method of making

mould is shown in the figure. The bearing halves are placed vertically on a base plate with a
hollow steel core in between them. The core is slightly small in diameter than the cranks journal.
The bearing halves having shims between them are bolte
d together. The core and base plate
are coated with graphite to prevent metal deposition, and the oil holes are properly covered.
Now the bearing is ready for remetalling.

The white metal is heated on a steel plate to a temperature of about 350° c to 450° c. The
molten metal is slowly poured into the mould, that is, into the space between the bearing and the
steel core. As the level of the metal rises, it is agitated
with an iron wire to permit the escaping of
air from the system. Pouring is continued till the bearing is full and an allowance of about 3 mm
metal is given above the bearing. Then the bearing is allowed to cool.

In some of the moulds, the molten met
al is poured in the steel core, from where it flows to
the bearing through small holes provided in the core at different heights. This avoids gas
bubbles completely and shrinkage of white metal. Now a days centrifugal methods have been
largely adopted for
depositing white metal. In this process, the big end cap are held by means of
a rotating chuck. The white metal in the molten form is poured in to the system, and due to
centrifugal action, it is deposited over the bearing surface.


is a small projected portion of the bearing beyond its seat. The crush on the
bearing shell used in a connecting rod is shown in the figure. When the cap is tightened, these
projected portions are squeezed

in to form a press fit between the bearing and th
e seating
which ensures a firm seating and good heating conductivity between the parts. The crush is
about 0.05 mm for the big end bearing.



Lack of oil:

When the oil supply to the bearing fails, the protective oil film between the
earing and journal will lose causing metallic contact, which in turn causes rapid wearing
of bearing.



Due to continuous usage, the fatigue failure may occur. This failure produces
radial and peripheral cracks in the bearing making the metal to fla
re out.


Foreign matter:
Even though the bearing material is designed to have
embed ability

one of its
, too much and too large sized particles finding their way to the
bearing will scratch out metals and obstruct oil flow in the bearing.


Tapered journal:

With tapered journals, the bearing is loaded on one side, heating the
metal to melt, subsequently wiping off the bearing metal on this side. Bent connecting rod
produces the same effect, but the metal is removed on opposite sides of the be
shells. Tapered journal wipes out metal only on the same side of both halves of the
bearing shells.


Radial ride:

The journal is

to the crank cheeks to reduce the stress concentration
on the crank material at the corners. When they are not finished to proper radius during
regrinding, the bearing will be fitted improperly leading to unequal loads and subsequent
failure. This failure

is indicated by the continuous scoring marks at the edges of the
bearing shells.


Ridging of shaft:

When the journals have camber wear, the ridges in it cut out channels in
the bearing leading to rapid bearing failure.


Improper location:

If the bearing

is not located properly in its seating (counter bore), the
flow of oil and flow of heat will not be uniform causing subsequent failure of bearing.


A crank shaft is said to be in good balance if no unbalanced
forces are developed during rotation that tend to be bend or distort the shaft. Unbalanced forces
in a crank shaft result in vibrations, high main bearing pressures and excessive strain on the
haft. The crankshaft is balanced statically and dynamically.


Static balance:

It is the state of balance when the shaft is at rest. It is determined by
placing the crank shaft on a pair of knife edges. If the shaft remains at any position in
which it is pla
ced and does not revolve, it is in good balance. An unbalanced crankshaft
will rotate as gravity pulls the heavy side downwards. Therefore, metal must be removed
from the heavy side to obtain perfect balance.


Dynamic balance: A special machine is used to
balance the crank shaft dynamically. This
test is performed when the shaft is revolving at different speeds. An indicator shows
where and how much the shaft is out of balance. The metal is removed from the sections
that are out of balance until the machine

indicates that the shaft is free from vibrations.
The machine performs both the static and dynamic tests at the same time. The crank
shaft is usually balanced with the flywheel attached to it because it is a part of revolving
mechanism in an engine. Befo
re doing, this the flywheel is to be balanced separately by
drilling holes in the rim in order to remove excess metal.

The problem of attaining smooth engine operation involves several other factors besides
crankshaft balancing. In an assembled engine, the

up and down movement of the piston and
connecting rod assembly and inertia forces are difficult to balance. However, they can be
balanced by the application of counter balance weights. These weights may be forged or cast
integrally with the crankshaft opp
osite to the crank pin or separate weights may be bolted to the
extension of the crank arm. All crank shafts have one or more critical speeds at which vibration
periods set up. In a revolving mass such critical speeds cause vibrations in the other parts of

engine which should be avoided under all conditions.



Hand turni
ng methods:

The journal is measured with a micrometer across several

in order to ascertain whether it has taper or ovality. It is possible to mak
e the
journal perfectly round by the careful use of smooth file and emery paper. It is then
subjected to lapping by an arrangement as shown in the figure.

The two halves of the lap is made with brass or copper to reduce weight. The
is mounted in a lathe and the lap is clamped on the journal after applying the
emery powder. A handle is provided for holding the lap the crank shaft is rotating between
the centers of the lathe.


Hand operated turning tools:

A very effective hand tool used for crankshaft machining is the Ammco tool which is
shown in the figure. It makes use of single cutter of very hard material such as tungsten carbide,
used along with a pair of guide faces. The guide fac
es are fixed at right angles to each other.
The tool makes contact with the journal along the diameter which passes through the contact
faces of the guides. The cutter can be adjusted for its depth of cut by adjusting the screw. The
crankshaft is mounted i
n a lathe and rotated after clamping the tool on the crank journal. The
handle is held in hand during the operation. Due to the three point contact of the guide and the
tool, a perfect cylindrical surface is obtained. Different sizes of tools are available

for various
crank journals. After this process, the journals are finished by honing operation, an arrangement
of which is shown in the figure shown below.

The finishing process of the machined crank journal is to polish the surface with a honing
one. Convenient clamping devices are used for this purpose which consist of two arms, hinged
together at one end. The lower jaw carries the honing stone and the upper jaw is provided with a
hard contact guide. The pressure on the honing stone is regulated
by a screw. The crank shaft is
rotated at about 1 to 1.5 rev/s and the metal is removed to a thickness of 0.0025 mm. The
attachment is held in hand during the operation.

Regrinding method


Portable grinding wheel attachment:

This arrangement is suitab
le only when the repair work is occasional and a few crankshafts are
available for regrinding. The grinding attachment, consists of a cylindrical grinding wheel driven
by an electric motor with provisions with the journal is made by means of a hook at the
end of
the drive. The crankshaft is mounted in a lathe and rotated. When the journal rotates, the
grinding wheel along with the hook follows its motion and a perfect cylindrical shape is obtained.


Crank shaft regrinding machine:

When a large number of crank shaft are to be reconditioned, it may be economical to use a
crank shaft regrinding machine. The main feature of this machine is that it has the component
known as work head throw block at both the head stock and tail
stock as shown in the figure.
This helps for the adjustment of different crank throws so that the crank journals are centered
with, reference to the center line of the machine. The throw blocks are provided with counter
weights. So that the crank shaft wil
l be in good balance when rotating. Suitable steady rests are
provided so as to reduce vibration. Large grinding wheel with radial grinding surface is used in
this operation. The grinding wheel is driven by an electric motor at different speeds. It is
ted on the cross slide mechanism along with the motor which has a speed of about 80
rev/s. Provision is made for different speeds and feeds. The crank shaft also rotates at relatively
slow speed of about 1
2 rev/s. Cutting fluid is essential during the

process. The machine is
provided with tools for easy mounting and measuring the diameter of journals.

Building up of crank journals

To increase the strength of crank shaft, the journals can be built
up by depositing metal over the bearing surfac
e. The metal is deposited to a
greater diameter
than the standard diameter. The journals are machined to a standard size and standard bearings
are used. Two methods are commonly used for this purpose.


Electro deposition method:

In this method metal is depo
sited over the journals by
electroplating. For this purpose, the crank shaft is to be prepared by protecting the parts
which do not require metal deposition. Nickel or chromium is the common material used
for deposition because it is corrosion resistant an
d gives better finish after regrinding.


Metal spraying method:

In this method, molten metal is sprayed over journal surfaces, be
using compressed air, through a nozzle. A steady flow of metal is maintained throughout
the process. During spraying, the shaft is mounted in a lathe and rotated slowly.


The flyw
heel must be tightly fitted on the crank shaft. If it is loose, it will cause a heavy
single knock, which can be detected by racing the engine slightly, then turning off the ignition
switch. Just when the engine is about to stop, switch on the ignition, an
d at this time, the
flywheel knock can be heard.

The clutch has to be removed for tightening the flywheel on the crank shaft. Before
removing the flywheel, punch marks should be made on it and the crankshaft flange. While
tightening, these marks should be
lined up. It will preserve the balance of the flywheel and crank
shaft assembly provided by the manufacture, and maintain the accuracy of the timing marks
stamped on the flywheel.


Usually the teeth on the ring gear become wor
n at
one or
more localized spots correspond
ing to the positions at which the crank shaft normally
tends to come to rest. Although the remainder of the teeth may be in good condition, the wear at
these points may be sufficient to necessitate the renewal of
ring gear. The method of removing
the ring gear depends upon the type of fit on the flywheel. If the ring is bolted to the flywheel it
can be easily removed by unscrewing the bolts.

In the majority of cases, the ring gear is shrink into place. It can

be expanded by heating to a
fairly high temperature. The procedure recommended is to support the flywheel in a shallow
trough of cold water with the ring gear resting on 3 or 4 equally spaced blocks to keep it just
clear of the surface of the water while
the flywheel itself is submerged. A oxyacetylene flame
should then be played around the circumference of the ring gear until the ring gear expands
sufficiently to allow the flywheel to drop out of it.

The seating on the flywheel should be cleaned up
before the new gear is fitted. The gear can
be heated in a furnace. If it is not available, the gear should be brightly polished in several
sections on the circumference and placed on a heavy iron plate which should be evenly heated
with oxyacetylene flame

The temperature can be justified by the alteration in

of the polished sections.
Heating should cease as soon as the color changes from purple to dark blue. The ring should be
wiped and rapidly dropped in to place on the flywheel. If necessary i
t may be gently tapped
home with a copper hammer. It is then allowed to cool normally to room temperature.




Trouble shooting of battery coil ignition system:

The quality and regularity of the spark obtained at the end of each spark plug wire provides
the effectiveness of the ignition operation. The following are the important troubles found with
the ignition system.

a) No spark:

The engine will not run whe
n no spark is produced at any of the spark plug wires.
For checking the spark, hold the end of the spark plug wire 5 mm away from the cylinder head
while the engine is being cranked. Repeat this test in turn at each plug wire. In case, no spark is

at any of the spark plug wires, it is possible that the coil to distrib
utor H.T wire might
have fallen
out of its socket. Some times the wire itself may be having damaged. Replace the
worn or damaged H.T wire. The next step is to check the primary circuit
. For this, by pass the
primary circuit between battery and ignition coil by means of an ammeter. Connect the ammeter
between battery terminal and the battery terminal of the ignition coil. Turn the ignition switch off
and crank the engine. Observe the amm
eter reading while cranking. It is possible that (1) the
engine starts (2) ammeter reads zero (3) the engine does not start.

(1)Engine starts:

If the engine starts, the trouble is in the primary circuit between the battery and
the battery terminal of the
ignition coil. Now stop the engine by removing the ammeter lead.
Make sure that the C.B points are in closed position when the engine is stopped. Now connect
one lead of the ammeter to the ignition coil and the other lead consecutively to each of the
ry circuit terminals starting at the battery and proceeding towards the coil till the ammeter
leads 3 t 7 ampere. The fault part of the circuit is between the terminal where zero reading is
obtained and the terminal where 3 to 7 ampere reading was last obt
ained. Repair or replace the
defective parts.

(2)Ammeter reads zero:

If, when by passing the battery to ignition coil circuit the ammeter reads
zero as the engine is cranked, the trouble is in the primary circuit between the battery terminal of
the coil an
d grounded side of the C.B point. Now ground the condenser insulated terminal and
note the reading. Either the ammeter reads zero or 7 to 9 amperes. If the reading is zero, the coil
is defective and replace it. Before ascertaining this, make sure that the
coil to distributor primary
wire is in good condition. If the ammeter reads 7 to 9 amperes. When the condenser is
grounded, the trouble is in the C.B points or the primary circuit contact to the brake arm
assembly. Adjust or replace the C.B points or repai
r the primary circuit contact.

Engine does not start: If, when by passing the battery to coil primary circuit, the engine does
not start and the ammeter reads 7 to 9 ampere as the engine is cranked, the trouble is in the
condenser or the ignition
secondary circuit. Now check the coil and condenser and replace if
found defective. Connect a jumper wire to the coil and hold the other end 5 mm away from the
cylinder head while the engine is being cranked. If, the spark obtained at the jumper wire is
tisfactory but the engine does not start, the distributor rotor, cap or terminal housing may be
having the trouble. Test the rotor for leakage of high tension circuit. Clean the distributor cap
with carbon tetra chloride to remove carbon tracks. Replace th
e cap if there is any crack

Spark at some wires: Test the spark from the end of each spark plug wire at an idle speed of
7 rev/sec. If the spark is obtained at some wires, inspect the distributor cap or terminal posts as
well as a spark plug wire fo
r damaged insulation bent or cracked plug electrodes, terminal, cap
or rotor electrode. Repair or replace if required.

c) Intermittent spark: Test the spark from the end of each spark wire at idle speed with 5mm gap.
A spark that does not consistently jump

this gap considered an intermittent spark. This trouble
occurs due to loose connections in the primary and secondary circuits. Tighten all connections. If
the trouble still exists adjust or replace C.B. points and reset timing. Test the coil, condenser an
spring tension of C.B. point (movable point). Ensure that there is no moisture, oil or any foreign
matter on the distributor cap and rotor.

d) Weak spark: The cause of weak spark at all spark plug wires are troubles in the entire primary
circuits, C.B. p
oints and high tension wire from coil to distributor due to leaks. It is possible that
the battery may be discharged and it does not crank the engine sufficiently.


a) Ignition coil:

The ignition coil is tested with a s
pecial coil testmeter. The following are the
main test conducted. 1) Resistance of primary winding. 2) Resistance of secondary winding. 3)
Internal short circuit and coil performance. Out of these, the first three test can also be
performed with the help o
f a multimeter, while the coil performance may be tested by putting all
other new and tested components in the ignition system and observing the spark. A weak spark
then indicates a faulty coil. A special oscilloscope is also used in modern automobile shop
s to
observe the coil output in a wave form, which indicates clearly where the fault lies in the ignition

b) C.B. points
: The spring tension of the movable point can be tested by a spring balance. To
rest the contact pressure, the hooked end of the

spring balance is clamped to the movable arm.
The spring balance is then pulled at right angles to the arm untile the C.B. points just begin to
open. The reading of the spring balance will now indicate the contact pressure. The C.B. point
gap can be check
ed and adjusted in three ways.

i) By feeler gauge: This method is used to check the gap when the C.B. points are in good
condition and not pitted. The distributor cap is removed and the engine cranked till the C.B. point
gap is maximum. The gap is then mea
sured by the feeler guage. If the gap is not correct, the
screw on the base plate carrying the fixed point is loosened and the offset screw turned inorder
to decrease or increase the gap. The fixing screw is then lightened.

ii) By dial guage: This method i
s adopted to check the gap if they are pitted. The dial
guage is clamped on the distributor body and the stem of the guage made to rest on the moving
arm at the back of the contact point. The engine is cranked slowly so that the gap is reduced to
zero. The

dial guage is then adjusted to bring the pointer to zero position, the engine is cranked
again to open the contact points to obtain maximum gap. The reading of the guage directly gives
the point gap.

iii) By dwell meter (synchronoscope) : Dwellmeter is an

electronic instrument which gives
the C.B. point gap in terms of the degree of camshaft rotation for the time during which the C.B
points remain closed.

c) Condenser:

i) short circuit test:

A direct current is passed through the condenser with a lam
p of suitable resistance in series
as shown in the fig. If the condenser is short circuited, the lamp will light when the switch is

ii) Open circuit test: A direct current of suitable voltage (above 100volt) is passed through the
condenser w
hich in series with a neon lamp as shown in the figure. When the Switch is closed, If
no flashing of the lamp occurs, an open circuit exists in the condenser.

iii) Capacity test: A condenser that is operating at over capacity will cause a build up of metal


ve side of the C.B points and that operating at under capacity will cause a build up of metal
on the +ve side of the C.B points as shown in the figure. The capacity of a condenser can also
be checked by using a megger meter.

d) Rotor
: The rotor ca
n be cleaned with a clean cotton dipped in a solvent. Then examine the
rotor tip for excessive burning. If the rotor tip is badly corroded, scrape it clean and check the
spring (if provided) on the rotor for sufficient tension to ensure a good contact with

the carbon
brush in the cap.

park plug

) Visual Inspection: If the insulator tip is brownish in color it indicates that the engine conditions
have been good and proper fuel has been used. A thick layer of oil on the body and electrodes
of the plug means badly worn out piston rings, excessively
rich air fuel mixture and improperly
adjusted ignition system. If the insulator tip is white in color and carbon is deposited on the plug
body, the probable cause may be a

air fuel mixture, excessively advanced ignition timing spark
plug used may be of low
er heat value etc.

ii) Gap


plug gap depend upon a number of factors including voltage develop by a coil,
the maximum engine compression and the characteristics of the engine as a whole . it is always
advisable to follow manufacture’s recommendati
on regarding the spark plug gap. As a general
rule, the point gap varies between 0.63mm

0.81mm. on high compression engines, the greater
compression pressure increases the resistance of air gap between the plug electrodes , and
therefore smaller gap of


is suitable
. If the gap is too small it will cause
difficult to starting and if the gap is too large there will misfire at high speed.

A round wire type of feller gauge should be used for checking electrode gap. Only the
side of eart
h electrode should be bent for adjusting the gap. Any attempt to bend the central
electrode is likely to crack the insulator and the render the plug is unserviceable.

ii) Testing for current leakage
: Increase the gap to say 105m and

run the engine at me
. If the current is leakage the terminal voltage at the plug will zero .

iv) Gas tightness test:

Compressed air passed through the spark plug which is screwed to a
special arrangement at the end of an a air tube . The plug is immersed in kerosene

and the air
bubbles coming out of the spark plug are collected in measuring jar placed inverted over the
plug. The air collected should not exceed 5*10^3 mm^3 in 60 sec.



A good spark plug must be able to function under al
l working condition of temperature
and pressure. it is designed to operate under 2000 v to 3000 v and to withstand pressure
as high as 4200


It must maintain air gap proper air gap between the electrodes.


It must proper gas tight


It must be corrosion resi


It must reduce electrode erosion caused by excessively long sparking.


It must have proper reach.


The seal of the spark plug must be able to withstand the temperature and pressure
created in a combustion chamber during power stroke.


It must offer ver
y high resistance to current leakage.


It should have reduced interference to radio and t.v from the ignition system.



Plug fouled by engine oil entering into the combustion chamber.


Plug fouled by too rich air fuel mixture.


ug badly covered with carbon due to poor ignition.


Incorrect plug gap


Burnt electrodes or broken lower insulator caused by over heating.


Cracked or broken insulator sealing.


Red, brown or yellow oxide deposits on the plug insulator that short the plug by


Accumulation of dirt or moisture on the outside of the insulator that short the plug by
grounding the high voltage.


The spark plug can be cleaned in a plug cleaning machine in a following manner.

Inserting the firing end
of the plug through the rubber adapter of the machine, lightly hold
down with finger tip and press the switch to blast cleaning compound towards the plug electrode
.the plug is rotated slowly with the finger so that it is pressed to remove the cleaning com
from the firing end of the plug. Finally, the electrode gap is adjusted according to the


The exact time at which spark occurs in the engine combustion chamber for igniting
compressed air fuel mixture is ca
lled ignition timing. These methods are used for checking the

1. Cork method: I
nsert a cork into the spa
rk plug hole of first cylinder
and crank the engine.
When the piston reach the T.D.C the cork will be pushed
. Crank the engine until the tim
mark on the engine pull
ey and the timing case coincide
. In

the position
, the C.B position just

2. L
ow voltage lamb methods: Connect the two leads of a low voltage lamb across the C.B
points begin to open. I f the C.B point opening is to give s
park in the first or fourth cylinders

of a
four cylinder engine
, the timing mark should coincide.

3. Timing light methods: in this method
, a timing light or stroboscopic light
is used for checking
the timing,

connect the leads of light to no
1 spark plug and the battery and start the engine.
Now flash the light beam on the timing

marks. When the light flashers
, the timing mark should be
coincide. The distributor should be rotated in the required direction to a
dvance or retard the


First of all bring the piston of first cylinder near the T.D.C at the

end of compression stroke
This can be done either by using cork or by viewing the position of rocke
r arms. To use the cork
remove the spark plug and insert the cork in the plug hole. Now crank the engine observe the
cork. Wh
en the piston reaches the T.D.C

the cork will be pushed out. Turn the engine further till
the timing mark on the pulley and the tim
ing case coincide. Remove the distributor cap and
disconnect the vacuum pipe from the vacuum
advancing unit. Loosen the clamp

bolt of the
distributor and switch on the ignition.

Rotate the distributor body in a

direction opposite to that of rotor direction

to advance the
timing. When the C.B point

begins to open, a

spark will be jumped across the points. Switch of
the ignition and tighten the clamp bolt after

holding the distributor firmly

Fit the rotor and connect
the H.T lead which fa
ce rotor tip to no.1

spark plug
. Connect

the other plug leads in the direction
of rotation of rotor according to the firing order. Now connect the vacuum pipe and start the


1. Centrifugal advancing unit: Turn the rotor in the req
uired direction so that the advancing
weights are in fully extended position. A ‘click’ sound would be heard as the weights return to the
rest position, on releasing the rotor. If no click sound is heared, insert the base plate and
advancing unit. Lubricat
e the different parts of the advancing unit after cleaning. A very rapid
advance is caused by weak spring.

2. Vacuum advancing unit: Turn the engine with the starter, keeping the choke valve closed and
the throttle valve wide open. If the breaker plate adv
ances and returns to the original position
when the starter is released, the unit is in good condition. If the breaker plate remains in
advanced position without returning it is an indication of broken internal spring. If the breaker
plate does not advance
, it is an indication of broken diaphragm.




1. No Spark

Burnt coil, coil or condenser short, burnt or pitted C.B points
and inoperative C.B points.

2. Weak Spark

Weak magnet, weak coil, weak
condenser, incorrect C.B point
gap and loose connections.

3. No spark when hot

Coil is short.

4. Misfire at low speed

Less C.B point gap, weak condenser and incorrect spark plug

5. Misfire at high speed

More C.B point gap and weak spring tension of

C.B point.


The two major units of gasoline fuel system are the fuel pump and the carburetor. The fuel pump
delivers fuel to the carburetor which mixes the fuel with air and delivers it to the engine.

1) Fuel starvation:

when the engine starts but fail to keep running or if caused by the fuel
system failing to deliver fuel occasionally, how ever the ignition system can suddenly fail.
Following are the reasons of fuel starvation.

a) Vacuum


Leakage in the fuel line br
eaks the vacuum created in the fuel pump and no fuel
will be pumped by it. The other parts which cause vacuum leaks are , loose connection in the
fuel line , broken or weak fuel pump diaphragm or linkage spring and a damaged sediment bowl.

b) Low tempera
ure: At freezing temperature
, water in the fuel pump and fuel line can freeze
and prevent the fuel from pumping.

c) High temperature: During hot weather
or when the engine is hot, two different kinds of
troubles are common. The

are ‘’ vapo
r lock’’ which

shout of fuel and ‘’percolation’’, which
floods the engine.

When the engine is stop while at it Is hot, heat from the engine may vapourise the fuel in
the line or fuel pump. This is known as vapour lock. No fuel will flow to the carburetor un
vapour lock is relived. For reliving vapour lock allows the engine to cool or pure water over the
fuel line and fuel pump. If the carburetor is not sufficiently insulated from the heat of engine, it
heated when the engine is stopped. Due to
s heat
, vapourized bubbles will be formed
in the fuel discharge tube and passages. These bubbles rise in the tubes, pushing the fuel
above them in to the carburetor throat causing the flooding of engine. This is known as
percolation. In up draft carbu
, this liquid fuel merely runs out and the chief result is lowered
the supply of fuel in the float chamber. But in down draft carburetor this liquid fuel spill in to the
engine casing flooding. This will be clearly visible viewing in to the carburetor thro
at after
removing the air cleaner. In order to remove the effect of percolation, hold the throttle valve wide
open and crank the engine to exhaust the rich mixture.

2) Flooding:

In addition to uneven running of the engine, a strong odour of petrol is prese
when the carburetor is flooding. The cause are a) defective choke action b) fuel percolation c)
high float level of carburetor d) excessive fuel pump pressure.

If the flooding is due to over chocking or fuel percolation, hold the
throttle valve
wide open and crank the engine to exhaust the rich mixture. If it’s due to excessive pump
pressure, test the push rod stroke and rocker arm free play. The pump pressure can also be
reduced by placing more gaskets between the pump and engine



The type of fuel pump test that can be made depends upon whether or not the engine
is running and the kind of equipment available. Six different types of fuel pump tests are as



If the suction pipe is emptied, a good fue
l pump should prime itself within 20 sec by
cranking the engine. If the engine cannot be cranked, the fuel pump mounting screws should be
removed and pull the pump away from the mounting surface. Now dry the suction line and work
the pump operating liver.
In most vehicles the pump should prime itself start pumping fuel in 30
full strokes or less. If the pump fails to do this, it is defective. Possibly the pump valves are not


Fuel delivery

Three methods of determining whether or not fuel is being

delivered to
carburettor are possible.


Accelerating pump discharge

Remove the air cleaner and while looking down the
throat of a downdraft carbure
tor, operate the accelerating pump. With each stroke of the pump,
a small stream of fuel to be squirted
in to the throat which indicates that the float bowl containing
fuel is not being delivered to the carburetor or the accelerating pump is not functioning.

Float bowl drain plug

If the float bowl is provided with drain plug, loosen it but do not

If fuel comes out, fuel is not being delivered to the carburetor.

tor fuel line

By disconnecting the fuel pump to carburettor fuel pipe at the
carburetor and cranking the engine it is easy to determine if fuel is being delivered to the
tor by the fuel pump.


Vacuum test

This test is recommended when the pump fails to prime and in general
checking operation intends to reveal source of trouble. A fuel pump in good condition will
develop a vacuum of 150
250 mm.

Disconnect the intake

line from the fuel pump and connect a vacuum gauge to the pump at
this point. Crank the engine and observe the vacuum gauge. The reading should climb to 150
mm to 250 mm in a few revolutions. When the reading stops advancing, stop cranking and note
how lo
ng it takes the vacuum reading to drop to or almost zero. The reading on a pump in good
condition should not return to zero in less than 60 seconds. If the vacuum test indicates that the
pump is good, reconnect the fuel line and run the engine long enough
to refill the carburetor
bowl before proceeding with the next test.


Delivery pressure test

The float mechanism in a carburetor is designed to shut off the fuel
at a given height for a given pressure. If the fuel being delivered at a lower pressure, the

level will be low. If the fuel is being delivered At a higher pressure, the float level will be high.

Disconnect the pump to carburetor fuel line at the fuel pump and connect a pressure gauge
to the pump outlet. Crank the engine and observe the

fuel pressure. The pressure will build up to
its maximum in a few revolutions. Most carburetor float mechanisms are designed for a pressure
of 21
35 kpa.


Stroke test:

Wear the eccentric of the fuel pump push rod will shorten the fuel pump stroke.
may reduce the pump output to the point where, under maximum speed or load, sufficient
fuel is not supplied to the carburetor. As the fuel pump wears, the clearance increases at each of
the pivot points from the rocker arm to the diaphragm, thus reducing t
he diaphragm stroke.


Volume test:

The volume test is performed by disconnecting the pump to carburetor fuel line
and measuring the discharge separately by means of a graduated container and stop watch,
while the engine is running in idle speed. It is to

be noted that the engine in this case will run
only by petrol in the carburettor float chamber. If the observed rate of flow is less than the
specified ones, the pump needs repair or replacement.


Almost all fuel pumps can be dismantled for repair. Repair kits are available to repair the
defective fuel pump, with complete instructions to carry out the repair work. The instructions
given in the service manual should be followed to repair a