Mech 10

Pistons, Rihgsr
and Connecting Rods
OBf ECTIVES! After studying Chapter 10, the reader
should be able to:
| . Prepare for ASE Engine Repair (A | ) certification test
content area"C" (Engine Block Diagnosis and Repair).
2. Describe the purpose and function of pistons, rings,
and connecting rods.
_
3. Explain how pistons and rods are serviced and
inspected.
4. Discuss connecting rod reconditioning procedures.
5. Explain how piston rings operate and how to install
them on a piston.
The connecting rod is connected to a part of the
crankshaft called a crank throw, crankpin, or
connecting rod bearing journal, which is another
swivel joint. The center of the crank throw determines
the amount by which the large end of the
connecting rod is offset from the crankshaft main
bearing centerline. This dimension of the crankshaft
determines the stroke of the engine.
S#{gY&, The stroke is the distance from the center of
the main bearing journal to the center of the connecting
rod journal times two.
ffi PISTON AND ROD REMOVAT
Idfll engine power is developed by burning fuel in the
presence of air in the combustion chamber. Heat from
the combustion causes the pressure of the burned gas
to increase. The force of this pressure is converted
into useful work through the piston, connecting rod,
and crankshaft.
ffi PURPOSE AND FUNCTION
oF ptsToNs, RtNGs,
AND COI.INECTING RODS
The piston forms a movable bottom to the combustion
chamber. It is attached to the connecting rod
with a piston pin or wrist pin. The piston pin is aIlowed
to have a rocking movement because of a
swivel joint at the piston end of the connecting rod.
The rod and caps should be checked for markings
that identify their location. If the rod and caps are
not marked, they should be marked before disassembly.
If number stamps are not available, punch
marks can be used.
CAUTION: If powdered-metal connecting rods are
being disassembled, do not use a number punch to
mark the rods (see Figure 10-1). The distortion caused
by the hammer blow can ruin the connecting rod. Most
vehicle manufacturers say that an electric engraving
tool can be used to identify the location ofthe rods in an
engine. Always follow the method recommended bv the
vehicle manufacturer.
The crankshaft is rotated until the piston is at
the bottom of its stroke. Remove the connecting rod
188

Pistons, Rings, and Connecting Rods
t89
Figure l0-2 A high-performance piston with valve reliefs
machined into the head of the piston to provide for valve
ctearance.
Figure l0-l Powdered metal connecting rods can be
identified by their smooth appearance.
nuts or cap screws and remove the connecting rod
cap. Place protective covers over the connecting rod
bolt threads before pushing the piston/rod assembly
out of the engine through the top of the cylinder. The
protectors help protect the crankshaft journal from
possible damage.
ffi PtsToNs
Recesses machined or cast into the tops of the pistons
for valve clearance are commonly called eyebrows.
These recesses are also called valve reliefs
or valve pockets. The depth of the eyebrows has a
major effect on the compression ratio and is necessary
to provide clearance for the valves if the timing
belt of an overhead camshaft engine should break.
See Figures 10-2 and 10-3.
Piston ring grooves are located between the piston
head and skirt. The width of the grooves, the
width of the lands between the ring grooves, and the
Figure | 0-3 Another style of piston with a recessed
head to not only provide valve clearance but also control
comoression ratio.
number of rings are major factors in determining
minimum piston height. The outside diameter of the
Iands is about 0.020 to 0.040 in. (0.5 to 1.0 mm)
smaller than the skirt diameter. See Figure 10-4.
See Figure 10-5 for an example of piston diameter
being measured. Always check the service manual
for the exact location and specifrcations for the
piston being checked.

t90
CHAPTER IO
Figure I0-4 A typical piston illustrating the various parts
and the names.
Figure | 0-6 The piston rings must have the specified
side and back clearance.
PISTON PIN
S}IAPE AT ROOM
TEIiPERATURE
Pt$TOil 80SS
Figure l0-5 All pistons are measured across the skirt.
F##gA; Some engines, such as the Honda high-fueleconomy
engine, use pistons with two rings: one compression
ring and one oil ring.
The piston ring groove must be deep enough to
prevent the ring from hitting the base of the
groove when the ring is pressed in so that it is flat
with the land face. This is called back spacing.
See Figure 10-6.
Cam Ground Pistons
Aluminum pistons expand when hot.To control the expansion
of the piston, the skirt of the piston is cam
SIIAPE AT OPERATII{G
TET'PERATURE IN A
RUNI{|NG ENGIf'|E
Figure l0-7 Pistons are often cam-ground to produce the
elliptical shape when the piston is at room temperature.
ground. With this design, the piston thrust surfaces
closely fit the cylinder, and the piston pin boss diameter
is fitted loosely. As the cam ground piston is heated,
it expands along the piston pin so that it becomes
nearly round at its normal operating temperatures. A
cam ground piston skirt is illustrated in Figure 10-7.

Pistons, Rings, and Connecting Rods
t9l
Piston Weight ls lmportantl
All pistons in an engine should weigh the same to help
ensure a balanced engine. Piston weight becomes a factor
whenever changing pistons. Most aluminum pistons
range in weight from l0 to 30 ounces (280 to 850
grams) (l oz. : 28.35 g). A typical paper clip weighs one
grom. If the cylinder has been bored, larger replacement
pistons are obviously required. lf the replacement pistons
weigh more, this puts additional inertia loads on
the rod bearings.
FORGED 2OOO
CAUTION: Some less expensive replacement pistons
are a great deal heavier than the stock pistons, even in the
same stock bore size. lf these heavy pistons are used, connecting
rod bearing failure can occur.
For the same reason, if one piston is being replaced,
all pistons should be replaced or at least checked and
corrected to ensure they have the same weight.
Hypereutectic Cast Pistons
A standard cast-aluminum piston contains aboutg7o
to l27o silicon and is called a eutectic piston. To add
strength, the silicon content is increased to about
167o, and the resulting piston is called a hypereutectic
piston. Other advantages of a hypereutectic
piston are its 257a weight reduction and lower expansion
rate. The disadvantage ofhypereutectic pistons
is their higher cost, because they are more difficult
to cast and machine.
Hypereutectic pistons are commonly used in the
aftermarket and as original equipment in many turbocharged
and supercharged engines.
Forged Pistons
High-performance engines need pistons with added
strength. They use impact-extruded forged pistons
whose design allows for great strength and is used
whenever the engine is going to be exposed to extreme
operation conditions such as racing. A forged
piston is denser than a cast piston and conducts heat
faster so it operates cooler. See Figure 10-8.
Piston Skirt Finish
For maximum life, the piston skirt surface finish is
important. Turned grooves or waves 0.0005 inch
(0.0125 millimeter) deep on the surface of some piston
skirts produce a finish that will carry oil for lu-
Figure l0-8 Notice the temperature difference between
a forged piston and a cast piston.
brication. Other piston skirts are relatively smooth.
A thin tin-plated surface (approximately 0.00005
inch or 0.00125 millimeter thick) is also used on
some aluminum pistons to help reduce scuffing and
scoring during occasional periods of minimum lubrication.
Some pistons have a Teflon coating on the
skirt to prevent scuffr.ng, as shown in Figure 10-9.
Piston Pins
CAST
Piston pins are used to attach the piston to the connecting
rod. Piston pins are also known as gudgeon
pins (a British term). The piston pin transfers
the force produced by combustion chamber
pressures and piston inertia to the connecting rod.
The piston pin is made from high-quality steel in
the shape of a tube to make it both strong and light.
See Figure 10-10.

192 CHAPTER IO
TAPSRED BOBS
STRAIGHT BORE
Figure l0-10 Cross-sectional piston pins. Most piston
pins are hollow to reduce weight and have a straight bore.
Some pins use a tapered bore to add strength.
Figure l0-9 A low-friction moly coating on the skirt of
this piston helps prevent piston scuffing when the engine is
cold.
t--
How Can a Piston Have
a Negative Clearancet
MAJOR
THRUST -------*
SURFACE
The specification for some DaimlerChrysler engines includes
a piston-to-cylinder wall clearance as small as
0.0005 in. negotive. This means that the piston is larger
than the bore! How is that possible? According to DaimlerChrysler,
a service technician cannot accurately measure
the diameter of the piston and determine the piston-to-cylinder
clearance due to the application of the
antifriction coating on the sl

Pistons, Rings, and Connecting Rods
t93
BTDC
COMPRESSION
STROKE
Figure l0-12 Enginerotationandrodangleduringthepowerstrokecausestheengine
to press harder against one side of the cylinder, creating a major thrust surface. In this
clockwise-rotating engine, as viewed from the front of the engine, the maior thrust
surface is on the left side.
by the pin offset. As the piston moves up in the cylinder
on the compression stroke, it is riding against the
minor thrust surface. When compression pressure becomes
high enough, the greater head area on the minor
side causes the piston to cock slightly in the cylinder.
This keeps the top of the minor thrust surface on
the cylinder. It forces the bottom of the major thrust
surface to contact the cylinder wall. As the piston approaches
top center, both thrust surfaces are in contact
with the cylinder wall. When the crankshaft
crosses over top center, the force on the connecting
rod moves the entire piston toward the major thrust
surface. The lower portion of the major thrust surface
has already been in contact with the cylinder wall.
The rest of the piston skirt wipes into full contact just
after the crossover point, thereby controlling piston
slap. This action is illustrated in Figure 10-12.
ru#Tffic Not all piston pins are offset. In fact, many engines
operate without the offset to help reduce friction
and improve power and fuel economy.
Piston Pin Retaining Methods
It is necessary to retain or hold piston pins so that
they stay centered in the piston. If piston pins were
not retained, they would move endwise and groove
the cylinder wall. Piston pins are retained in one of
two common ways. The piston pin may be full floating,
with some type of stop located at each end. Fullfloating
piston pins in automotive engines are re-
What's That Knocking Noiset
A worn piston pin or an enlarged piston pin hole in the
piston causes a double knock sound when the engine is
running. Unlike other knocking sounds, the sound of a
worn piston pin will not disappear when the cylinder is
grounded out and prevented from firing. Obviously, the
engine will have to be disassembled and replacement piston(s)
installed to correct the knocking noise.
tained by lock rings located in grooves in the piston
pin hole at the ends of the piston pin. See Figures
10-13 and 10-14.
The modern method of retaining the piston pin
in the connecting rod is to make the connecting rod
hole slightly smaller than the piston pin. The pin is
installed by heating the rod to expand the hole or by
pressing the pin into the rod. This press or shrink fit
is called an interference fit.
ffi COMPRESSION RllrlGS
A compression ring is designed to form a seal between
the moving piston and the cylinder wall. Combustion
chamber pressure during the compression, power, and

194 CHAPTER IO
CIRCLIP
r
='., ,e".
€a**,
MACHINED SLOT
Figure l0-13 Circlips or snap rings hold full-floating
piston pins in place.
Big Problem, No Noise
Sometimes the piston pin can "walk" off the center of the
piston and score the cylinder wall. This scoring is often
not noticed because this type of wear does not create
noise. Because the piston pin is below the piston rings, little
combustion pressure is lost past the rings until the
groove worn by the piston pin has worn the piston rings.
Troubleshooting the exact cause ofthe increased oil
consumption is difficult because the damage done to the
oil control rings by the groove usually affects only one
cylinder.
Often, compression tests indicate good compression
because the cylinder seals, especially at the top. More
than one technician has been surprised to see the cylinder
gouged by a piston pin when the cylinder head is removed
for service. ln such a case, the cost of the engine
repair immediately increases far beyond that of normal
cylinder head service.
Figure !8*[4 This high-performance connecting rod uses
a bronze bushing in the small end of the rod and an oil
hole to allow oil to reach the full-floating pin.
COMPRSS$IOtt, FORCE
,*,+-+ P ,-*.*\
U\a\L\&l\bb \
exhaust strokes is applied to the top and back ofthe
ring. This pressure will add the force to the ring that
is required to seal the combustion chamber during
these strokes. Figure 10-15 illustrates how the combustion
chamber pressure adds force to the ring.
Top compression rings are usually rectangular
or barrel shaped. The barrel is 0.0003 inch per
0.100 inch (0.0076 millimeter per 0.254 millimeter)
of the piston ring width. See Figure 10-16. A piston
ring with a taper face would contact the cylinder
wall at the lower edge of the piston ring. See Figure
l-0-17. Positive twist will give the same wall contact
as the taper-faced ring and will also provide a
Fignrn e lG*[5 Combustion chamber pressure forces the
ring against the cylinder wall and the bottom of the ring
groove. These are the two sealing surfaces that the top
ring must be able to seal for maximum engine power.
line contact seal on the bottom side ofthe groove. By
chamfering the ring's lower inner corner, a reverse
twist is produced. This seals the lower, outer section
of the ring and piston ring groove, thus improving oil
control. Reverse twist rings require a greater taper
face or barrel face to maintain the desired ring-faceto-cylinder-wall
contact. See Figure 10-18.

Flsi*r-os, Ring:. e;rd C*nn*crir-rg Rnds
t9s
RECTANGULAR
POSIT'VE TORSIOT.IAL
TWIST
BARREL FACE
REVERSE TORSIOIIIAL
TWIST
Figure E ** I 6 The rectangular and the barrel face are the
most commonly used top compression rings because they
provide the best seal.
Figure [&*fl& Torsional twist rings provide better
compression sealing and oil control than regular taper face
rings.
TAP€R FACE
Figara"+: !G:E? The taper face ring provides good oil
control by scraping the cylinder wall. lf this design ring
were accidentally installed upside down, the tapered face
would pump oil into the combustion chamber.
ffi*mg Gap
The piston ring gap will allow some leakage of gases
past the top compression ring. This leakage is useful in
providing pressure on the second ring to develop a dynamic
sealing force. The amount of piston ring gap is
critical. Too much gap will allow excessive blowby, the
leakage of combustion gases past the rings. Too little
gap, on the other hand, will allow the piston ring ends
to butt when the engine is hot. Ring end butting increases
the mechanical force against the cylinder wall,
causing excessive wear and possible engine failure.
Figur* !&*E€ Chromium facing can be seen on the right
side of the sectional view of the piston ring.
materials are present in the air. During manufacture,
the chromium-plated ring is slightly chamfered
at the outer corners. About 0.0004 inch (0.010 millimeter)
of chrome is then plated on the ring face.
Chromium-faced rings are prelapped or honed before
they are packaged and shipped to the customer. The
frnished chromium facing is shown in a sectional
view in Figure 10-19.
#ffinem$aeBE:+ trE1s #F'E ffi**tgs
A chromium facing on cast-iron rings greatly increases
piston ring life, especially where abrasive
Metybd*ffic&#e P$s€*se #€nmgs
Early in the 1960s, molybdenum piston ring faces
were introduced. These rings proved to have good

t96
CHAPTER IO
Gap the Second Ring Wider
Than the Top Ring
In the past, it was standard operating procedure that vehicle
and piston ring manufacturers specified a wider piston
ring gap on the top ring and a narrower gap for the
second compression ring. The reason given was the top
ring was exposed to greater heat and, therefore, would
expand more than the second ring.
Today many vehicle manufacturers, as well as piston
ring manufacturers, are specifying that the second ring
gap should be greater than the top ring gap. The reason
given is that the larger second ring gap prevents combustion
pressure between the rings that can cause the top
ring to be lifted up off the bottom of the ring groove. lf
the top ring is raised up, it cannot properly seal the combustion
gases. With this in mind, many high-performance
engine builders are specifying a standard ring gap for the
top ring and a ring gap of 0.002 to 0.004 in. greater for
the second ring.
service life, especially under scuffing conditions.
Most molybdenum-faced piston rings have a groove
that is 0.004 to 0.008 inch (0.1 to 0.2 millimeter) deep
cut into the ring face. This groove is filled with
molybdenum, using a metallic (or plasma) spray
method, so that there is a cast-iron edge above and
below the molybdenum. A sectional view of a molybdenum-faced
ring is shown in Figure 10-20.
A molybdenum-faced ring, when used, will be
found in the top groove, and a plain cast-iron or
chromium-faced ring will be found in the second
groove.
Moly-Chrome-Carbide Rings
Rings with a moly-chrome-carbide coating are also
used in some original equipment (OE) and replacement
applications. The coating has properties that
include the hardness of the chrome and carbide
combined with the heat resistance of molybdenum.
Ceramic-coated rings are also used when additional
heat resistance is needed, such as in some heavyduty,
turbocharged, or supercharged engines.
Oil Control Rings
Steel rails with chromium or other types of facings
are used on most oil control rings. The rails are
backed with expanders and separated with a
spacer, seen in Figure 10-21.
Figure l0-20 Molybdenum facing can be seen on the
right side of the sectional view of the piston ring.
IIiIi
CONNECTIHG RODS
The connecting rod transfers the force and reciprocating
motion of the piston to the crankshaft. The small
end of the connecting rod reciprocates with the piston.
The large end rotates with the crankpin. See Figure
10-23. These dynamic motions make it desirable to
keep the connecting rod as light as possible while still
having a rigid beam section. Lightweight rods also reduce
the total connecting rod material cost.
Connecting rods are manufactured by casting,
forging, and powdered (sintered) metal processes.
Forged connecting rods have been used for years and
have a thick parting line along the length of the rod.
They are always used in high-performance engines
and are generally used in heavy-duty engines. The
cost of cast rods is lower than that of forged rods,
both in the initial casting and in the machining. Cast
rods can be identified by a thin parting line along the
length of the rod. Generally, the forging method produces
lighter weight and stronger, but more expensive,
connecting rods.
Powdered Metal Connecting Rods
Some production engines, such as the General Motors
Northstar, have been switched from forged to pow-

Pistons, Rings, and Connecting Rods
t97
Figure l0-2 I This typical three-piece oil control ring uses a hump-type stainless steel
spacer-expander. The expander separates the two steel rails and presses them against
the cylinder wall.
PISTON
CONl'lECllNG
ROD
ROD
SOLT
Figure l0-22 The gapless ring overlaps, while the
conventional ring design uses a gap.
dered metal (PM) rods, which proved to be stronger.
Each ofthe rods is blended into a tapered I-beam section.
The large split-ring form for the crankshaft end
is machined after the cap is assembled on the rod.
:\:'i::l'.:":i-:+::ni:i.:11:.++:=.i::::!iFa!:::::!ilrl=l:€*:=+:i-!:l++E+:=+*q:i*:*;l.:€iigEffiT
I iriri.:T+:t: Most powdered (sintered) metal connecting i
, rods are broken at the parting end ofthe big end ofthe !
i connecting rod. This rough, broken surface helps en- j
: sure a perfect match when the pieces are bolted to- I
, sether' se:-li*:."
1o-?,n- ""..- -" -- " : i
The hole will be a perfect circle. Therefore, the rod
caps must not be interchanged. Assembly bolt holes
are closely reamed in both the cap and connecting
rod to ensure alignment. The connecting rod bolts
Figure l0-23 A typical connecting rod and related
engine parts. The connecting rod is probably the most
highly stressed part in the engine. Combustion forces try
to compress it and when the piston stops at the top of the
cylinder, inertia forces try to pull it apart.
have piloting surfaces that closely frt these
reamed holes.
Connecting Rod Design
In some engines, offset connecting rods provide the
most economical distribution of main bearing space

98
CHAPTER iO
BALANCE
PAD
Figure | 0-24 Most powdered metal cast connecting
rods are broken at the parting line.
and crankshaft cheek clearance. Some V-6 engines
have the connecting rods offset by approximately
0.100 inch (2.54 millimeters).
Connecting rods are made with balancing bosses
so that their weight can be adjusted to specifications.
Some have balancing bosses only on the rod cap. Others
have a balancing boss above the piston as weII.
Some manufacturers put balancing bosses on the
side ofthe rod, near the center ofgravity ofthe connecting
rod. Typical balancing bosses can be seen in
Figure 70-25. Balancing is done on automatic balancing
machines as the final machining operation
before the rod is installed in an ensine.
Connecting Rocl Service
As an engine operates, the forces go through the
large end of the connecting rod. This causes the
crankshaft end opening ofthe rod (eye) to gradually
deform. The large eye of the connecting rod is resized
during precision engine service. See Figure 70-2G.
Step I The parting surfaces of the rod and cap are
smoothed to remove all high spots before resizing.
A couple of thousandths of an inch of metal
is removed from the rod cap parting surface. The
amount removed from the rod and rod cap only
reduces the bore size 0.003 to 0.006 inch (0.08 to
0.15 millimeter).
F{O?S; Powdered metal connecting rods cannot be reconditioned
using this method. Most manufacturers
recommend replacing worn powdered metal connecting
rods.
Figure l0-25 Some connecting rods have balancing
bosses (pads) on each end of the rod.
Step 3 The cap is installed on the rod, and the nuts
or cap screws are properly torqued.
S**p 3 The hole is then bored or honed to be perfectly
round, as shown in Figure 10-27.
Sf*p 4 The connecting rod is checked for twist (misalignment).
See Figure 10-28.
ffi PTSTON AND ROD ASSEMBTY
To assemble the piston and rod, the piston pin is put
in one side of the piston. The piston and rod are
placed on a press, using adapters and supports. This
setup is shown in Figure t0-29. The pin is pressed
into the rod until it is centered. The press-fit of the

Pistons, Rings, and Connecting Rods
t99
Figure l0-27 A connecting rod being reconditioned.
SURFACE
PLATE
Figure l0-26 The rod bearing bores normally stretch
from top to bottom causing the rod bearing to wear most
near the parting line.
I
BENO
INDICATOR
The Might-As-Wells
One of the hardest questions a technician or vehicle
owner faces is,"How much work should be done to make
a proper repair without incurring too much cost?" The
technician wants to make a proper repair to prevent an
early failure (and a customer comeback). The vehicle
owner does not want to spend any more than is necessary
for the repair service.
When the engine is disassembled, many small procedures,
such as connecting rod reconditioning, are often
left unperformed because of additional effort and cost.
Connecting rod reconditioning is one of many operations
considered important enough to be included in a proper
engine repair or overhaul. Many engine rebuilders also
recommend replacing all connecting rod bolts whenever
the engine is disassembled. The major expense is removing
the engine from the vehicle; therefore, the technician
"might as well" do a complete and thorough engine service
job and convince the customer that the added expense
of "doing it right" is money well spent.
pin in the small eye of the rod will hold the pin securely
in place during engine operation. In precision
engine shops, the small eye of the connecting rod is
heated before the pin is installed. This causes the rod
eye to expand so that the pin can be pushed into
CONTACT
BAf,S
CONNECTING
ROD
Figure l0-28 A rod alignment fixture being used to
check a connecting rod to see if it is bent or twisted.
place with little force. The pin must be rapidly
pushed into the correct center position.
Full-floating piston pins operate in a bushing in
the small eye of the connecting rod. The full-floating
piston pin is held in place with a lock ring at each
end. The lock rings should always be replaced with
new rings.
There is usually a noteh on the piston head indicating
t}i'e front. Using this will correctly position
the piston pin offset toward the right side of the engine.
The connecting rod identification marks on
pushrod inline engines are normally placed on the
camshaft side.

CHAPTER IO
Figure l0-30 Using a connecting rod bolt stretch gauge
to measure the amount the rod bolt stretches to tighten
the fastener to its ultimate strength.
:#ffd '
Figure l0-29 A press used to remove and install
connecting rods to the pistons.
Use a Rod Bolt Stretch Gauge
When a fastener is torqued to specification, it may or
may not be applying the clamping force that is necessary.
Many high-performance engine builders use a connecting
rod bolt stretch gauge to measure the actual amount the
rod bolt is being stretched. This process results in uniform
clamping force on all connecting rod cap bolts. See
Figure l0-30.

Pistons, Rings, and Connecting Rods 201
lEf ffi
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ffi
Lr F
P8-l New piston rings, piston ring expanders, piston
groove cleaner, and a piston ring grinder are all
necessary items to fit piston rings to a particular
P8-2 Remove the old piston rings from the piston
using a piston ring expander tool.
' _ . 'l'
f*! a.
P8-3 Carefully clean the piston ring grooves using a
piston ring groove-cleaning tool.
P8-4 The oil drain holes behind the oil control rings
should be cleaned.
" '+.q.
P8-5 Whenever fitting piston rings to pistons, always
be certain to properly assemble the piston on the
connecting rod. The rod should be marked with its
cylinder position in the engine.
P8-6 Piston rings should be checked for proper size
and application. Consult the instructions for the
meaning of the marks (data) on the rings.

202 CHAPTER IO
*
P8-7 Measure the side clearance between the piston
ring and the piston ring groove using a feeler (thickness)
gauge. Compare the readings to factory specifications
(usually between 0.001 in.-0.003 in.).
P8-8 Insert a piston ring into the proper cylinder and
use a piston upside down to help position the ring
squarely in the cylinder bore.
P8-9 Use a feeler gauge to measure the piston ring
end gap and compare ro the factory specifications
(usually 0.004 in. per inch of bore).
P8- l 0 lf the piston ring end gap is too close, use a
grinder to remove material from the ends of the piston
rings until the proper end gap has been achieved.
P8-l I Carefully install the piston rings onto the piston
using a piston ring expander.
P8-12 Be sure to position the gap of the rings
according to factory recommendations before installing
the piston in the cylinder.

Pistons, Rings, and Connecting Rods 203
T SUMMARY
1. The connecting rods should be marked before disassembly.
2. Pistons are cam ground so that when operating temperature
is reached, the piston will have expanded
enough across the piston pin area to become round.
3. Replacement pistons should weigh the same as the
original pistons to maintain proper engine balance.
4. Some engines have an offset piston pin to help reduce
piston slap when the engine is cold.
5. Piston rings usually include two compression rings at
the top ofthe piston and an oil control ring below the
compression rings.
6. If the ring end gap is excessive, blowby gases can
travel past the rings and into the crankcase.
7. Many piston rings are made of coated cast iron to provide
proper sealing.
8. If the connecting rod is twisted, diagonal wear will be
noticed on the piston skirt.
9. Powdered metal connecting rods are usually broken at
the big end parting line. Because of this rough junction,
powdered metal connecting rods cannot be re-
;;ditloned-they
must be replaced if damaged or
10. The piston and the connecting rod must be correctly
assembled according to identifying notches or marks.
r REVTEW QUESTTONS
1. Describe the procedure for correctly removing the piston
and rod assembly from the engine.
2. Why are some piston skirts plated or coated?
3. Describe the effect ofthe piston pin offset as it controls
piston slap.
4. Why is it important to keep the connecting rod cap with
the rod on which it was originally used and to install it
in the correct way?
5. Describe how connecting rods are reconditioned.
6. How is the piston pin installed in the piston and rod assembly?
X ASE CERTIFICATION.TYPE
QUESTTONS
1. Connecting rod caps should be marked (if they were not
marked at the factory) before the piston and connecting
rod assembly is removed from the engine _.
a. Because they are balanced together
b. Because they are machined together
c. To make certain that the heavier rod is
matched to the heavier piston
d. To make certain that the lighter rod is
matched to the lighter piston
2. Many aluminum pistons skirts are plated with _.
a. Tin
b. Lead
c. Antimony
d. Terneplate
3. A hypereutectic piston has
a. A higher weight than a eutectic piston
b. Ahigher silicon content
-.
c. Ahigher tin content
d. Ahigher nickel content
4. Protective covers should be placed over the connecting
rod threads to help prevent damage to the _.
a. Connecting rod
b. Crankshaft
c. Cylinder wall
d. Camshaft
5. Full-floating piston pins are retained by _.
a. Lock rings
b. A drilled hole with roll pin
c. An interference fit between rod and piston pin
d. An interference fit between piston and piston
pin
6. A misaligned connecting rod causes what tlpe of engine
wear?
a. Cylinder taper
b. Barrel-shaped cylinders
c. Ridge wear
d. Angle wear on the piston skirt
7. Piston damage is most likely to be caused by _.
a. Valves hitting the piston head
b. Abnormal combustion
c. Lugging the engine during operation
d. High engine speeds that can break piston
heads
8. The diameter of the piston is measured _.
a. Across the top (head) ofthe piston
b. Across the piston pin
c. Across the thrust surface
d. Between the top and second piston ring
9. Full-floating piston pins use an interference fit between
the piston and the piston pin.
a. True
b. False
10. A worn piston pin causes what type of problem?
a. Engine burns an excessive amount of oil (blue
smoke)
b. Engine produces a knocking noise that will
disappear ifthe cylinder is grounded out
c. Engine produces a double knocking noise that
will not disappear if the cylinder is grounded
out
d. Engine knock when cold only