chap28 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 28, the reader should
be able to:

•

Prepare for Engine Repair (A1) ASE certification test
content area “C”(Engine Block Diagnosis and Repair).

•

Describe the purpose and function of pistons, rings, and
connecting rods.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 28, the reader should
be able to:

•

Explain how pistons and rods are constructed and what to
look for during an inspection.

•

Discuss connecting rod reconditioning procedures.

•

Explain how piston rings operate and how to install them on
a piston.

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KEY TERMS:
back clearance • balancing bosses (pads) • bleed hole • blowby
cam ground • connecting rod bearing journal • compression
rings • crank throw • crankpin
dish • ductile iron • double-knock • eyebrows • flat-top piston •
full floating • grooves • gudgeon pins
heat dams • hypereutectic • identification marks • inertia forces
• interference fit • lands • left-hand rule • lock rings
major thrust surface • notch • oil control ring
Continued
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KEY TERMS:
piloting surfaces • piston • piston pin • piston ring • piston ring
expanding tool • pop-ups • positive twist
recesses • reverse twist • ring gap • scraper ring • skirt • side
clearance • slipper skirt • slots • spit hole • struts
taper face ring • valve pockets • valve reliefs • wrist pin

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All engine power is developed by burning fuel in the presence
of air in the combustion chamber.
Heat from the combustion causes the burned gas to increase in
pressure.
The force of this pressure is converted into useful work
through the piston, connecting rod, and crankshaft.

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PURPOSE AND FUNCTION OF PISTONS,
RINGS, AND CONNECTING 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 connecting rod is connected
to a part of the crankshaft called
a crank throw, crankpin, or
connecting rod bearing
journal.
Figure 28–1
All pistons share these parts in common.

This dimension of the crankshaft determines the stroke of
the engine.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Continued

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NOTE: The stroke is the distance from the center of the main bearing
journal to the center of the connecting rod journal times two.

Piston rings seal the space between piston and cylinder wall,
keeping the pressure above the piston. When the pressure builds
up in the combustion chamber, it pushes on the piston.

The piston pushes on the piston pin and upper end of the connecting
rod. The lower end of the connecting rod pushes on the crank throw.
This provides the force to turn the crankshaft.
Inertia is the force that causes the crankshaft to continue rotating.
This action will bring the piston back to original position, where it
will be ready for the next power stroke. While the engine is running,
the combustion cycle keeps repeating as the piston reciprocates
(moves up and down) and the crankshaft rotates.
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PISTON AND ROD REMOVAL
After oil pan and cylinder head(s) are removed, the piston and
rod can be removed:
Step #1 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.
Figure 28–2
Punch marks on connecting rod and

rod cap to identify their location in the
engine.
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CAUTION: Powdered metal connecting rods should only be marked
with a permanent marker to avoid damage to the rod.

Figure 28–3 Powdered metal connecting rods
can be identified by their smooth appearance.

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Step #2 The crankshaft is turned until the piston is at the bottom
of its stroke. This makes connecting rod nuts or cap screws easily
accessible. They are removed, and the rod cap taken off. This
may require light tapping on the connecting rod bolts with a softfaced hammer.
Step #3 Protectors should be placed over the rod bolt threads to
protect the threads and the surface of the crankshaft journal. The
piston and rod assembly is pushed out, with care being taken to
avoid hitting the bottom edge of the cylinder with the rod.

The rod caps should be reattached to the rod after the assembly has

been removed from the cylinder. They are not interchangeable.
The assembly must be handled carefully. The rings are carefully
removed from the piston to avoid damage to piston or the ring. The
best way to remove them is to use a piston ring expanding tool.
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PISTON DESIGN
When the engine is running, the piston starts at the top of the
cylinder. As it moves down, it accelerates until it reaches
maximum velocity slightly before halfway down.
The piston comes to a stop at the bottom of the cylinder at 180

degrees of crankshaft rotation. During the next 180 degrees of
crankshaft rotation, the piston moves upward.
It accelerates to reach a maximum velocity slightly above the
halfway point and then comes to a stop at the top of the stroke.
The piston starts, accelerates, and stops twice in each crankshaft
revolution.
Continued
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NOTE: A typical piston in an engine at 4000 rpm accelerates from 0 to
60 miles per hour (97 kilometers per hour) in about 0.004 second

(4 milliseconds) as it descends about halfway down the cylinder.

Reciprocating action of the piston produces large inertia forces.
Inertia causes a part that is stopped to stay stopped or a part in
motion to stay in motion.
The lighter the piston can be made, the less inertia force that is
developed. Less inertia will allow higher engine operating speeds.
For this reason, pistons are made light as possible while still strong.
The piston operates with its head exposed to the hot combustion
gases, whereas the skirt contacts the relatively cool cylinder wall.
This results in a temperature difference of about 275°F (147°C)
between the top and bottom of the piston.
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Piston Weight Is Important!
All pistons in an engine should weigh the same to help ensure a balanced
engine. Piston weight becomes a factor when changing pistons. Most
aluminum pistons range in weight from 10 to 30 ounces (280 to 850
grams) (1 oz = 28.35 grams). A typical paper clip weighs 1 gram.
If the cylinder has been bored, larger replacement pistons are required. If
replacement pistons weigh more, this puts additional inertia loads on rod
bearings. To help prevent rod bearing failure on an overhauled engine,
replacement pistons should not weigh more than original pistons.
For the same reason, if one piston is being replaced, all pistons should be
replaced or checked and corrected to ensure the same weight.

CAUTION: Some less-expensive replacement cast pistons or high
performance forged pistons are much heavier than the stock pistons, even
in the stock bore size. This means the crankshaft may need heavy metal
added to the crankshaft counterweights for the engine to be balanced.
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PISTON HEADS
Because the piston head forms a portion of the combustion
chamber, its shape is very important to the combustion process.

Low-cost, low-performance engines have flat-top pistons. Some
come so close to the head that recesses are cut in the piston top.
Pistons used in high-powered engines may have raised domes or
pop-ups on the piston heads. These are used to increase the
compression ratio.
Pistons used in other engines may be provided with a depression or
a dish. Varying depths provide compression ratios required by
different engine models. Several piston head shapes are shown in
Figure 28–4.
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Figure 28–4 Piston head shapes: (a) flat (b) recessed, (c and d) pop- up, and (e and f) dished.

(a)

(b)

(c)

(d)

(e)

(f)

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Recesses machined or cast into tops of pistons for valve clearance
are called eyebrows, valve reliefs, or valve pockets. The depth has
effect on compression ratio and provides clearance for the valves if
the timing belt of an overhead camshaft engine breaks.

Without them pistons could hit valves near TDC if valves are not
operating (closing) due to nonrotation of the cam. An engine
designed to not have pistons hitting valves, is called freewheeling.
NOTE: Newer engines do not use valve reliefs because this requires that
the thickness of the top of the piston be increased to provide the necessary
strength. The thicker the top of the piston, the lower down from the top of

the top piston ring.
To reduce unburned hydrocarbon (HC) exhaust emissions, engineers
attempt to place the top piston ring as close to the top of the piston as
possible to prevent the unburned fuel from being trapped (and not burned)
between the top of the piston and the top of the top piston ring.
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CAM GROUND PISTONS
Aluminum pistons expand when they get hot. A method of
expansion control was devised using a cam ground piston skirt.

With this design, piston thrust surfaces closely fit the cylinder, and
the piston pin boss diameter is fitted loosely
As the piston is heated, it
expands along the piston
pin so it becomes nearly
round at normal operating
temperatures.
Figure 28–5 Piston cam shape. The
largest diameter is across the thrust
surfaces and perpendicular to the
piston pin (lettered A).

A cam ground piston skirt
is shown here.
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Figure 28–6 A piston diameter is measured across the thrust surfaces

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PISTON FINISH
Finish on pistons varies, but all are designed to reduce scuffing,

a condition where metal of the piston contacts the cylinder wall.
When the piston stops at the top of the cylinder, welds or transfer
of metal from one part to the other can take place.
Scuffing can be reduced by coating
piston skirts with tin 0.0005 inch
(0.0125 mm) thick or a molygraphite coating.

Figure 28–7 A moly - graphite coating on the
skirt of this piston from a General Motors 3800
V-6 engine helps prevent piston scuffing when
the engine is cold.
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PISTON HEAD SIZE
The piston top or head smaller in diameter than the rest of the
piston. The top is exposed to more heat and expands more than
the rest of the piston.
Most pistons have
horizontal separation
slots that act as heat dams.
They reduce heat transfer
from the hot piston head to
the lower skirt.

Figure 28–8
Piston skirt cam shape.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Because the slot is placed
in the oil ring groove, it
can be used for drainback
and expansion control.
Continued

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PISTON STRUT INSERTS
A major development in expansion control occurred when the
piston aluminum was cast around two stiff steel struts. This
keeps the piston skirt thrust surfaces from expanding more
than the cast-iron cylinder in which the piston operates.
Pistons with steel strut inserts allow good piston-to-cylinder
wall clearance and allow the cold operating clearance to be as
small as 0.0005 inch (one-half thousandth of an inch) (0.0127
millimeter).
This small clearance will prevent cold piston slap and noise.

Continued
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With newer engines, number and thickness of the piston rings
have
decreased and the cast-aluminum piston skirt has been reduced
to a
minimum by using an open-type slipper skirt.
Figure 28–9 Steel struts cast inside pistons
help control expansion.

Examples of the slipper
skirt piston are shown
in Figure 28–10.
Continued
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Figure 28–10a & b
Two sectional views of a slipper-skirt-type piston that uses a steel expansion strut.

Continued
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HYPEREUTECTIC PISTONS
A standard cast-aluminum piston contains about 9% to 12% silicon
and is called a eutectic piston. To add strength, the silicon content is
increased to about 16%, and the called a hypereutectic piston.
Other advantages of a hypereutectic piston are its 25% weight
reduction and lower expansion rate. The disadvantage of
hypereutectic pistons is higher cost, as 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.

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