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7/31/2019 4.Pistons Rings Rods
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Pistons, Rings, and
Connecting Rods
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Pistons
The piston's primary
responsibility is to take
thermal energy created
by the ignition of fuel
and air, and transform it
into linear motion. Linear motion acts on the
crankshaft journal and
becomes rotary motion.
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A typical pistonillustrating thevarious partsand the names.
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Piston Heads (Crown)
Domed
Flat Top
Recessed (valve reliefs)
Dished
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A domed piston with valvereliefs or valve pockets.
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A Flat Top piston
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A Flat Top piston with valvereliefs or valve pockets.
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A Dished piston with valvereliefs or valve pockets.
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Piston Skirts
Moly Graphite
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Piston Skirts
Finished
perpendicular
to the head
Scuffing
promotes
blow-by
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A low-friction moly coating on the skirt ofthis piston helps prevent piston scuffingwhen the engine is cold.
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Piston Composition @ Process
Iron – heavy machinery or “old school”
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Piston Composition @ Process
Aluminum - cast
Pour aluminum into a
mold Light-weight
economical
Some silicone added
General usage Brittle
Somewhat
unpredictable expansion
qualities
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Piston Composition @ Process
Hypereutectic
Cast aluminum
with a high siliconcontent
Light-weight
Higher performance
Less brittle
Predictable
expansion qualities
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Piston Composition @ Process
Aluminum - Forged Can be made lighter
weight (smaller) thancast because it’sstronger
Can withstand abuse
Newer designs have
predictableexpansion qualities
Silicon & Nickeladded
Greater piston to wallclearance
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Notice the temperaturedifference between a forgedpiston and a cast piston.
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Pistons are often cam-ground toproduce the elliptical shape whenthe piston is at room temperature.
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Piston diameterbeing measured
using a micrometer.
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Piston (wrist) Pins
High-Quality steel
Usually Hollow
Cross-sectional pistonpins.
Most piston pins are
hollow to reduceweight and have astraight bore.
Some pins use atapered bore to add
strength.
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Piston pin is offset toward
the major thrust surface.
TECH
TIP
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Engine rotation and rod angle during thepower stroke causes the engine to pressharder against one side of the cylinder,creating a major thrust surface.
In this clockwise-rotating engine, asviewed from the front of the engine, themajor thrust surface is on the left side.
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Piston Pin Retaining Methods
Full Floating
Lock Rings
Circlips or snap ringshold full-floatingpiston pins in place.
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Piston Pin Retaining Methods
Interference Fit
Pressed
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Piston Rings
Compression
Oil – usually unidirectional
Ring expander
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Compression Ring Composition
Pearlitic
Nodular Iron
Ductile Iron – flexible
Cast Iron
Chromium - .0004” - durable
Molybdenum – reduced scuffing
Chrome-moly
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The preferred material for compression rings is a
low-alloyed, heat-treated nodular cast iron
(KV1/GOE 52). This material is characterized by
a high bending strength of min. 1300 MPa and a
high modulus of elasticity attributable to a
martensitic microstructure and spheruliticgraphite structure.
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•In the 2nd groove, alloyed grey cast irons are used in aheat-treated condition.
•Besides having a high bending strength and modulus of
elasticity, an increased hardness of 320 to 470 HB is
produced in order to obtain the required wear resistance in
the uncoated condition.
•The demand for high wear strength is also met by the use
of a tempered, alloyed cast iron (GOE 44). This has the
benefit of a high bending strength of min. 800 MPa and
high modulus of elasticity.•The good wear resistance results from the combination of
a fine-pearlitic matrix structure and finely dispersed,
precipitated secondary carbides.
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Unalloyed grey cast iron is used for 2-piece oil
rings in the 3rd groove. These ring materials (STD
/ GOE 12, GOE 13) are characterized by a fine-lamellar graphite structure in a pearlitic matrix and
have good conformability due to a relatively low
modulus of elasticity.
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•Reduced width piston rings in gasoline engines to match
reductions in the overall height of pistons, and increasing
combustion pressures in diesel engines call for materials withincreased strength characteristics.
•These challenges are met by the use of high-chromium alloyed
steels and spring steels.
•The greater durability under increased stresses is demonstratedby the improved fatigue strength manifested as form stability in a
comparison of S/N curves for different piston ring materials
(spherulitic, heat-treated cast iron versus heat-treated 18%
chromium steel).
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•The wear resistance derives from finely distributed
chromium carbides of the type M23C6 and M7C3 embedded
in the tempered martensite matrix.•For improved wear resistance these steels are mainly
used in a nitrided condition or with a peripheral coating.
•The steels mentioned are used chiefly as compression
ring materials for gasoline engines and truck diesel enginesas well as for the steel rails and expander-spacers of oil
control rings and for 2-piece profiled steel oil rings.
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Pearlitic Rings
Pearlite is a two-phased, lamellar (or layered) structurecomposed of alternating layers of alpha-ferrite (88 wt%) and
cementite (12%) that occurs in some steels and cast irons. The eutectoid composition of Austenite is approximately
0.8% carbon ; steel with less carbon content will contain acorresponding proportion of relatively pure ferrite crystallitesthat do not participate in the eutectoid reaction and cannot
transform into pearlite. The appearance of pearlite under the microscope resembles
mother of pearl (also a lamellar structure), from which ittakes its name.
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Nodular (ductile) Iron
Ductile iron, also called ductile cast iron,
spheroidal graphite iron, nodular cast iron, isa type of cast iron invented in 1943 by Keith
Millis.
While most varieties of cast iron are brittle,
ductile iron is much more flexible and elastic, dueto its nodular graphite inclusions.
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Chromium rings
Chromium is a steely-gray, lustrous, hard
metal that takes a high polish and has a highmelting point.
It is odourless, tasteless, and malleable.
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Chromium facing can be seen on the rightside of the sectional view of the piston ring.
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Molybdenum Rings
Molybdenum (pronounced
/məˈlɪbdənəm)
It has the sixth-highest melting point
of any element, and for this reason
it is often used in high-strength steel
alloys.
Molybdenum was discovered in1778 by Carl Wilhelm Scheele and
first isolated in 1781 by Peter Jacob
Hjelm.
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Molybdenum facing can be seenon the right side of the sectionalview of the piston ring. TECH
TIP
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This typical three-piece oil control ring uses ahump-type stainless steel spacer-expander.
The expander separates the two steel railsand presses them against the cylinder wall.
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Ring Gaps
Ring gap must be checked prior to engine
assembly
Loose
Tight
Butt gap
Gapless?
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The gapless ring overlaps, while theconventional ring design uses a gap.
Frequently
Asked Question
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Gapless Ring
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Combustion chamber pressure forcesthe ring against the cylinder wall andthe bottom of the ring groove.
These are the two sealing surfacesthat the top ring must be able to sealfor maximum engine power.
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The piston rings musthave the specified sideand back clearance.
Fitting Piston Rings
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The rectangular and the barrelface are the most commonly usedtop compression rings because
they provide the best seal.
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The taper face ring provides good oil controlby scraping the cylinder wall.
If this design ring were accidentally installedupside down, the tapered face would pump oil
into the combustion chamber.
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Torsional twist ringsprovide better compressionsealing and oil control than
regular taper face rings.
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Cutting Rings
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Connecting Rods
Some designs utilize spit holes or bleed
holes
Cast iron – general
duty
Forged steel –
stronger – generallyheavier
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Connecting Rods
Cast aluminum
Forged aluminumLightweight
Stretch easily
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Connecting Rods
Powdered metal Pre-balanced
Fractured parting line
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Connecting Rods
H-Beam Design
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A typical connecting rod and related engineparts. The connecting rod is probably themost highly stressed part in the engine.
Combustion forces try to compress it andwhen the piston stops at the top of the
cylinder, inertia forces try to pull it apart.
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Some connecting rods have balancingbosses (pads) on each end of the rod.
Rod caps are unidirectional and mustbe reinstalled in the same rod position.
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The rod bearing boresnormally stretch from topto bottom causing therod bearing to wear mostnear the parting line.
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Rod Bore Measurement Use a bore gauge and rod fixture in vise
Check out-of-round
Rod caps must be torqued to mfg. specs.
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Connecting Rod Installation
The
chamfered
side of the
bore will
always lead
towards the
crankshaftside on a v-
type motor.
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A press used to removeand install connectingrods to the pistons.
Connecting rod
End
of
show
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HighPerformance
Ti
Using a connecting rod bolt stretch gauge to
measure the amount the rod bolt stretches to
tighten the fastener to its ultimate strength.
Connecting rod clamp vise
Rod bolt stretch gauge
Connecting rod show