This document discusses various components of internal combustion engines including pistons, piston rings, connecting rods, and their materials and functions. It provides details on piston design types, materials, manufacturing processes, ring materials and compositions. It also describes connecting rod designs, materials, measurements and installation procedures. The key components and their roles in transforming combustion energy to rotational motion are summarized.

1. Pistons, Rings, and
Connecting Rods

2. 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.

3. A typical piston
illustrating the
various parts
and the names.

4. Piston Heads (Crown)
 Domed
 Flat Top
 Recessed (valve reliefs)
 Dished

5. A domed piston with valve
reliefs or valve pockets.

6. A Flat Top piston

7. A Flat Top piston with valve
reliefs or valve pockets.

8. A Dished piston with valve
reliefs or valve pockets.

9. Piston Skirts
 Moly Graphite

10. Piston Skirts
 Finished
perpendicular
to the head
Scuffing
promotes
blow-by

11. A low-friction moly coating on the skirt of
this piston helps prevent piston scuffing
when the engine is cold.

12. Piston Composition @ Process
 Iron – heavy machinery or “old school”

13. Piston Composition @ Process
 Aluminum - cast
 Pour aluminum into a
mold
 Light-weight
 economical
 Some silicone added
 General usage
 Brittle
 Somewhat
unpredictable expansion
qualities

14. Piston Composition @ Process
 Hypereutectic
Cast aluminum
with a high silicon
content
Light-weight
Higher
performance
Less brittle
Predictable
expansion qualities

15. Piston Composition @ Process
 Aluminum - Forged
 Can be made lighter
weight (smaller) than
cast because it’s
stronger
 Can withstand abuse
 Newer designs have
predictable expansion
qualities
 Silicon & Nickel
added
 Greater piston to wall
clearance

16. Notice the temperature
difference between a forged
piston and a cast piston.

17. Pistons are often cam-ground to
produce the elliptical shape when
the piston is at room temperature.

18. Piston diameter
being measured
using a micrometer.

19. Piston (wrist) Pins
 High-Quality steel
Usually Hollow
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.

20. Piston pin is offset
toward the major thrust
surface.
TECH
TIP

21. Engine rotation and rod angle during the
power stroke causes the engine 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
major thrust surface is on the left side.

22. Piston Pin Retaining Methods
 Full Floating
Lock Rings
Circlips or snap rings
hold full-floating
piston pins in place.

23. Piston Pin Retaining Methods
 Interference Fit
Pressed

24. Piston Rings
 Compression
 Oil – usually unidirectional
Ring expander

25. Compression Ring Composition
 Pearlitic
 Nodular Iron
 Ductile Iron – flexible
 Cast Iron
Chromium - .0004” - durable
Molybdenum – reduced scuffing
Chrome-moly

26. 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 spherulitic
graphite structure.

27. •In the 2nd groove, alloyed grey cast irons are used in a
heat-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.

28. 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.

29. •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 with
increased strength characteristics.
•These challenges are met by the use of high-chromium alloyed
steels and spring steels.
•The greater durability under increased stresses is demonstrated
by 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).

30. •The wear resistance derives from finely distributed
chromium carbides of the type M23
C6
and M7
C3
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 engines
as well as for the steel rails and expander-spacers of oil
control rings and for 2-piece profiled steel oil rings.

31. Pearlitic Rings
 Pearlite is a two-phased, lamellar (or layered) structure
composed 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 a
corresponding proportion of relatively pure ferrite crystallites
that 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 it
takes its name.

32. Nodular (ductile) Iron
 Ductile iron, also called ductile cast iron,
spheroidal graphite iron, nodular cast iron, is
a 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, due
to its nodular graphite inclusions.

33. Chromium rings
 Chromium is a steely-gray, lustrous, hard
metal that takes a high polish and has a high
melting point.
 It is odourless, tasteless, and malleable.

34. Chromium facing can be seen on the right
side of the sectional view of the piston ring.

35. Molybdenum Rings
 Molybdenum (pronounced
/mə l bdənəmˈ ɪ )
 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 in
1778 by Carl Wilhelm Scheele and
first isolated in 1781 by
Peter Jacob Hjelm.

36. Molybdenum facing can be seen
on the right side of the sectional
view of the piston ring. TECH
TIP

37. 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.

38. Ring Gaps
 Ring gap must be checked prior to engine
assembly
Loose
Tight
 Butt gap
 Gapless?

39. The gapless ring overlaps, while the
conventional ring design uses a gap.
Frequently
Asked Question

40. Gapless Ring

41. 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.

42. The piston rings must
have the specified side
and back clearance.
Fitting Piston Rings

43. The rectangular and the barrel
face are the most commonly used
top compression rings because
they provide the best seal.

44. The taper face ring provides good oil control
by scraping the cylinder wall.
If this design ring were accidentally installed
upside down, the tapered face would pump oil
into the combustion chamber.

45. Torsional twist rings
provide better compression
sealing and oil control than
regular taper face rings.

46. Cutting Rings

47. Connecting Rods
Some designs utilize
spit holes or bleed
holes
 Cast iron – general
duty
 Forged steel –
stronger – generally
heavier

48. Connecting Rods
 Cast aluminum
 Forged aluminum
Lightweight
Stretch easily

49. Connecting Rods
 Powdered metal
 Pre-balanced
 Fractured parting line

50. Connecting Rods
H-Beam Design

51. 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.

52. Some connecting rods have balancing
bosses (pads) on each end of the rod.
Rod caps are unidirectional and must
be reinstalled in the same rod
position.

53. The rod bearing bores
normally stretch from top
to bottom causing the
rod bearing to wear most
near the parting line.

54. Rod Bore Measurement
 Use a bore gauge and rod fixture in vise
 Check out-of-round
Rod caps must be torqued to mfg. specs.

55. Connecting Rod Installation
 The
chamfered
side of the
bore will
always lead
towards the
crankshaft
side on a v-
type motor.

56. A press used to remove
and install connecting
rods to the pistons.

57. High
Performance
Tip
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
End
of
show