Pistons, Rings, and

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 _at_ Process

• Iron heavy machinery or old school

13
Piston Composition _at_ Process

• Aluminum - cast
• Pour aluminum into a mold
• Light-weight
• economical
• Some silicone added
• General usage
• Brittle
• Somewhat unpredictable expansion qualities

14
Piston Composition _at_ Process

• Hypereutectic
• Cast aluminum with a high silicon content
• Light-weight
• Higher performance
• Less brittle
• Predictable expansion qualities

15
Piston Composition _at_ Process

• Aluminum - Forged
• Can be made lighter weight (smaller) than cast
  because its 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.
TECHTIP
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 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 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 /m?'l?bd?n?m)
• 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.
TECHTIP
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
Fitting Piston Rings
The piston rings must have the specified side and
back clearance.
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
(No Transcript)