PISTONS

1
CHAPTER 11

• PISTONS RODS
• Page 286 Classroom Manual
• Page 286 Lab Manual

2
PISTONS

• Pistons undergo tremendous stresses.
• Start and stop twice each revolution.
• Pistons are made of cast aluminum.
• Pistons are made of forged aluminum.

Page 286
3
SKIRT TYPES

• Today slipper-skirt is the most common style.
• Cut always are to clear crank counterweights.
• The surfaces 90 to wrist pin are thrust surfaces.

Page 287
4
BARREL SKIRT
5
PISTON EXPANSION CONTROL

• Aluminum pistons expand about twice as much as
  cast block.
• Piston can be tapered, because they run cooler at
  bottom.
• Piston is given a cam-ground shape, .0005 to
  .0025
• Cast pistons have struts of spring loaded steel.

Page 289
6
PISTON HEAD RING GROOVES

• Piston head is .019 to .048 smaller than the
  skirt.
• Head is round the skirt is cam ground, (oval).
• Sealing the ring groove to the ring is important.
• Holes in oil ring groove return oil from cylinder
  wall to pan.

7
PISTON SKIRT SURFACE

• Skirt is purposely left slightly rough
  (machined).
• The machined surface is good for retaining
  lubrication.
• DO NOT GLASS BEAD PISTONS.

8
PISTON PIN OFFSET

• Piston pins are often offset up to .060.
• Power stroke - major thrust side.
• Piston pin is offset toward the major thrust
  side.
• Rod pushes piston toward minor thrust side on
  compression stroke.
• Direction of rotation determines major thrust
  side.

Page 290
9
PISTON HEAD SHAPES

• Often manufactures use different piston head
  shapes to vary compression.
• Compression height distance from center of pin
  to piston top.
• Pop-up or dome pistons can only be installed one
  direction.

Page 292
10
CAST FORGED PISTONS

• Cast aluminum pistons are the most common type.
• Forged pistons are for high-performance or
  heavy-duty.
• Forged pistons are 70 stronger than cast.
• Hypereutectic refers to the silicone content of
  the piston.

11
PISTON CLEARANCE

• Cast pistons are installed with less clearance
  than forged pistons.
• Cast pistons have about .0015 to .002
  clearance.
• Used pistons should have less than .003
  clearance.

Page 293
12
PISTON WEIGHT

• Pistons are precisely balanced.
• Balance pads are under piston pins.
• Oversize pistons are supposed to weigh the same
  as originals.

13
PISTON WEAR PROBLEMS

• Scuffing
• Scoring
• Cracking
• Pin boss wear
• Ring Land Wear