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PISTONS

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PISTON HEAD SHAPES Often manufactures use different piston head shapes to vary compression. Compression height = distance from center of pin to piston top. – PowerPoint PPT presentation

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Title: 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
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