Pistons

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Pistons
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Piston Design Considerations

• Pistons must
• Contain cylinder pressure
• Transmit the pressure created by combustion to
  force on the connecting rod
• Provide a place for oil control and compression
  rings to be located

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Piston Design Considerations

• Pistons must
• Be rigid enough to not deform under the
  tremendous pressures and forces encountered
• Be ductile enough to absorb pressure peaks and
  not shatter
• Retain the proper shape under the extreme
  temperatures encountered

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Piston Construction

• Materials
• Cast iron (used in very old engines)
• Cast aluminum (most common)
• Forged aluminum
• Hypereutectic alloys (high silicon content
  aluminum)
• Carbon Graphite (being tested)

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Piston Construction
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Piston Construction

• Piston head
• Round
• Approx. .040 undersized
• Ring grooves
• Ring lands
• Drain holes and slots
• Wrist pin boss

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Piston Construction

• Reinforcing struts cast pistons contain steel
  struts to control expansion and aid in wrist pin
  support
• Lock ring grooves for full-floating piston pins
• Piston skirt
• Full skirt
• Partial skirt

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Piston Construction

• Cam ground pistons.
• Most pistons are cam ground, this means they are
  not perfectly round.
• The diameter is larger on the non-thrust sides,
  because they tend to expand more as the
  temperature of the piston increases.
• Piston taper pistons may be tapered from top to
  bottom

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Piston Construction

• Piston Pin Offset
• The piston pin is offset towards the major thrust
  side of the piston to reduce piston wear, and
  piston noise
• The major thrust side is the side that is loaded
  during the power event
• The minor thrust side is the side loaded during
  the compression event

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Major/Minor Thrust
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Piston Direction
Pistons should be positioned so arrow, dot or
other marking faces the front of the engine
(except reverse rotation engines)
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Piston Head Design

• Pistons may have a dished, flat or domed design

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Piston Inspection

• Abnormal skirt wear
• Scoring
• Melted aluminum or pits in the head indicates
  detonation
• Cracks check ring lands, pin boss, and skirt
  closely
• Pin bore check for scoring and wear