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CHAPTER 11

CHAPTER 11. PISTONS & RODS Page 286 Classroom Manual Page 286 Lab Manual. 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. SKIRT TYPES.

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CHAPTER 11

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  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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