CHAPTER 11PISTONSSKIRT TYPESBARREL SKIRTPISTON EXPANSION CONTROLPISTON HEAD & RING GROOVESPISTON SKIRT SURFACEPISTON PIN OFFSETPISTON HEAD SHAPESCAST & FORGED PISTONSPISTON CLEARANCEPISTON WEIGHTPISTON WEAR PROBLEMSCHAPTER 11PISTONS & RODSPage 286 Classroom ManualPage 286 Lab ManualPISTONSPistons undergo tremendous stresses.Start and stop twice each revolution.Pistons are made of cast aluminum.Pistons are made of forged aluminum.Page 286SKIRT 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 287BARREL SKIRTPISTON 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 289PISTON 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.PISTON SKIRT SURFACESkirt is purposely left slightly rough (machined).The machined surface is good for retaining lubrication.DO NOT GLASS BEAD PISTONS.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 290PISTON 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 292CAST & 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.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 293PISTON WEIGHTPistons are precisely balanced.Balance pads are under piston pins.Oversize pistons are supposed to weigh the same as originals.PISTON WEAR PROBLEMSScuffingScoringCrackingPin boss wearRing Land