Figure 12-17.-Piston pin types.expanded by the heat generated during operation, itbecomes round because the expansion is proportional tothe temperature of the metal. The walls of the skirt arecut away as much as possible to reduce weight and toprevent excessive expansion during engine operation.Many aluminum pistons are made with split skirts sothat when the pistons expand, the skirt diameter will notincrease.The two types of piston skirts found in most enginesare the full trunk and the slipper. The full-trunk-typeskirt, more widely used, has a full cylindrical shape withbearing surfaces parallel to those of the cylinder, givingmore strength and better control of the oil film. Theslipper-type (cutaway) skirt has considerable relief onthe sides of the skirt, leaving less area for possiblecontact with the cylinder walls and thereby reducingfriction.PISTON PINS.— The piston is attached to theconnecting rod by the piston pin (wrist pin). The pinpasses through the piston pin bosses and through theupper end of the connecting rod, which rides within thepiston on the middle of the pin. Piston pins are made ofalloy steel with a precision finish and are case hardenedand sometimes chromium plated to increase theirwearing qualities. Their tubular construction gives themmaximum strength with minimum weight. They arelubricated by splash from the crankcase or by pressurethrough passages bored in the connecting rods.Three methods are commonly used for fastening apiston pin to the piston and the connecting rod: fixedpin, semifloating pin, and full-floating pin (fig. 12-17).The anchored, or fixed, pin attaches to the piston by ascrew running through one of the bosses; the connectingrod oscillates on the pin. The semifloating pin isanchored to the connecting rod and turns in the pistonpin bosses. The full-floating pin is free to rotate in theconnecting rod and in the bosses, while plugs orsnap-ring locks prevent it from working out against thesides of the cylinder.PISTON RINGS.— Piston rings are used onpistons to maintain gastight seals between the pistonsand cylinders, to aid in cooling the piston, and to controlcylinder-wall lubrication. About one-third of the heatabsorbed by the piston passes through the rings to thecylinder wall. Piston rings are often complicated indesign, are heat treated in various ways, and are platedwith other metals. Piston rings are of two distinctclassifications: compression rings and oil control rings.(See fig. 12-18.)The principal function of a compression ring is toprevent gases from leaking by the piston during thecompression and power strokes. All piston rings are splitto permit assembly to the piston and to allow forexpansion. When the ring is in place, the ends of the splitjoint do not form a perfect seal; therefore, more than onering must be used, and the joints must be staggeredaround the piston. If cylinders are worn, expanders (figs.12-15 and 12-18) are sometimes used to ensure a perfectseal.The bottom ring, usually located just above thepiston pin, is an oil-regulating ring. This ring scrapes theexcess oil from the cylinder walls and returns some ofit, through slots, to the piston ring grooves. The ringgroove under an oil ring has openings through which theoil flows back into the crankcase. In some engines,additional oil rings are used in the piston skirt below thepiston pin.12-16
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