Figure 12-18.-Piston rings.Connecting RodsConnecting rods must be light and yet strongenough to transmit the thrust of the pistons to thecrankshaft. Connecting rods are drop forged from a steelalloy capable of withstanding heavy loads withoutbending or twisting. Holes at the upper and lower endsare machined to permit accurate fitting of bearings.These holes must be parallel.The upper end of the connecting rod is connected tothe piston by the piston pin. If the piston pin is lockedin the piston pin bosses or if it floats in both the pistonand the connecting rod, the upper hold of the connectingrod will have a solid bearing (bushing) of bronze orsimilar material. As the lower end of the connecting rodrevolves with the crankshaft, the upper end is forced toturn back and forth on the piston pin. Although thismovement is slight, the bushing is necessary because ofthe high pressure and temperatures. If the piston pin issemifloating, a bushing is not needed.Figure 12-19.-Crankshaft of a four-cylinder engine.The lower hole in the connecting rod is split topermit it to be clamped around the crankshaft. Thebottom part, or cap, is made of the same material as therod and is attached by two or more bolts. The surfacethat bears on the crankshaft is generally a bearingmaterial in the form of a separate split shell; in a fewcases, it may be spun or die-cast in the inside of the rodand cap during manufacture. The two parts of theseparate bearing are positioned in the rod and cap bydowel pins, projections, or short brass screws. Splitbearings may be of the precision or semiprecision type.The precision type bearing is accurately finished tofit the crankpin and does not require further fittingduring installation. It is positioned by projections on theshell that match reliefs in the rod and cap. Theprojections prevent the bearings from moving sidewaysand prevent rotary motion in the rod and cap.The semiprecision-type bearing is usually fastenedto or die-cast with the rod and cap. Before installation,it is machined and fitted to the proper inside diameterwith the cap and rod bolted together.CrankshaftAs the pistons collectively might be regarded as theheart of the engine, so the crankshaft might beconsidered the backbone (fig. 12-19). It ties together thereactions of the pistons and the connecting rods,transforming their reciprocating motion into rotarymotion. It transmits engine power through the flywheel,clutch, transmission, and differential to drive yourvehicle.The crankshaft is forged or cast from an alloy ofsteel and nickel. It is machined smooth to provide12-17
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