Connecting rod bolts are the securing link between the piston assembly and the crankshaft. The rod bolts, because of the need for great strength, are generally made of heat-treated ahoy steel. Fine threads with close pitch are used to give maximum strength and to permit secure tighting.

The majority of rod bolts used are machined to provide high fatique resistance by having a large portion of the body of the bolt turned to a diameter of less than the root diameter of the thread. Thus, all the body, except the ends and the center portion (which acts as a dowel), is machined to the smaller diameter. (Refer to the bolts in fig. 4-16.) As an additional precaution in some connecting rods, the mating surfaces between the foot of the rod and the connecting rod cap are serrated to help the bolts resist side forces. (Refer to figs. 4-15 and 4-17.)

CRANKSHAFT

As one of the largest moving parts in an engine, the crankshaft changes the movement of the piston and the connecting rod into the rotating motion that is needed to drive such items as reduction gears, propeller shafts, generators, and pumps.

As the name implies, the crankshaft consists of a series of cranks (throws) formed as offsets in a shaft. The crankshaft is subjected to all the forces developed in an engine. Because of this, the shaft must be of especially strong construc-tion. It is usually machined from forged alloy or high-carbon steel. The shafts of some engines are made of cast-iron alloy. Forged crankshafts are nitrided (heat-treated) to increase the strength of the shafts and to minimize wear.

While crankshafts of a few larger engines are of the built-up type (forged in separate sections and flanged together), the crankshafts of most modern engines are of one-piece construction (fig. 4-19).