Connecting rods must be light and yet strong enough to transmit the thrust of the pistons to the crankshaft. Connecting rods are drop forged from a steel alloy capable of withstanding heavy loads without bending or twisting. Holes at the upper and lower ends are machined to permit accurate fitting of bearings. These holes must be parallel.

The lower hole in the connecting rod is split to permit it to be clamped around the crankshaft. The bottom part, or cap, is made of the same material as the rod and is attached by two or more bolts. The surface that bears on the crankshaft is generally a bearing material in the form of a separate split shell; in a few cases, it may be spun or die-cast in the inside of the rod and cap during manufacture. The two parts of the separate bearing are positioned in the rod and cap by dowel pins, projections, or short brass screws. Split bearings may be of the precision or semiprecision type.

The precision type bearing is accurately finished to fit the crankpin and does not require further fitting during installation. It is positioned by projections on the shell that match reliefs in the rod and cap. The projections prevent the bearings from moving sideways and prevent rotary motion in the rod and cap.

The semiprecision-type bearing is usually fastened to or die-cast with the rod and cap. Before installation, it is machined and fitted to the proper inside diameter with the cap and rod bolted together.

As the pistons collectively might be regarded as the heart of the engine, so the crankshaft might be considered the backbone (fig. 12-19). It ties together the reactions of the pistons and the connecting rods, transforming their reciprocating motion into rotary motion. It transmits engine power through the flywheel, clutch, transmission, and differential to drive your vehicle.

The crankshaft is forged or cast from an alloy of steel and nickel. It is machined smooth to provide

bearing surfaces for the connecting rods and the main bearings. It is case-hardened (coated in a furnace with copper alloyed and carbon). These bearing surfaces are called journals. The crankshaft counterweights impede the centrifugal force of the connecting rod and assembly attached to the throws or points of bearing support. These throws must be placed so that they counter-balance each other.

Crankshaft and throw arrangements for four-, six-, and eight-cylinder engines are shown in figure 12-20. Four-cylinder engine crankshafts have either three or five main support bearings and four throws in one plane. As shown in the figure, the four throws for the number 1 and 4 cylinders (four-cylinder engine) are 180 from those for the number 2 and 3 cylinders. On six-cylinder engine crankshafts, each of the three pairs of throws is arranged 120 from the other two. Such crankshafts may be supported by as many as seven main bearingsone at each end of the shaft and one between each pair of crankshaft throws. The crankshafts of eight-cylinder V-type engines are similar to those of the four-cylinder in-line type. They may have each of the four throws fixed at 90 from each other (as in fig. 12-20) for better balance and smoother operation.

V-type engines usually have two connecting rods fastened side by side on one crankshaft throw. With this arrangement, one bank of the engine cylinders is set slightly ahead of the other to allow the two rods to clear each other.