Conventional Rods

The conventional rod is sometimes referred to as the normal or standard rod because of its extensive use by many manufacturers. The rod illustrated in figure 4-16 is typical of those used in many in-line and V-type engines. When used in V-type engines, two rods are mounted on a single crankpin. The two cylinders served are off-set so that the rods can be operated side by side.

Rods are generally made of drop-forged, heat-treated carbon steel (alloy steel forging). Most rods have an I- or H-shaped cross section which provides maximum strength with minimum weight. The bore (hub, eye) at the piston end of the rod is generally forged as an integral part of the rod (fig. 4-16); however, the use of semifloating piston pins eliminates the need for the bore. (See figs. 4-11 and 4-15.) The bore at the crankshaft end is formed by two parts, one an integral part of the rod and the other a removable cap. (See fig. 4-16.) Rods are generally drilled or bored to provide an oil passage from the crankshaft to the piston end of the rod.

The bore of the crankshaft end of a conventional rod is fitted with a precision bearing of the shell type. (See fig. 4-16.) In design and materials, rod bearings are similar to the main journal bearings, which are discussed in connection with crankshafts later in this chapter. Connecting rod bearings of most engines are pressure-lubricated by oil from adjacent main bearings, through drilled passages. The oil is evenly distributed over the bearing surfaces by oil grooves in the shells. Bearing shells have drilled holes which line up with an oil groove in the rod bearing seat. Oil from this groove is forced to the piston pin through the drilled passage in the rod. Figure 4-17 illustrates a drilled type of connecting rod, which in this example is used in conjunction with a “cocktail shaker” type of piston.

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