The bearings used in connection with most piston pins are of the sleeve bearing or bushing type. These bearings may be further identified according to locationthe piston boss piston pin bearings and the connecting rod piston bearings.

The bearings or bushings are made of bronze or similar material. Since the bushing material is a relatively hard-bearing metal, surface-hardened piston pins are required. The bore of the bushing is accurately ground in line for the close fit of the piston pin. Most bushings have a number of small grooves cut in their bore for lubrication purposes (fig. 4-16). Some sleeve bushings have a press fit, while others are cold shrunk into the bosses. Bearings of the sleeve bushing type for both the bosses and the connecting rod are shown in figure 4-16. Note that the bosses are a part of an insert.

If the piston pin is secured in the bosses of the piston (stationary) or if it floats (full-floating) in both the connecting rod and piston, the piston end

Figure 4-16.Piston and connecting rod (Fairbanks-Morse).

of the rod must be fitted with a sleeve bushing. Pistons fitted with semifloating pins (fig. 4-15) require no bearing at the rod end.

Sleeve bushings used in the piston end of connecting rods are similar in design to those used in piston bosses. Generally, bronze makes up the bearing surface. Some bearing surfaces are backed with a case-hardened steel sleeve, and the bushing has a shrink fit in the rod bore. In other bushings, the bushing fit is such that a gradual rotation (creep) takes place in the eye of the connecting rod. In another variation of the sleeve-type bushing, a cast bronze lining is pressed into a steel bushing in the connecting rod.