LO 7-6 Identify and explain the purpose, function, and construction of piston pins

Full-Floating Piston Pins

The piston pin design common to most diesel engines is a full-floating piston pin. Both the piston and the connecting rod rotate on the pin. Although this requires a bushing in the small end of the rod, more surfaces are rotating with less load, and they have an oil film to better minimize wear. This type of pin is held in place by spring clips or, in the instances of some newer engines, Teflon caps that prevent the pin from coming out of the bore and scratching the cylinder Figure 39. Some wrist pins may have an interference fit when at room temperature, but the pin bore will increase in diameter with temperature in a running engine to allow the pin to turn freely in both the piston and the connecting rod. This type of pin needs to be heated when removing and replacing it Figure 40. A trapezoidal pin bushing is most commonly used with this design because the bearing surfaces that absorb combustion forces are wider on the connecting rod and piston; this improves piston and connecting rod load-bearing capabilities Figure 41.

FIGURE 07-39: Ends of the piston pin clip must be installed vertically to prevent dislocation by inertia forces when the engine is operating at high speed. Note the paint mark on the rod indicating service weight.

FIGURE 07-40: Hollow and solid wrist pins. Lower cylinder pressures mean that hollow pins can be used.

FIGURE 07-41: A. The trapezoidal style of connecting rod has a wider load-bearing surface on top of the pin than B. the parallelogram style of connecting rod.