Gudgeon Pin (Wrist Pin)

The gudgeon pin, called the wrist pin or piston pin in the US, is a hardened hollow steel pin that passes through the piston and the small end of the connecting rod. It lets the rod swing back and forth as the piston moves up and down the cylinder.

The pin is held in place by circlips at each end. In two-stroke engines the small end usually runs on a needle roller bearing, while four-strokes typically use a plain bush or run directly in the rod.

A worn pin or small-end bearing causes a light knocking sound. On two-stroke dirt bikes the small-end bearing is usually replaced whenever the piston is changed, and new circlips should always be fitted.