Piston

The piston is a cylindrical part, usually cast or forged aluminium, that slides up and down in the cylinder. Combustion pressure pushes it down, and the connecting rod turns that motion into rotation of the crankshaft. Piston rings seal the gap to the cylinder wall and control oil, and a gudgeon (wrist) pin joins the piston to the rod.

On a four-stroke the piston also compresses the charge and pushes out exhaust gases. On a two-stroke it opens and closes the ports, so its shape and skirt are part of the engine's timing.

Pistons wear, especially in high-revving off-road engines, where makers specify replacement after a set number of running hours. Signs of wear include blue smoke, loss of compression, piston slap noise and hard starting. Overheating or lean running can seize or hole a piston.