Piston rod

Piston rods are usually attached to the crosshead and the piston by a transverse slot and a tapered key or gib. Driving this key sideways tightens the attachment. Using a transverse key allows relatively easy dismantling for maintenance. Some smaller pistons are retained by a threaded piston rod and a large nut, although the crosshead nearly always uses a key. As the precise length of the piston rod is important for timing the engine's valvegear, the attachment tightens the piston down onto a fixed step or location in the piston rod and the length (and valve timing) is not adjustable. This length requires the precision of the manufactured rod but, unlike the rod driving the valves, does not need to be adjusted even more precisely by a fitter during the engine's erection.