A piston is a component o reciprocating ingines, reciprocating pumps, gas compressors an pneumatic cylinders, amang ither seemilar mechanisms. It is the muivin component that is contained bi a cylinder an is made gas-tight bi piston rings. In an ingine, its purpose is tae transfer force frae expanding gas in the cylinder tae the crankshaft via a piston rod an/or connecting rod. In a pump, the function is reversed an force is transferred frae the crankshaft tae the piston for the purpose o compressing or ejecting the fluid in the cylinder. In some ingines, the piston an aw acts as a valve bi covering an uncovering ports in the cylinder wall.