Editing Piston (section)

===Steam engines===
[[File:Piston, sprung ring (Heat Engines, 1913).jpg|thumb|Cast-iron [[steam engine]] piston, with a metal piston ring spring-loaded against the [[cylinder (engine)|cylinder]] wall.]]
[[Steam engine]]s are usually double-acting (i.e. steam pressure acts alternately on each side of the piston) and the admission and release of steam is controlled by [[D slide valve|slide valves]], [[piston valve]]s or [[poppet valve]]s. Consequently, steam engine pistons are nearly always comparatively thin discs: their diameter is several times their thickness. (One exception is the [[trunk engine]] piston, shaped more like those in a modern internal-combustion engine.) Another factor is that since almost all steam engines use crossheads to translate the force to the drive rod, there are few lateral forces acting to try and "rock" the piston, so a cylinder-shaped piston skirt isn't necessary.