are few lateral forces acting to try and "rock" the piston, so a cylinder-shaped piston skirt isn't necessary. Pumps Piston pumps can be used to move liquids or compress gases. For liquids For gases Air cannons There are two special type of pistons used in air cannons: close tolerance pistons and double pistons. In close tolerance pistons O-rings serve as a valve, but O-rings are not used in double piston types. See also Air gun Fire piston Fruit press Gas-operated reloading, using a gas piston Hydraulic cylinder Piston motion equations Shock absorber Slide whistle Steam locomotive components Syringe Wankel engine, an internal combustion engine design with a rotor instead of pistons Notes References Bibliography External links Piston Engines Essay How Stuff Works – Basic Engine Parts Piston Motion Equations Engine technology