They consist of a fixed cylinder capped by a moving piston. The principal difficulty was in machining the piston and cylinder. There is a short cylinder which contains a piston and its associated return spring. Tubas are made with either piston or rotary valves. The increased pressure and temperature can cause problems such as melting the piston or valves. As differential pressure increases, a magnet attached to either a piston or rubber diaphragm moves. This made the inertia starter a common choice for aircraft with large radial piston engines. The bore was reduced to , resulting in an engine of piston displacement. They act both as a piston and cylindrical crosshead. Turbochargers are used on piston engines. See the airgun article for information on how spring piston and pneumatic airguns work. Gas sealing is achieved by the use of piston rings. Jet engines are much more reliable than piston engines. Designers have a number of problems to overcome in designing axial piston pumps. One type is a tube with a floating piston.