A cylinder in the central working part of a reciprocating engine, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or iron before precision features are machined into it.
A piston is seated inside each cylinder by several metal piston rings which fit around its outside surface in machined grooves; typically two for compressional sealing and one to seal the oil. They are made of spring steel and make near contact with the hard walls of the sleeve, riding on a thin layer of lubricating oil which is essential to keep the engine from seizing up. This contact, and the resulting wear, explains the need for the hard lining on the inner surface of the cylinder.