A cylinder in an internal combustion engine or external combustion engine is the space within which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or 'block'. A cylinder block is typically cast from aluminum or cast iron before precision features are machined into it. The cylinders may then be lined with 'sleeves' of some harder metal, or given a wear-resistant coating such as Nikasil. In some engines, especially French designs, the cylinders have "wet liners", which means that they are not part of the main casting and coolant is free to flow around the outside each individual cylinder. Wet lined engines have better cooling and more even temperature distribution, but the block is somewhat less rigid. The cylinder block sits between the engine crankcase a