The cylinders included in Motorsports air-cooled kits are produced and machined to original equipment tolerances, designed to provide increased performance, durability and longevity. Some applications are available as either a slip-in or machine-in design. The slip-in cylinders are simply a larger internal bore replacement. The machine-in cylinders require the engine cases be machined to a larger bore diameter to accept their larger spigot diameter. The larger spigot diameter is preferable for extreme applications as with highly boosted turbo or competition use engines.