The cylinder head sits above the cylinder block and consists of a platform containing part of the combustion chamber that locates the valves and spark plugs. The cylinder head is key to the performance of the internal combustion engine as the shape of the combustion chamber, inlet passages and ports determines a major portion of the volumetric efficiency and compression ratio of the engine. Most of the parts are made of Aluminum with the exception of most commercial vehicles, so PCD tooling with specialised geometries for high-speed machining is required.