Valve guides play a critical role in the function of all types of internal combustion engines. They are responsible for the proper function and alignment of engine valves which are required for introducing air for combustion of the post-combustion exhaust gases from the cylinder. Valve guides perform 2 functions, providing a liner between a cast iron or aluminium block and the steel valve stems, and maintaining the orientation of the valve stems. The first is fairly obvious as valves wouldn't last long sliding against the rough surface of the engine block.
