In most types of reciprocating engines, a valve guide is provided for each poppet valve in the cylinder head. Along with the valve spring, it serves to positively locate the valve so that it may make proper contact with the valve seat. A valve guide is a tube-shaped piece of metal, pressed into the cylinder head, with the valve reciprocating inside it. Guides serve also to conduct heat from the combustion process out from the exhaust valve and into the cylinder head where it may be taken up by the cooling system. Bronze is commonly used, as is steel, a balance between stiffness and wear on the valve is essential to achieve a useful service life.
As valve guides wear, their ability to positively locate the valve to the valve seat decreases. As the valves lose their ability to seal the combustion chamber properly, the engine can lose performance and start to burn oil, leaking from the top of the cylinder head into the intake and exhaust manifolds. Modern engines are fitted with valve stem oil seals which can be replaced if oil leakage occurs.
