A valve guide is a cylindrical piece of metal, pressed or integrally cast into the cylinder head of most types of reciprocating engines, to positively locate a poppet valve so that it may make proper contact with its valve seat. Along with a corresponding valve spring, it is one of the components of any engine’s valve train.

Guides also serve 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.

The clearance between the inner diameter of the valve guide and the outer diameter of the poppet valve stem is critical for the proper performance of an engine. If there is too little clearance, the valve may stick as oil contaminants and thermal expansion become factors. If there is too much clearance, the valve may not seat properly and excessive oil consumption can occur.