A Valve Guide is a cylindrical piece of metal, pressed or
integrally cast into the cylinder head, with the valve
reciprocating inside it.
Guides also serve to conduct heat from the combustion process
out from the exhaust valve and into the cylinder head where it
is taken up by the cooling system.
The hollow between the inner Diameter of the valve guide and the
outer diameter of the valve stem is important for the good
performance of the engine.
If the hollow is large the valve may not seat properly and
excessive oil consumption can occur and overtime will deform.