Valve guides

Valve guides are small tubes, usually made of bronze, cast iron or another wear-resistant alloy, pressed into the cylinder head. Each valve stem slides up and down inside its guide, which keeps it centred so the valve head closes squarely on its seat and passes heat to the head.

A valve stem seal on top of each guide stops oil from being drawn down into the combustion chamber. When guides or seals wear, the engine may burn oil, giving blue smoke at start-up or when closing the throttle, and valves may seal poorly.

Guide wear is checked during a cylinder head overhaul by measuring the play between stem and guide. Worn guides can be replaced by a workshop, followed by recutting the valve seats.