Engine valves

Valves in a four-stroke engine are mushroom-shaped parts that seal the intake and exhaust ports in the cylinder head. Intake valves let the air-fuel mixture into the combustion chamber; exhaust valves let the burnt gases out. Most modern motorcycle engines have two or four valves per cylinder, with the intake valves slightly larger.

The camshaft opens the valves, either directly through bucket tappets, through finger followers or rocker arms, or, in pushrod engines, through pushrods and rockers. Springs close them again; Ducati's desmodromic engines close them with a second cam.

Valves are made from heat-resistant steels, sometimes titanium in high-performance engines. Keep valve clearances within specification, as a valve that does not close properly can burn and lose compression. Two-stroke engines generally do not have poppet valves.