Editing Four-stroke engine (section)

=== Valve train ===
The valves are typically operated by a [[camshaft]] rotating at half the speed of the [[crankshaft]]. It has a series of [[Cam (mechanism)|cam]]s along its length, each designed to open a valve during the appropriate part of an intake or exhaust stroke. A [[tappet]] between valve and cam is a contact surface on which the cam slides to open the valve. Many engines use one or more camshafts "above" a row (or each row) of cylinders, as in the illustration, in which each cam directly actuates a valve through a flat tappet. In other engine designs the camshaft is in the [[crankcase]], in which case each cam usually contacts a [[push rod]], which contacts a [[rocker arm]] that opens a valve, or in case of a [[flathead engine]] a push rod is not necessary. The [[overhead cam]] design typically allows higher engine speeds because it provides the most direct path between cam and valve.

==== Valve clearance ====
Valve clearance refers to the small gap between a valve lifter and a valve stem that ensures that the valve completely closes. On engines with mechanical valve adjustment, excessive clearance causes noise from the valve train. A too-small valve clearance can result in the valves not closing properly. This results in a loss of performance and possibly overheating of exhaust valves. Typically, the clearance must be readjusted each {{convert|20000|mi|km}} with a feeler gauge.

Most modern production engines use  [[hydraulic lifters]] to automatically compensate for valve train component wear. Dirty engine oil may cause lifter failure.