Editing Four-stroke engine (section)

==== 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.