Valve Clearance (Valve Lash)

Engines with mechanical followers need a specified clearance, set with shims or screws during service. Engines with hydraulic lifters adjust the clearance automatically. The specification is given separately for intake and exhaust valves, usually for a cold engine.

Clearance is measured with feeler gauges between the cam and follower or between the rocker and valve stem. Adjusting it on shim-type engines means removing the camshaft and fitting shims of a different thickness.

The gap ensures that the valve can sit firmly on its seat even when the valve stem lengthens as it heats up. Too little clearance holds the valve slightly open; too much makes the mechanism noisy.

Too much clearance causes ticking noises; too little can burn exhaust valves and cause misfires. Engines that need adjustment have a check interval in the service schedule. Because the job takes time on some engines, it is easy to forget; the service schedule gives the interval, often a fixed distance for motorcycles and some petrol cars.