Valve clearance is the slack or lash in the drivetrain which must be taken up by the increasing lift of the cam lobe is it rotates. The valve itself does not begin to move until the valve clearance is eliminated.

Valve clearance is essential to allow the valvetrain components to expand as they get hot without causing damage, but if the valve clearances are too large there will be an increase in noise and wear. Excessive valve clearances also alter the valve timing and overlap, affecting engine performance.

In modern engines valve clearances are often automatically regulated by hydraulic tappets. If the engine uses solid tappets the valve clearances must be measured and set by hand - either by inserting thin shims between the valve and tappet or by using a screw adjustment mechanism within the tappet.