Tappet clearance is a space between the top of the valve stem and the rocker arm. Its purpose is to allow for some mechanical expansion and lenghtening of the valve stem and push rods as the engine warms up. This clearance is also called valve lash.

If insufficient valve lash is set when the engine is cold the valves will not properly close when the engine warms up. If too much lash is provided (additional clearance) then even after the engine warms up there will be some clearance which will result in lost motion. Lost motion mean that as the cam tryes to open the valve the push rod and rocker arm moves to first take up the clearance before touching the valve to open the valve. The result is late opening of the valve.

Excess lash can beheard in an idling engine as a series of ticking sounds that sould disappear as the engine warms up. Over time mechanical wear causes an increase in lash usually with the symptomatic ticking sound in the engine (one or more valves can produce the sound). The fix was to remove the valve cover and mechanically adjust the lash with an adjustment screw on the rocker arm.


Modern engines now operate with zero lash through the use of hydraulic lifters. Lifters use engine oil pressure to automatically remove the mechanical clearance of the valve operating components. It is possible for dirt in engine oil to foul the tight clearances in a hydraulic lifter and the lash will not be taken out while the engine is running resulting in the ticking noise. The hydraulic lash adjuster would then need to be replaced