Hydraulic lifters eliminate the clatter and the need for periodic adjustments by maintaining zero clearance when the engine is running. They do this by using oil pressure against a spring-loaded plunger inside the lifter body. Oil fills the cavity under the plunger when the valve is closed.

How do tappets work?

A hydraulic tappet, also known as a "hydraulic lash adjuster", contains a small hydraulic piston that becomes filled with pressurised engine oil. the piston acts as a hydraulic spring that automatically adjusts the tappet clearance according to the oil pressure.

How do you test hydraulic tappets?

Remove one hydraulic lifter and sit it face up on the bench. Use an old pushrod to shove down on the plunger top of the lifter as hard as you can. If the lifter plunger does not move or barely moves, place it back into the egg carton and mark it with an "Ok" symbol. Test each lifter in this fashion.