Hydraulic tappet

A hydraulic valve lifter, also known as a hydraulic tappet or a hydraulic lash adjuster, is a device for maintaining zero valve clearance in an internal combustion engine. Conventional solid valve lifters require regular adjusting to maintain a small clearance between the valve and its rocker or cam follower. This space prevents the parts from binding as they expand with the engine's heat, but can also lead to noisy operation and increased wear as the parts rattle against one another until they reach operating temperature. The hydraulic lifter was designed to compensate for this small tolerance, allowing the valve train to operate with zero clearance—leading to quieter operation, longer engine life, and eliminating the need for periodic adjustment of valve clearance.

The hydraulic lifter, situated between the camshaft and each engine's valve, is a hollow steel cylinder encasing an internal piston. This piston is held at the outer limit of its travel with a strong spring. The lobed camshaft rhythmically presses against the lifter, which transmits the motion to the engine valve one of two ways:

Oil under constant pressure is supplied to the lifter via an oil channel, through a small hole in the lifter body. When the engine valve is closed (lifter in a neutral position), the lifter is free to fill with oil. As the camshaft lobe enters the lift phase of its travel, it compresses the lifter piston, and a valve shuts the oil inlet. Oil is nearly incompressible, so this greater pressure renders the lifter effectively solid during the lift phase.

As the camshaft lobe travels through its apex, the load is reduced on the lifter piston, and the internal spring returns the piston to its neutral state so the lifter can refill with oil. This small range of travel in the lifter's piston is enough to allow the elimination of the constant lash adjustment.

The basic idea is to set the lifter plunger to the center of its travel. That gives the most leeway for temperature expansion/contraction. The amount of turns needed to set the plunger in the middle of its travel will vary from engine to engine because of differences in the thread pitch, rocker ratio, and lifter design.

One approach is to use an identical lifter to the ones being installed that has been made "solid". Making a lifter solid is a process where the lifter piston is prevented from moving - this can be done with small welds made to the lifter, or by dis-assembling the lifter, removing the internal spring, and installing washers to take up the space and prevent the lifter piston from compressing. Some lifter manufacturers also sell solid versions of the hydraulic lifter being installed.

...
Wikipedia