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 required regular adjusting to maintain a small clearance between the valve and its rocker or cam follower. This space allowed for thermal expansion, and prevented the parts from binding. This clearance space meant noisy operation and earlier wear, as the parts would rattle against one another until the parts heated up and expanded. The hydraulic lifter was designed to compensate for this 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:
- through a pushrod which actuates the valve via a rocker mechanism; or
- in the case of overhead camshafts, via direct contact with the valve stem.
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 un-compressible, 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.
Hydraulic lifter pre-load
When hydraulic lifters are installed, a certain amount of pre-load (or lifter internal piston compression) is required. A mechanic normally uses 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.
The mechanic installs the solid lifter and rotates the engine crank until the cam is on the "base circle" (not on any intake or exhaust cam lobe). An adjustable-length push-rod checking tool is then installed on the valve location being checked. The pushrod length checking tool is extended until there is no slop (zero lash). Then, the checking tool is removed and measured. The mechanic then adds .030-.090" of additional length, depending on the specifications of the lifter being installed.  A more typical pre-load range used and recommended by most medchanics is in the neighborhood of .030-.060" The length of the adjustable length push-rod plus the desired pre-load chosen then becomes the desired push-rod length that should be installed, which will keep the hydraulic lifter in the middle of its total piston-travel range. Example: Adjustable push-rod at zero lash = 9.0". Desired pre-load = .050". 9.0" + .050" = 9.050" push-rods should be installed.
The first firm to include hydraulic lifters in its design was the Cadillac V 16 engine (Model 452) first offered in 1930. Hydraulic lifters were popular on automobiles designed in the 1980s, but most newer cars have reverted to bucket-and-shim mechanical lifters. Although these do not run as quietly and are not maintenance-free, they are cheaper and rarely need adjustment because the wear caused by operation is spread over a large area.
There are a number of potential problems with hydraulic lifters. Frequently, the valvetrain will rattle loudly on startup due to oil draining from the lifters when the vehicle is parked. This is not considered significant provided the noise disappears within a couple of minutes, typically it usually only lasts a second or two. A rattle that does not go away can indicate a blocked oil feed or that one or more of the lifters has collapsed due to wear and is no longer opening its valve fully. The affected lifter should be replaced in the latter situation.
It is a myth that in certain circumstances, a lifter can "pump up" and create negative valve clearance. The engine oil pump cannot generate enough pressure to cause "pump-up". The problem is due to weak valve springs which permit float at high engine speeds. The followers attempt to take up what they see as extra clearance. As this speed is maintained, the lifter will continue to expand until the valve is held off its seat when it should be closed. Maintenance of the valve springs at the correct strength is therefore very important to avoid engine damage.
Tappets should be fitted while FULL of oil/diesel liquid: the original-equipment manufacturers specify that the installer should be unable to compress them BEFORE re-fitting, given that these components are intended to take up the slack in the valve train. The reason they tap when faulty is because they cannot keep the gap correctly.