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:
- 1) through a pushrod which actuates the valve via a rocker mechanism; or
- 2) 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 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 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, the OEM manufacturers specify that you should be unable to compress them BEFORE re-fitting - They are there to take up the slack in the valve train. The reason they tap when faulty is because they cannot keep the gap correctly.
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