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A hydraulic tappet, also known as a hydraulic valve lifter or 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:
- 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 or rocker arm.
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.
Hydraulic lifter pre-load
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 number 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.
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 mechanics 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 cars 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.
As the whole process is actuated by hydraulic pressure at engine start, there is no need for service or adjustment. Another advantage is cheaper operation, as there is no need for service and charges associated with tappet maintenance. Usually hydraulic tappets survive through the whole of the engine life without any service requirements.
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 a significant issue provided the noise disappears within a couple of minutes; typically it usually lasts only 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.
Hydraulic tappets require more complex and more expensive cylinder head design. A number of subcompact car manufacturers prefer solid valve lifters due to lower design and production cost rather than hydraulic tappets.
Generally, hydraulic tappets are more sensitive to engine oil quality and frequency of oil changes, as carbon sludge and residues may easily lock up the tappets or block oil channels, making the clearance setting ineffective. This has negative impact, especially on the engine camshaft and valves due to excessive wear if the clearance setting is not working correctly. As mentioned, one may avoid this by using the manufacturer-recommended grade of engine oil, and by not exceeding the prescribed oil change interval.
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.
Hydraulic lifters can also create "valve bounce" at high RPM, which is undesirable for performance uses. This is why engine tuners prefer solid lifters instead.
Used hydraulic lifters should be drained of oil before installation, to prevent them from holding open the valves on startup and potentially causing damage to the valve-train/pistons. This is easily accomplished by compressing them in a vise. Oil pressure will build quickly upon startup and they will set themselves to the proper height.
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