A hydraulic tappet maintains zero clearance between the rocker arm and valve stem in an engine. It uses engine oil to operate and is quieter than solid lifters. However, it can cause engine damage in heavy-duty applications, so anti-pump lifters have been developed. Advances in technology have made performance-oriented hydraulic lifters possible on production vehicles.
A hydraulic tappet is a device used to open and close valves in an internal combustion engine. Mounted on the camshaft lobe, the hydraulic grip, or lifter as it is commonly called, actuates the valve via a pushrod and rocker arm. The function of the hydraulic grip is to maintain zero clearance, or clearance, between the tip of the rocker arm and the tip of the valve stem. Set with a slight preload, the hydraulic grip uses engine oil to maintain the zero play setting while the engine is running.
Before the invention of the hydraulic plug, valves were opened and closed with solid lifters. These solid lifters required a light whipping when adjusting the valves. The whiplash allowed expansion of the valve tip due to engine heat and ran with a slight noise or rattle when cold. The advent of the hydraulic grip solved this noise by operating with a controlled preload, or zero whip, at the valve tip. When the lifter was on the downstroke, it was filled with engine oil through an oil hole in the side of the lifter; Since it was on the upstroke, the oil got trapped and caused the lifter to act like a solid lifter and push the valve open.
The hydraulic grip design works well for everyday driving and maintains quiet valvetrain operation. However, in heavy duty applications, the hydraulic plug falls short and often causes engine damage. As the engine speed increases, the oil has a difficult time coming out of the lifter. With the lifter body filled with oil, the preload gives way to the added oil and forces the valve to stay slightly open. This can cause contact between the piston and the valve head, which is tantamount to catastrophic engine failure.
In an attempt to overcome this problem, some aftermarket companies have developed anti-pump lifters. These lifters are designed to operate at high engine speeds without filling with oil. This creates a lifter that works in the same way as the hydraulic versions, and requires none of the readjustment of solid-type lifters; This lifter also has none of the pumping problems of the typical hydraulic lifter.
The solid lifter is noisy and requires frequent readjustment to maintain proper valve lash. The hydraulic handle runs much quieter and requires no readjustment. Advances in camshaft lobe design, as well as lifter technology, have made performance-oriented hydraulic lifters possible on production vehicles.
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