A hydraulic tappet is a device used to open and close valves in an internal combustion engine. Riding on the camshaft lobe, the hydraulic tappet, or lifter as it is commonly called, actuates the valve via a push rod and rocker arm. The function of the hydraulic tappet is to maintain zero lash, or gap, between the tip of the rocker arm and the tip of the valve stem. Adjusted with a slight preload, the hydraulic tappet uses engine oil to maintain the zero lash setting while the engine is running.
Prior to the invention of the hydraulic tappet, valves were opened and closed with solid lifters. These solid lifters required a slight lash to be left when adjusting the valves. The lash allowed for expansion of the valve tip due to engine heat and operated with a slight noise or rattle when cold. The advent of the hydraulic tappet solved this rattle by operating with a controlled preload, or zero lash, on the valve tip. As the lifter was on the down stroke, it filled with engine oil through an oil hole on the side of the lifter; as it was on the up stroke, the oil was trapped and caused the lifter to act like a solid lifter and push the valve open.
The design of the hydraulic tappet works fine for everyday driving and maintains quiet operation of the valve train. In high-performance applications, however, the hydraulic tappet falls short and often causes engine damage. As the engine speed increases, the oil has a difficult time exiting the lifter. With the lifter body full of oil, the preload gives way to the added oil and forces the valve to remain open slightly. This can cause contact between the piston and the valve head, equaling catastrophic engine failure.
In an attempt to overcome this problem, some aftermarket companies have developed anti-pump up lifters. These lifters are designed to operate at high engine speeds without filling full of oil. This creates a lifter that operates in the same manner as the hydraulic versions, requiring none of the readjustments of solid-type lifters; this lifter also has none of the pump-up problems of the typical hydraulic lifter.
The solid lifter is noisy and requires frequent readjustment to maintain proper valve lash. The hydraulic tappet operates much more quietly and requires no readjustment. Advancements in camshaft lobe design as well as in lifter technology have made performance-oriented hydraulic lifters possible in production vehicles.