Hydraulic tappet

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.