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. The conventional means of adjusting valve actuation always requires a small clearance to be left between the valve and its rocker or cam follower to allow for thermal expansion and wear. The hydraulic lifter was designed to ensure that the valve train always cooperates with zero clearance, leading to quieter operation and eliminating the need for periodic adjustment of valve clearance.