Hydraulic tappet

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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 clearance 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 this, allowing the valve train to operate with zero clearance at all operating temperatures once the oil is warm—leading to quieter operation, longer engine life, and eliminating the need for periodic adjustment of valve clearance. During operation, it fills with oil, which provides hydraulic resistance to compression, eliminating "play" (also known as "lash" ^[1]) from the system and creating an effective zero-tolerance system.