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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 .
In an internal combustion engine, a tappet (also called a 'valve lifter' or 'cam follower') [3] [4] [5] is the component which converts the rotation of the camshaft into vertical motion to open and close an intake or exhaust valve. The principal types of tappets used in automotive engines are solid, hydraulic, and roller. [6] [7]
The tappet bore where a hydraulic tappet would normally reside becomes the cylinder, and the retainer assembly becomes the piston. Pressurized air ( nitrogen ) is pumped into this cylinder which then causes the piston/retainer to rise to the top of cylinder, causing the valve to form an airtight seal with the seat.
K series motors have both hydraulic tappet or hydraulic valve lifters, solid lifters and 1.5 ratio rockers with an adjustment thread for tappet clearance. 7K engines were released with only the hydraulic valve lifters from factory, whereas 4K and 5K engines were made with both hydraulic and solid lifters (depending on year and which model vehicle.)
Depending on the design used, the valves are actuated by a rocker arm, finger, or bucket tappet. Overhead valve engines use rocker arms, which are actuated from below indirectly (through the pushrods) by the cam lobes. Overhead camshaft engines use fingers or bucket tappets, which are actuated from above directly by the cam lobes. [3]
The word poppet shares etymology with "puppet": it is from the Middle English popet ("youth" or "doll"), from Middle French poupette, which is a diminutive of poupée.The use of the word poppet to describe a valve comes from the same word applied to marionettes, which, like the poppet valve, move bodily in response to remote motion transmitted linearly.
The common valve spring system is satisfactory for traditional mass-produced engines that do not rev highly and are of a design that requires low maintenance. [1] At the period of initial desmodromic development, valve springs were a major limitation on engine performance because they would break from metal fatigue.
The engines all employ hydraulic tappets which operate in aluminum guides that are machined into the crankcase. The tappets are built from four parts, a cam follower body, cup, cylinder, and piston and operate with clearances of 0.03 in (1 mm) to 0.11 in (3 mm). The pushrods are steel and feature pressed-in ball ends. [1]