In mechanical engineering, a tappet is a projection which imparts a linear motion to some other component within an assembly. In automotive mechanics, a 'tappet' is a somewhat archaic term which is falling into disuse being largely replaced by other terms such as rocker-arm. Properly speaking, a tappet is only that part of a rocker-arm which makes contact with an intake or exhaust valve stem above the cylinder head of a gasoline or diesel engine. In an overhead cam engine, the rocker-arm pivots on a fixed shaft while one projection of the rocker-arm rides on an eccentric cam lobe of the rotating cam shaft. This creates an oscillating linear motion at the 'tappet' side of the rocker-arm, opening and closing a valve as the tappet 'taps' on the valve stem. The closing of the valve is typically accomplished by an compression spring placed between the valve collet (or retainer) and the cylinder head above the combustion chamber. The Ford Essex V6 engine Mechanical engineering is a very broad field of engineering that involves the application of physical principles for analysis, design, manufacturing, and maintenance of mechanical systems. ... It has been suggested that Car engine be merged into this article or section. ... Overhead cam (OHC) piston engines place the camshaft above the cylinder heads and drive the valves or lifters directly instead of using pushrods. ...

Traditionally, the nominal distance (clearance) between the tappet surface and the valve's contact surface was maintained by means of an adjustment screw on the tappet. Today, this is typically accomplished by introducing shims into this space to create the necessary clearance or by hydraulic adjusters. Hydraulics is a branch of science and engineering concerned with the use of liquids to perform mechanical tasks. ...

This traditional mechanism for opening and closing valves, while tried and true, has its drawbacks. Valve clearances periodically require adjustment as the contact surfaces of both tappet and valve stem wear. Also the problem of valve float has for decades plagued high-rpm engines. This problem occurs when a valve spring cannot close the valve quickly enough at high engine rpm's. While this has largely been solved with modern metallurgy, engines in the 20,000 plus rpm range can still exhibit valve float. Metallurgy is a domain of materials science and of materials engineering that studies the physical and chemical behavior of metallic elements and their mixtures, which are called alloys. ...

The Ducati motorcycle company partially solved these problems with their desmodromic valve train. The desmodromic principle attempts to minimize wear by minimizing clearances between contacting surfaces, and eliminating the 'tapping' action of valve actuation. The real advantage to a desmodromic valve however, is that it's positively closed by the mechanism instead of allowed to close by spring action. Mercedes successfully built and raced desmodromic Grand Prix engines in the 1950s, but never put them into production. Ducati Motor Holding is an Italian motorcycle manufacturer. ... Desmodromic valves are those which are positively closed by a leverage system, rather than relying on the more conventional springs to close the valves. ... This page is about the Mercedes-Benz brand of automobiles and trucks from the DaimlerChrysler automobile manufacturer. ... Georges Boillot winning the 1912 French Grand Prix in Dieppe, France Grand Prix motor racing has its roots in organized automobile racing that began in France as far back as 1894. ... // Events and trends This map shows two essential global spheres during the Cold War in 1959. ...

In internal combustion engines of the future, the use of tappets in the mechanical opening and closing of valves may disappear completely in favor of electronically controlled linear actuators. Motors utilizing this technology already exist (though production costs are high). The advantages here are many; valves can be opened or closed dynamically, maximizing power output and fuel consumption based on changing conditions, and without respect to the mechanical limitations of a camshaft; valves can be opened or closed almost instantaneously, eliminating valve float; engines can be used as very effective downhill-brakes, much like the engine brake of a semi-trailer truck; the seals and bushings of a traditional valve train could potentially be eliminated, reducing the overall complexity of the motor. A linear actuator is a device that develops force and motion along a straight line. ...

Failure of tappets

Over time, tappets fail and will break. The problem with this is it no longer closes and opens the valves properly. This causes the affected cylinder to constantly misfire, with the car being noticeably slower than it was previously, the idle speed being lower, copious amounts of white smoke rather than the normal low amount of slightly bluish smoke for petrol engines, and black smoke for diesel engines, and the car sometimes shuddering.

On a V8 engine, the failure of one tappet would not be too noticeable unless you were aiming for high acceleration.

On a V6 engine, the failure of one tappet would be noticeable, as the engine would not be as responsive.

On a 4-cylinder engine, for example, a diesel, the failure will be extremely noticeable, with reduced idle speeds, and increased shuddering as the vehicles' engine attempts to keep running with a quarter of its engine power missing.