Tappet

Early versions of the Newcomen engines from 1712 had manually operated valves, but by 1715 this repetitive task had been automated through the use of tappets.

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.

This minimizes wear caused by the camshaft contacting the same point on the base of the tappet each valve cycle, which can result in grooving.

[citation needed] The base of most plain tappets is given a slight convex profile to soften contact of the leading edge of the camshaft lobe.

An alternative to the tappet is the “finger follower”, which is a pivoting beam that is used to convert the camshaft rotation into opening and closing of a valve.

Finger followers are used in some high-performance dual overhead camshaft (DOHC) engines, most commonly in motorcycles and sports cars.

Too large a gap results in wear from misaligned parts and compromised engine performance, and too small can lead to bent pushrods or burnt valves.

Loose set screws can cause catastrophic engine failure, which has led to fatal aircraft crashes.

Although the movements of the piston are small and infrequent, they are sufficient to make the valve actuation self-adjusting so that there is no need to manually adjust the clearance of the tappets.

When starting a cold engine, with low oil pressure, hydraulic tappets are often noisy for a few seconds, until they position themselves correctly.

[7] In a sidevalve engine— a common design for car engines until the 1950s— the valves are mounted at the sides of the cylinder and face upwards.

The linear sliding tappet side often had a high rate of wear and demanded careful lubrication with oil containing zinc additives.

Where a reciprocating action is produced, such as for a pneumatic drill or jackhammer, the valve may be actuated by inertia or by the movement of the working piston.