tappet

A tappet is a crucial component within an internal combustion engine responsible for operating the valves that control the flow of air and fuel into, and exhaust gases out of, the cylinders. More technically known as a valve lifter or cam follower, it sits between the camshaft—which rotates to open and close the valves—and the valve itself. As the camshaft lobe pushes on the tappet, it transmits that motion upwards to push the valve open.

There are different types of tappets: solid ones, which need periodic manual adjustment, and hydraulic ones, which adjust themselves using oil pressure. A properly functioning tappet helps keep valve timing correct, which is important for engine performance and efficiency.

If a tappet becomes worn, dirty, or loses hydraulic pressure, it can create too much space between moving parts. This can lead to a noticeable clicking or tapping sound from the engine. While the noise is sometimes minor, ongoing tappet noise can point to a mechanical problem that needs attention.

While it is mainly an automotive term today, "tappet" was also once used for a small peg, stopper, or similar projecting part. In modern English, though, you will almost always see it in relation to engines and the small but important parts that keep them running smoothly.