Definition
A small cylindrical component in a reciprocating engine's valve train that sits between the camshaft lobe and the pushrod (or directly on the valve stem in some designs), transferring the lifting motion of the cam to open the engine's intake or exhaust valve. In most aircraft piston engines the tappet is a hydraulic unit that also takes up clearance in the valve train automatically.
Plain English
A part inside the engine that the camshaft pushes against to open and close the valves. It acts as the messenger between the spinning camshaft and the valve.
Context Anchor
Seen when studying aircraft piston engines, especially the parts that open and close the intake and exhaust valves.
Derivation
From the verb 'tap' — the part literally taps against the cam lobe as the camshaft rotates. The '-et' ending suggests a small device that does the tapping.
Why Pilots Care
Correct tappet clearance keeps valves opening and closing properly; wrong clearance can cause power loss, valve damage, or engine roughness.
Example Sentence 1
The mechanic found a collapsed hydraulic tappet during the annual inspection, which explained the engine's rough idle.
Example Sentence 2
Worn tappets can produce a ticking sound and reduce engine efficiency.