Tappet

[Sarah]

A lever or projection moved by some other piece, as a cam, orintended to tap or touch something else, with a view to producechange or regulate motion. G. Francis. Tappet motion, a valve motionworked by tappets from a reciprocating part, without an eccentric orcam, -- used in steam pumps, etc.

[Sarah]

Moving part of an engine that rattles when loose resulting in engine failure. Slang usage as someone who's tappets are slightly loose (not quite with it - a.k.a someone who is a few sandwiches short of a picnic)

South African variation is of a tappet being a [redneck] in the american sense of the word