Tappet /(?)/

Tap·pet

Tappet

n.
  1. A lever or projection moved by some other piece, as a cam, or intended to tap or touch something else, with a view to produce change or regulate motion. (Mach.)

Phrases & Compounds
Tappet motion
a valve motion worked by tappets from a reciprocating part, without an eccentric or cam, -- used in steam pumps, etc.