What is "tappets"

Usage examples of "tappets".

And in the center of it all, an enormous steam Wurlitzer pounded and thrummed, flywheel spinning, slide valves popping, with shafts and belts connected to an incredible Rube Goldberg concoction of rocking cranks, syncopating levers, undulating cams, whirling gear trains, and nodding tappets, all acting out its cycle of interlocked motions with a complexity and ingenuity that astonished even Hunt.

He stumbled back against the engine, head between the clashing tappets, and rolled down to the grating.