Rockers, pushrods and tappets

Younger mechanics have hardly heard the word “tappets”, but they featured prominently in the lives of mechanics 40 years ago. In those days many cars on the road (VW Beetles, Chevs, Chrysler Valiants, British Fords, etc) had overhead valve engines with a camshaft in the block. Riding on the lobes of the rotating camshaft were cam followers (“lifters”), short chunks of hard steel linked to the rockers in the cylinder head by means of relatively long, slender rods called pushrods. The pushrods, moving up and down in step with the rotating cams, were responsible for making the rockers rock by pushing upward on one end of the rocker so it pivots, forcing the opposite end downward to push a valve open against the tension of the valve spring. An engine with such a valve-train layout is known as a cam-in-block or pushrod engine. Few are still produced, exceptions being American engines like the Chrysler Hemi and GM V8 in the Chev Corvette. A valve-train layout such as described above has to make provision for thermal expansion of its components, especially the pushrods, otherwise the valves may no longer close fully as the engine warms up. Thus a gap was designed between the top of the valve stem and the tip of the rocker arm pressing down on the valve. This “valve clearance” had to be checked and adjusted periodically to compensate for wear in the system. And it was this routine known as “setting the tappets”. The specified gap varied between manufacturers and engine designs. It was traditionally stated in “thou”, short for thousandths of an inch, each thou being equal to 25 microns. Checking was done with a feeler gauge on cold engines. On Beetles, where regular valve adjustment was critical, the gap was as little as four thou, ie 100 microns (0,1mm) on some models. Comparing this with the thickness of newsprint paper, generally around 70 microns, indicates the precision required of the mechanic where he lay under the Beetle’s aircooled boxer, feeler gauge in hand, trying to decide if the resistance as he pulled the blade through the gap is just right. On Ford’s Kent engine, fitted to early Cortinas, it was easier — the gap was eight thou for inlet and 18 thou for exhaust valves, and you could adjust “the tappets” while standing upright. This whole ritual has become largely obsolete. This raises the question: “What has become of the tappets?” Hereby is an interesting tale to which we shall return.