So What Is Valve Lash Anyways?
It’s basically a fancy way of saying “the gap between your cam and the thing your cam hits when it swings around.” That would either be a shim, a lifter, a rocker arm, etc. In the Miata, the head is set up like this, with the the cams directly above the lifters:
Why Is Correct Lash Important?
If the gap between the cam and the lifter is too large, not only will you be annoyed by valve tick, but the lobe will “smack” the lifter each time it comes around instead of smoothly following the contour of the cam lobe, which can cause damage to the valvetrain. As the valve closes, the cam lobe will also “let go” of the lifter too early, causing the valve to slam closed which is hard on the valve and the seat. Worst of all… you’re losing valve lift… which means less power! On the other hand, if the gap is too tight, as the various metals on the head and its parts all expand and contract with heating and cooling, that gap can close up and become zero or less (“negative lash”) which means the valve won’t close all the way. This can cause misfires due to loss of compression, and not letting the valves seat all the way can cause damage (“burn a valve”) due to extreme heating.