ORIGINAL: moocow_v

Valve lash is the measurement of air space between the rocker arm and the tip of the valve stem during that portion of cam revolution where the lifter is riding on the base circle, the valvetrain is relaxed, and the valve spring is holding the valve fully closed. so zero lash is no clearance

ORIGINAL: mjr46

^^^^^^^good description..however what I believe he is refering to and how most people use this term is when the valvetrain is being set up.......this is what zero lash is when refering to the adjustment of the valvetrain....first you roll the cylinder that you will be setting the rockers up on to tdc so best to start with cylinder #1 cause it's the first in the firing order..and you set the push rod down in the hole and then if you have stud mount rockersset the roller rocker down on the stud and then begin adjusting the poly lock down until you start to feel a slight drag on the push rod while spining it as you were adjusting down the poly lock....this is known as 0 lash and no clearence as moocow states.....after o lash you give the poly lock 1/4 to 1/2 a turn more and this is known as setting the preload.....after that is done tighten down the lock on the poly adjuster and your done