Zero lash is:NO PLAY.
Loosen the rocker nut to a point where you can feel play (clearance) between the pushrod and the rocker.
Now tighten the nut,SLOWLY.
At the EXACT moment that there is no play (clearance) between the rocker and the pushrod,STOP.
YOU ARE NOW AT ZERO LASH.
When you are dealing with a hydraulic cam and lifter you should only give the lifter 1/4-1/2 turn preload (turning the rocker nut down an aditional amount) after you reach the zero lash point.
A lot of manufacturers say to give the lifter 3/4-1 full turn,DO NOT DO IT!!!
[This message has been edited by mutant 68 (edited 12-31-2000).]