I am installing new valvetrain on my ZZ502. The lifters are hydraulic-roller with stamped steel rockers mounted on studs. When I go through the TDC, zero lash plus 3/4 quarter turn (per GM specs), I find that the rockers are tight at first, then minutes later they become a little loose. Could someone define "zero lash" for me, I may be. Is zero lash when the pushrod will no longer rotate between your fingers, or is it just no play up-and-down? Appreciate the help.

Bako Steve
70 Chevelle

Do you have the intake off still? You can watch it easier to make sure you are not compressing the plunger that way. To answer your question though is yes to both. As you are tightening the nut I feel for no play up and down then you feel the push rod stop to rotate, thats zero lash. But you are correct when you do the 3/4 preload the rocker will seem tight but after the lifter bleeds out a bit they feel loose again.

Dave

Steve,
I personally hate the "pushrod stops turning" method in books. With new lifters that have not been filled with oil, you can tighten them down almost until the plunger is bottomed out and yet still turn the pushrod.

I always tell my customers to wiggle the pushrod up and down till they just feel that they have all of the play out, that is zero lash. Also, like Dave said, they usually will loosen up some as the lifter bleeds down.

By the way, be careful using the TDC method on all but the mildest camshafts. On most performance cams both valves will be slightly open at TDC and it is not the correct point to adjust them.

I always use the Exhaust Opening - Intake Closing method. When the exhaust valve just starts to open you adjust the intake, and when the Intake valve has opened all the way and then closed about 1/2 way ... that is when you adjust the exhaust. This is the method that almost all of the cam companies recommend.

Bill,
Thanks for the information. Very helpful.
Steve