Does anyone know the torque spec for the rocker nut on a GM aluminum fast burn head (from zz383 crate engine)?

 

there isn't one. it doesn't work that way. With the one you're adjusting on the base of the lobe, spin the pushrod with the thumb and finger of one hand while turning the tool with the other. When you begin to feel drag on the pushrod, that's zero lash, from there turn the nut another 180 degrees. Then move on to the next one.

 

The Chev adjustable valve train has no torque on the rocker nut. The rocker nut and ball inside the rocker or a full roller rocker is a pivot point that maintains an adjustable geometry between the pushrod and the valve. The rocker nut is a special pinched nut that won't back itself off.

1/2 to 3/4 turn from zero lash. Don't readjust after that because the pushrod will be able to spin again once the lifter bleeds down. The 1/2 to 3/4 turn from zero lash is adding roughly .030" - .060" preload inside the lifter.

Rotate the engine by hand and do each cylinder one at a time then the piston is at TDC of that cylinder with both valves closed. Make sure it's on the compression stroke and not the exhaust stroke. The exhaust stroke will have a slight valve overlap.