If your heads were shaved and/or the deck was cut a bit, and/or aftermarket cam is on a slightly different base circle, and/or pushrods are aftermarket and have a slight different lenght, factory studs and factory way of adjustment (ie no adjustment, just torquing to 20 lbs) can NOT BE USED. You then MUST go to adjustable valvetrain if any of the above have been done to your motor. That is, using 7/16 big block chevy studs and a rocker nut that "locks".