I recently had new rocker studs installed in my 1965 gto 389 TriPower heads and I am in the process of installing Scorpion 1.5 roller rockers. I have checked the push rod length using the recommended method (using a black marker to color the valve stem tip, setting the lash, rotating the crank to allow the valves to open and close several times and then checking to see if the mark left by the roller rocker is in the center of the valve tip). It seems that some marks are in the center but other marks are just slightly off center.....not sure why a few random valves would be just slightly off center. My question is: should I really be that concerned about the few random valves that are just slightly off center?? Could it just be that a few of the lifters are not fully pumped up or something like that?
Also, if dry hyd lifters they will collapse some as well. I think the recommended method was to use a hydr lifter but pack it full of foil and make sure it was some certain distance down inside itself or something? I don't recall. Anyways i made one of those lifters and used it on like 4 random valves per side to be sure and buttoned it up.
When changing push rod length look for the most narrow pattern. As long as the pattern is not close to the edge of the valve stem, it should be fine.
Normal preload of a hydraulic lifer is around .050" or turning the adjusting nut one turn past zero lash. I'm old so someone speak up if my math is fuzzy.
There are a few ways to preform the check. Imo, the easiest way is to gut then build and old hydraulic lifter with washers and or tin foil until the plunger hight is .050 short of the lifter plunger hight to be used.
Related- "I've seen way too many guys end up with the wrong pushrod length because they start chasing the tip patterns,without actually knowing what's happening with the rocker geometry,and what is causing the pattern they're seeing."