I prefer spacers and shims, minumum of .015 clearances between each pair cold Main thing is to get the roller tip to have full contact on all of the tip of all the valves I had one motor on the dyno that had the weak springs, it would allow the rocker arm to push over towards the middle and the rocker arm tip was pushing down on the retainer instead of the staying on valve tip I repalced those springs with stronger ones to fix that set, it got spacers and shim when I took the motor back to the shop