The hole is under 9/16 but bigger than 1/2 for the 1/2 inch bolts. The factory bolts are back cut so that oil can get to the rockers. With a 1/2 inch stud things get tighter so I run a 9/16 drill bit through the hole and use 9/16 K&S brass tubing and put a small coat of JBweld on it also. Just don’t forget to redrill the two holes for rocker oiling after the JB weld sets
