When drilling to repair stripped holes, I often use the VGS seat & guide machine. I made this little pilot to go in a good hole and level it up against the spindle, this one is 3/8-16, and I also have one made to go in 7/16 stud holes. I tipped it in the valve refacer and also put on a chamfer while there.
Big Don Kincaid
Big Don’s Racing Engines
Point, TX