I bought a Miller Bobcat welder for real cheap at an auction because it had 3 broken head bolts and several other missing parts.
To repair the broken bolts I first removed the head to have a look at possibly simply welding a washer and a nut on the to remove them. No such luck this meant I would have to drill them out.
So I found a piece to steel tubing that fit very tightly in the holes of the head Actually it was tight enough to require tapping in place with a hammer the inside diameter was small enough that the drill bit I needed to use would almost drill out the entire body of the bolts leaving a thin shell and the threads.
after drilling out the bolts all I had to do was use a small seal pick to break the remaining thread piece out
the Guide served to hold the bit centered as well as a depth gauge for my drill