When I was reinstalling the head studs, one was a little loose, so I decided to helicoil it. Well, I didn't research it well enough and, afterwards, an experienced Model A’er had to remind me to use the head as a guide to drill the hole for the tap straight. My two helicoils were 4 deg from square and the head wouldn't go on. I removed them and used this insert to recover from my error:
www.mcmaster.com/90259ai54. The specs say it needs a 33/64" tap drill, which I don't have, but 1/2" worked fine. In order to use the head as a guide, the holes in the head had to be reamed to 11/16" (as I recall), but that doesn't hurt anything, in my opinion. For checking squareness of tap, I used a 4" square I have. I would start the tap, turn enough to get a bite and then check for square in two directions, then adjust the tap and continue to bias the horizontal force in the direction needed and repeat every half turn or so. I used a nut and bolt to install the insert.