I have been doing the same to the heads that I have. I tapped 7 of them successfully using my 5/16 - 18 thread tap and then broke the tap in the hole on #8

I have now broken it off below the surface (it keeps crumbling) and resists easy removal. My next step is to use my Dremel and a tiny grinder bit to get all the rest of it out. I will use a proper pre drill bit and go up to a 3/8 - 16 tap on one hole in this head. Just a little hiccup in the process. I know better, I just wanted to hurry the process and it bit me in the A**