So I messed up when tapping threads for head bolts, the bottom piece end of the thread hole was cracked and broke off. It’s just the bottom piece without any threads on it, is it repairable?


Left side

Right side

Bottom piece

 

One option, maybe...

Go ahead and tap all the way through the damaged spot so it's threaded through.
Buy a short set screw that has the right threads.
Test fit it into the hole, making sure it can thread all the way through.
Remove and clean the crap out of everything.
Apply red thread locking compound to the threads and to the set screw.
Run it in, leaving the set screw poking out the bottom but a few threads still engaged.
Let the loctite cure.
Grind the bottom down to the correct profile using a dremel or similar, but go slow not to overheat and kill the red loctite.
Lastly, clean and smear some 2 part epoxy over it for extra insurance.

That's roughly what I'd do.

 

Just use silicone on the bolt when you assemble the engine. Studs work better because it’s less difficult to get the sealant in threads. There are engines that have bolts that go into the water jacket area, smb Chevy etc.