Size of threads that hold lock ring in 617 head?

[yuke]

Would any one know the size and pitch of the threads that hold the lock ring in that holds the prechamber down? I would like to know because as I was trying to pull my prechamber out which I never succeeded at I messed some of the upper threads up and now I can't get the lock ring started. So if I knew the thread size I could locate a tap and clean them up. I want to at least salvage the head out of this engine that the prechamber blew down into the cyl. It's a pretty big tap for sure I'm thinking bigger then the inside of the prechamber threads that the injector screws into.