The exhaust ports I've encountered are a tapered pipe thread.
IIRR it should be a 1" NPT.
You might have to shorten the tap a bit on the grinder if it bottoms out before you clean up the threads.( don't turn the tap blue) keep it cool.
Epay is yer friend as you don't want to "buy" a pipe tap new,
or borrow one from a friend.
(and if you borrow one don't shorten it) Ha,LOL!
lube it and turn, stop, back out 1/2 turn and proceed again.
The port is rusted/and has carbon in it as you know.
Not rocket science
