Hi MikeM,
I Have had stripped 6mm treads in a 650 head I made special studs.
This is how i did it, Drill stud hole to 6mm, tap tread to 7mm, take 7mm round rod and tread, turn down the bit that sticks out to 6mm and tread it, done. Or you could see if the 7mm tread will go through the exhaust clamp, 7mm nut, job done.
No Heat, No twist
Just a thought.
Works for me.
Paull