I'm (slowly) rebuilding an '84 22r. Putting on my new exhaust manifold last weekend and lower rear stud just kept spinning. Take everything back off again to investigate and it was helicoiled in the past and stipped out again. Tried chasing the threads in hopes of putting in another helicoil but they have been to trashed to use the needed 10mm x 1.25 tap.
Was thinking I'd just size up to an 11 or 12mm, but they are proving next to impossible to find.
I found a threaded metal insert I can put in which will give me fresh 10mm x 1.25 threads to work with, but it requires tapping out a 9/16" hole to install the insert. I'm concerned that a hole this size is going to be too much for this area of the head. Any input?

 

FWIW,

At this stage, guys just move up to a 7/16 stud. Cheap and effective. Not sure with your current heli-coil situation though.