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Nice fix, I have used a heli coil in a pinch on a exhaust stud bolt hole it has stayed put for at least 30,000 or so miles. Whenever I have the head off I,m going to use one of those to fix to right.
This is the #1 thread on Google for fixing stripped exhaust studs for Miatas, so I figured I'd add my info here rather than starting a new thread:
I couldn't find Keen-serts for anything even approaching reasonable prices, and TimeSerts require the special tool to cut the shoulder the insert needs which also was super expensive (the kit someone mentions is like $400+).
I tried a helicoil repair kit, but for some reason I had a really tough time getting the coil insert to thread properly, which meant I wasn't able to get the stud to install properly back into the head.
I needed a new solution, so:
I was able to find a 10 pack of E-Z Lok inserts on ebay for $15 shipped (located under title: E-Z LOK 650-10F Thread Insert, M10x1.25, 33/64L, Pk10). These install with a large flat-head screwdriver, so no special tools required.
I got a stubby 31/64" drill bit for $15 on amazon (easier to drill with the engine in the car), and this set of tapered/plug/bottoming taps for $27:
(Irwin Tools 2648-3)
You'll need a bottoming tap to avoid the issue Schuyler had.
I re-drilled the head, tapped with each successive tap, and was able to easily install the insert:
Hope this helps anyone else who comes across this thread. The previous posters had a lot of info on the methodology, but no specific part numbers or costs, so hopefully this will give some guidance.