What do you think of this method of bolt hole repair?
 

He drills out the stripped bolt hole, taps it for one size larger, degreases them, uses a center punch to ding up the threads on a grade 2 bolt, creating a burr and a lock thread between the threads on the top half of the part that will go into the hole, use Lock Tight, install the bolt, cut it flush, carefully grind off the excess, punch the center, and tap it for the correct bolt:
https://www.youtube.com/watch?v=Ykk0bP4DChE

Sounds like a home-brew Helicoil
https://i.ytimg.com/vi/xigq7aPpff0/maxresdefault.jpg
guidemanualjean99.z19.web.core.windows.net/helicoil-design-guide.html

Indeed, but a grade 2 bolt and a cheap tap kit are cheaper.

That's a pretty small margin of error.