Steel male threads bolting directly into female
aluminum threads are always a disaster. No matter how careful you are,
with enough cycles the female threads eventually fatigue and give up
the ghost: you torque down that fastener to spec and just before the
wrench clicks, the aluminum yields and the threads pull out in a perfect
coil, and you're toast.
To make matters worse, the studs in this case have UNF threads on both ends! Usually a stud has coarse threads on the permanent side, and fine threads for the nut. With UNF threads, there's just not enough aluminum in there to last. I'm proactively replacing all of them with UNC threads, as the others will eventually fail too.
I've tried repairing aluminum like this before with Heli-Coils but have never been very satisfied. This time, my mechanic recommended Time Fastener Company's Time-Sert, a one-piece steel insert, as being much better. He showed me the kit, it looked good, so I tried it: now I'm sold, goodbye Heli-Coil. No, I'm not associated with the company nor do I get a kick-back, it's just good stuff. The kit cost about $60 in 2004.
All operations are performed by hand, with the tooling installed in a tapping T-wrench. This is especially comforting when drilling out the threads, which is easy enough to do by hand, but with power tools you could quickly ruin the casting, then you'll cry. The only tricky bit is keeping the tooling perpendicular; try holding a square against the face as a reference while you drill and tap.