I recently got a similar frame and when I asked my LBS if they could put a Helicoil insert they said no (too small). I could do it myself, but I'd rather have someone experienced do it and my LBS is obviously not that someone.

Other than clearance, is there any advantage to a threaded hole vs an unthreaded hole with a nut on the other side? For all the trouble with threading, possible stripping, etc I'm surprised that they're threaded on so many frames...unless there's some hidden advantage?