Thank you for the link, Trav. I deal with worn out, sheared bolts, or just plain rusted solid. I once used a deep reach spark plug to tap a hole in an exhaust manifold after I had to drill out a sheared banjo bolt for the air injection. It worked great. IIRC I used a 3 corner file to cut the flutes.
I use cutting taps to clean threads. If you absolutely need to clean to the last thread then cut off the end of the tap. You won't mess the hole up if you're careful you will be able to tell if it's following the threads. When you feel resistance then back it out a couple turns and then start going again. Repeat at each time you feel resistance. This is just a precaution to keep the tap from getting jammed really tight due to Filthy threads and then you have risk of breaking the tap off in the hole trying to turn it out.
I often use a wrench but whenever the hole is in a recessed area you cannot beat Lisle tap sockets.
Tapered cutting taps are really good about starting into and following threads. The flutes catch the dirt. And a bolt will easily thread into the hole after running a cutting tap through it.
