If you are not using specialized bits you need the hardest ones you can buy. Surprisingly, after tons of shopping, I found the craftsman bit set to be the best don't bother with depot or lowes. At least you have a system of keeping the drilling work square that's probably the biggest issue. In the end, don't be surprised if you end up having to use these:
++ TIME-SERT Metric Kits ++ thread repair kits and inserts repair stripped threads damaged threads repaired threads threaded inserts for metal thread inserts for metal aluminum inserts threaded inserts for aluminum
far better result than heliocoils, even though I expected one I never had an exhaust leak post repair. With the amount sticking out you have, maybe you could weld two nuts on there and go "nuts".
Threaded Mode
