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:
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".
I tried welding a nut on and only snapped more of the bolt off. Can you buy time-serts long enough? I was planning on just stacking the helicoils to get the correct depth.
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1997 Boxster 4.2L Audi V8 Bi-Turbo
2003 911 C2
NASA HPDE Instructor