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So to begin, I’ve already committed to drilling the 3/12 exhaust bolts that broke and installing a helicoil .
All were snapped too deep to weld. I’ve tried a HSS, Cobalt, dremel, LH Drill bits and I have yet to make it hardly past where the bolt snapped.
Your options are welding a nut to it (very effective) or using carbide drill bits in a mill/drill press with fixtures.
Dont even try to drill them by hand, the bolts are hardened and your bit will want to shoot off into the soft aluminum.
If you really want to drill them and they are deep in there, I have a way to center drill them. Take a 8x1.25 bolt and cut it down to where there are 2-3 threads showing then center drill it with a small bit. Then thread it in the hole and run that same bit through to center drill the bolt. I'll post a pic of the one I made in a second
You can use a steel sleeve as a bushing when drilling into an aluminum hole, such as this. Cut up a pushrod and stick it down into the hole when drilling to keep the drill bit off the aluminum threads. Be creative and look at other tubing types/sizes to put in your toolbox, for crap just like this.
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Drilling broken exhaust bolts 
