Thanks to Eric from Perform-x for this tip.
We've all had the issue of having to drill out broken studs or bolts.
If it's broken above the surface, at least there's something you can grab hold of and with a long soak using a good penetrant, or some heat (if it's not aluminum) on the surrounding surface to expand the metal away from the stud or bolt, you may be lucky to get that sucker out of there.
But, if it's broken flush or below the surface, now you have the potential to be a problem.
If your plan is to drill out the broken piece and retap the hole, you had better get your first drilled hole 'dead center' or you will be soon drilling outside of the threads as you use multiple drill bits to get up to your tap size. Here's a new tool that will help with that http://www.brokenbolt.com/blog/tag/stud-extractor/
The next option is to use an extractor kit and they come in many designs. The most common use a reverse drill bit and a reverse extractor bit that you lightly hammer into the hole so the sharp edges of the bit bite into the broken bolt and then simply unscrew the broken piece.
Well, anyway, that's what they say.
Personally, I haven't had too much success with these. The only time they were successful was when these left handed extractors were brand new and the edges were still sharp. When they lose their grip, the sharp edges of the bit usually round off and they are now good for the garbage. Although, the reverse drill bits themselves worked many times just by themselves because of the grip it gets during the drilling. That is, if the broken piece is not too tight.
Here's a tip that has helped me out the most.
Torx bits.
We all have spare torx bits in our boxes, so why not put them to use.
Once you've drilled the hole, choose a torx bit that's just a little larger than the hole and hammer it into the hole. Being hardened steel with six edges will give you a positive grip on the broken piece and short enough to use a socket or wrench to remove.
Note; Don't use too small a torx bit because the smaller they are, the more likely they can break off in the hole.
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