I'll second Grant's suggestion of welding on a nut one or two sizes larger than the broken bolt/stud. I use MIG myself for this purpose, but the materials techs I talked to about it were surprised this works as they prefer the arc welder for that purpose.
The risk in this case of buggering it is low since the broken bolts are in a cast steel. The mig will not weld the cast without suitable prep. Clean the exposed broken stud until shiny, then situate a nut over the stud. Set the heat of the mig to the highest possible. Fill the hole of the nut, trying to get as much weld as possible onto the stud before the hole starts to fill up. As the whole thing cools, the weld will shrink, making the welded portion of the stud shrink with it. Between the heat and the shrinkage, the broken bit will likely work free. Work it back and forth a bit until either starts to move, or you break the nut off. If you break the nut off, clean the area a little and do it again.
I have not met the bolt yet that will not come out this way, and when you are talking smaller bolts, it is often amazing how small the amount of weld that hits the broken bolt will still bring it out. Sometimes I have had to do it a second time after the nut shears off, but never had to do a third.
The drilling through method will work too, but will take longer, and if you are not on exactly at 90°, you risk drilling through the threads.
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