There are bolt extractors, worth a purchase. You drill out you bolt but leave some in, then you tighten these extractors in and the tighten in a left hand thread. They jam in and screw out as you tighten. As long as you drill the bolt central, then you will be fine. If you have and some bolt is still in, try one of these first before threading. I had to buy a set when I snapped a bleed nipple on my vw caliper.