Well my first rule is never heat anything I will undoubtedly have to drill later when for example my weld fails or twists off the last bit to grab onto which they almost always do on this type work thats #1.
Rule #2 (& 5 are close) is always drill a small hole exactly in the center and all the way straight thru and spray oil it inside and if at all possible I do that when I am shutting the lights off for the day.
Rule #3 is use sharp bits! I always have a brand new box of cobalts just for this they dont drill anything but broken exhaust studs etc.
Rule #4 drill up a step at a time until its tap size for that hole and tap it with a good USA made tap. Now sometimes if I did my job right I use a reversed bit and at the last step of drilling sometimes just sometimes the rest will back out and only minimal thread cleaning is required.
Rule #5 is sometimes I take a cut off wheel and make a sideways cut like for a screwdriver bit to fit in on a flush bolt and I extend it into the casting if necessary and air/sprayoil all the chips off good, fact I almost always do that first if I think I can get away with it and then drill my pilot just depends on how stuck they may be.
Then I get a nice sharp flat head screwdriver and a wrench on it and give that a try. Sometimes a torch can help heat the metal around but as soon as you go a bit too far forget the easy drilling you have tempered steel then and only cobalts will drill only reasonably well.
Then when the drill or tap breaks I get out my Rescue bit and drill them out thats a last resort that bit is a monster find them online or Snap-On truck.
1/8" x 3" Double Ended Rescue Bit - The Original Rescue Bit Store
(notice I didn't get the welder out on a flush broken bolt? there is a time but not now

) hth :thumbsup: