[pen15]

I have removed many broken exhaust studs in aluminum heads by Placing a flat washer slightly smaller than the broken bolt over it and welding thru the center onto the stud. Much easier than the nut method because you don't have to reach all the way thru the nut. When the washer is welded securely pick any random nut to weld thru the center to the washer. Wait 30 seconds for weld to harden and back it out. Never failed.