My Beautiful Buell is out of commission... Help! :)

So one of the studs that holds the front header into the exhaust port on the front cylinder has sheared off! I am in the process of drilling it out to use an extractor but so far it doesn't seem to want to budge! I know that drilling all the way to the bottom can help loosen it up so it'll come out, but I don't know how far the bottom of the stud is and I don't want to drill into the aluminum cylinder head. Does anyone know how deep the threaded holes for these stud go into the head?