Good luck, man. I'm not sure what kind of bike you have, but on my cb650, (both of them, actually...) the steel bolt had organically welded (electrolytically welded?) itself to the aluminum housing and the only way to remove it was to carefully grind off the top of the bolt. I'm not kidding. We'd ground the bolt (and the washer-like disk under the bolt head) down so thin it was like tinfoil and it was STILL not coming off. Once the head of the bolt was off, though, and the housing could be removed, the remains of the bolt spun right out with finger pressure. It was the darnedest thing.
I sincerely hope that's not the case for you, but if it is, be very careful about how you grind it off- if you damage the housing, it may leak with the new bolt. Some have luck with heat... I didn't.