Thanks for the help, ya'll. I actually figured out a solution earlier today. The size was indeed 5/8-11. Problem is, no bolts at the standard stores are shorter than 2". I just cut a length of threaded rod to 3/4" and used a dremel to cut a slot for a flat head screwdriver and screwed that puppy in (using a bit of copper RTV on the threads). Problem solved.
I don't quite understand the design. I wonder why the hole goes all the way to the exaust port??
Anyway, I'm happy now. No more exaust leak.
