Extracting a broken bolt is a little like two pocipines makeing love. It must be done very carefully.
Sometimes you can get the bolt out by tapping it with a center punch, hitting the edge counterclockwise. It is worth trying. It is only tight for a short part of the way out and then it comes out like any other bolt.
If that doesn't work, then it must be drilled out. You need to drill the hole dead center in the middle of the bolt. This is the main thing you must do carefully. You must also be careful that you keep the drill straight so that the hole is in the middle of the bolt. Start with a drill bit that is less than half the diameter of the bolt, and then go larger and larger until you have removed about 2/3 of the diameter of the bolt.
Now try a bolt extractor or ez-out. Since the head is aluminum, it is important that you keep trying with an ez-out. The reason for getting the hole in the bolt as big as possible is that you want the strongest ez-out as possible. If it is too small, it is likely that you will break it. If it were an iron head, you could keep drilling larger and larger intil you are nearly the size of the bolt and then clean the threads with a thread tap. With aluminum that is very risky.
As to what size the tap would be, match it up to one of the other bolts.
I have had good luck with Sears Craftsman tools. Actually, the torque wrench I have is a Craftsman.
Good luck.
