May I suggest one of these, if you can't get the broken bolt out without harming the threads:
http://www.timesert.com/
LOTS easier to install and more reliable.
DISCLAIMER:
To say I am not fond of Heli-Coils is an understatement....the aerospace community loves 'em, to such an extent that they've brainwashed lots of folks to mistrust any threads in aluminum. I've had guys who should know better ask for Heli-Coils in 6061-T6 aluminum for a no-load part that's only going to get a few cycles of assembly/disassembly. Granted, on some materials, especially magnesium, you've got no option but to use an insert, as mag is not very strong stuff.
Personal experience, they're a total PITA to install, not very reliable in a marginal drilling/tapping situation like you're facing, and they make total mayhem when they gall onto the bolt. Time-Serts are a far better alternative IMHO. Just a satisfied customer.......