i wouldn't mess with heli-coil, i have had lots of luck with those on bolts with oil pumps and such, types of bolts that don't go all the way through, and you have to drill the hole to the size of the coil anyways, lots of mess..but thats a big hole your working with, personally i would retap it and get a bigger bolt, from home depot or something, and the metal shavings isn't as bad. Dip the tap in oil or something so you get most of the shavings form the tapping to stick to the tap instead of a lot getting in the motor.