In this case I would not use a Heli-Coil but rather a Time-Sert, (It is a solid piece threaded on the inside and the outside instead of a spring coil). The kit comes with a two step tap. The tap starts in what is left of the existing threads and draws itself through, cutting and retapping a larger hole that will accept the Time-Sert. Use grease or lard (yes plain ole lard from the grocery store) on the tap and remove and clean it every rotation or two, to keep the "chippage" in the cylinder to a minimum. The best way is to remove the head, but if you're careful..............