As others have said, a tap. But be aware there are two kinds of taps. One has a pointed end and the threads are gradual. Use that kind. The other kind of tap, called a bottom tap, looks blunt and threads start very quickly. It is called a bottom tap for it is meant to tap a blind hole to the bottom.
Is there access on the back side? If so, start your tap on the back, not the front, then it will clean out the bad threads.
Just for your info. Helicoils are weak. Don't use them. It is much better to use a Time-sert which is stronger. (
++ TIME-SERT Official Threaded inserts for stripped threads, blown out sparkplugs,).