[Piotrsko]

Anything that sizes the minor diameter smaller will tend to work but not at the strength as the original parent metal. Ever notice that helicoils are hardened spring steel or that they work by enlarging the bearing area? If you had a full set of taps / dies /drill bits you might NOT need helicoils for repair. If your whole engine is tapped for 8-125 metric screws, and you just stripped the 3rd manifold bolt because the idiot before you used a impact wrench on monster, helicoils are a godsend and you don't need every size.