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YEa heli-coils have their own taps that aren't to metric or SAE spec it's it's own thing.
Just a general piece of FYI, most metric threads (standard too but using as example) the hole is drilled about 1mm smaller than the thread spec size..for example, M6 x 1.0 is drilled 5.2mm, M8 x 1.25 is drilled about 7mm etc..etc.. this is for room to cut threads for the overall diameter with threads included, or to have meat to stick into the threads of the bolts