[Jax]

I was at Hilco this morning and noticed they carry Perma Coil...the tool is the expensive part...the coils themselved weren't too expensive...also they carried one that was specifically for spark plugs...probably made of a different material...

[Black_peter]

and what do you mean when you say you might reach for a 5mm tap and grab something else? you kinda lost me there. a 5mm is a regular tap, like any other. so is a 7mm.

I think I'm coming from SAE helicoils.. For example an 8-32 helicoil tap looks like a 10-32 tap but it's not. I guess it makes sense that metric helicoil taps would (because the can be) regular sizes..
Like a 6mm helicoil uses a 7mm tap..