Date registered: Nov 2014
Vehicle: 1990 350SDL, "Grandpa's Roadster" Project Car
Location: New Hampshire
Mentioned: 9 Post(s)
Quoted: 824 Post(s)
The helicoil changes size (gets smaller) when you thread it into the specially tapped hole. This is what helps keep them from unthreading when you remove the fastener. It is not reasonable to assume that, because one threads into the other on the bench that it will do the same thing in use. If that were the case, then the M8 helicoil tap would be exactly the same as a standard M10 tap, and it is not. If you don't believe me, screw the M8 helicoil onto an M8 bolt and you will see it is a very loose fit.
If you're planning to use M10 hardware, you can just drill and tap the original hole for M10; no need to go oversize then back to the M10 helicoil.