I’ve only encountered M7 fasteners on the studs holding the camshaft bearing caps on a Volkswagen Polo engine. At work when we changed from imperial fasteners to metric, it was quickly discovered that M6 x 1 was not a good replacement for 1/4 UNF. The fitters found it all too easy to shear the M6. After that the smallest size used was M8 x 1,25 for holding parts together. M6 x 1 was only regarded as good for holding on nameplates and the like, not for anything important.

It is a pity that M7 and also M9 were designated as 2nd choice sizes, I think they would have been a better replacement for 1/4 and 3/8 UNF. We did use M14 x 2, another 2nd choice thread, to hold fuel injection pumps to the cambox. M12 being too small and M16 being too large.

In manufacturing, if you need to make lots of specials, there is no problem. It’s only if you want “off the shelf” that you find yourself limited. If you make the threads yourself, then there is no real problem.

Neil

On the DS series cars, for general purposes Citroen had standardised on M5 (hex head screws), M7 and M9. Certainly M7 covered the majority of engine and other mechanical uses that now seem to need a mix of M6 and M8. M9 was adequate and neater than M10 for common transmission fixings. I have the impression that M7 was indeed used for con rod bolts.