External oil filter head - what is the thread in the mounting holes?

Wasn't the whole bolt-on filter head a French car part originally ? The risk of course is that one of the later suppliers will have assumed an Imperial thread and retro-manufactured...a bit like Wassells years ago when they put BSF threads in Commando camshafts.

Richard is correct in that the original oil filter design was pinched from something like a French Citreon 2CV. In which case metric threads for the filter body and mounting bolts would make sense...........However.........

5/16"  is about 7.98 mm

8 x 1.25  roughly equals  5/16" x 18 tpi UNC which also roughly equals 5/16" x 18 tpi BSW.

I stand to be corrected on any of the above..

A real original norton commando filter housing and good after market ones made to original spec are 5/16 UNF.

Beware of aftermarket ones as the spin-on thread have on more than one occasion , NOT been square with the gasket surface.

Beware also the pushed in pipe spigots that come out, go for the screwed in version if possible.

I worked on an elcheapo aftermarket one where the internal black paint had flaked off and very nearly blocked off the outlet!