French Metric Threads

There always seem to be curved balls thrown at anybody trying to restore old vehicles. This is one that i new to me.

I have just received a query from a friend and ex-colleague in South Africa. He is restoring a 1937 MG SA and has come up with information regarding early metric threads.

I repeat the relevant parts of his email below.

It concerns something I’ve never hear of before, which is Pre- ISO Metric French Metric Threads. I’ve been looking at the purchase of a 1937 MG SA convertible. It is powered by a MG 6 cylinder engine which has some components made by a company called Hotchkiss. Apparently Hotchkiss was a French company who migrated to the UK during WW 1. MG purchased engine parts from them and eventually bought the company. The company fixings standard was a thing called French metric, which used thread sizes different to the later SI system. In order for British Mechanics to use their Imperial spanners, the bolts and nuts were conveniently equipped with BSF spanner size heads!. So forgetting about the spanner sizes, the question is whether any bolts can be obtained with these original threads. I assume that Hotchkiss was only one of many French engineering firms and that fasteners with these threads must be found on many older French cars and machines. So as my consulting engineer on all matters Francais, could you please cast some light on this unexpected twist to a very British car marque.

Like my friend, I had never heard of this but looking on the internet reveals that this was/is an interesting problem for restorers.

Can any members shed more light on this and, in particular, whether any of these fasteners are available?

Roger_G