Whilst removing my dynamo in order to get it upgraded to 12V I thought I would check the primary chain tension - to my horror I discovered it was as tight as an iron bar, relieved that I had decided to check it I started to look through my spanners & sockets for the correct size to loosen the two bolts holding the gearbox in place, to my chagrin I found that I didn' have either a spanner or a socket of the correct dimensions, I even tried my metric sizes. eventually discovered it is 25/32 AF, who would have such a peculiar sized spanner in their toolbox, obviously not me. The good news is that there is a plethora of the beasts on Fleabay and I am now the proud possesser of a brand new Britool ring spanner and a brand new Britool socket of the required sizes. Strangely (I thought) I noticed on Fleabay that there were quite a few spanners for around a tenner but with over double that cost for postage - closer investigation revealed that there is a considerable number of tools on Fleabay UK that are presently in the States. Perhaps 25/32 AF is more popular in America?
I have a spanner that size and that is the only thing I use it on.
I think the engine shaft timing pinion nut is the same.
I have at least 2 sockets of this size but no spanner, which made adjusting the primary chain tension a pain as access for a socket is tight to say the least. (B31 plunger).
I solved it by making up two elongated nuts from 19mm hex bar. This makes it easier to get at the nuts, especially the front one and 19mm ring spanners are easier/cheaper to find than 25/32.