I am further confused. Having removed the worm wheel without drama, I can now clearly see that this C15F engine does not have a plain bush main bearing. My understanding was that the roller bearing was fitted to the later G model, and not the F.
Rupert's book mentions that the plain bush was sometimes upgraded with an aftermarket needle roller assembly. From my picture, is it possible that this is a needle roller bearing? Or is it possible that somehow the 'G' model roller bearing has been fitted? Would that mean that a 'G' crankcase side has been fitted? (The engine was rebuilt in 2016 having been blown up in 1982, and this certainly involved new bearings, big end and oil pump. The engine has never been run since.)
I guess I need to follow the instructions for splitting the crankcases as per the G model?
That is not a "G" case.
I think your bike has been fitted with a needle roller bearing conversion to replace the bush. Alpha did such a conversion and no doubt others. I fitted one to a B40 many years back. The needle roller goes into the same hole as the bush it replaces. There was a washer either end of the bearing which provided a rudimentary seal and oil was fed into the crank in the same way as the bush. Worked very well too. The below is from an old Alpha Bearings catalogue.
I was wrong - it is a plain bush. Should have gone to Specsavers!
Is the grub screw I have highlighted the non-return valve?