i had to realise that the primary chaincase is not at all oiltight.
is there any reasonable chance to make it oiltight or should i think of alternative approaches to lubricacte / cool the primary chain?

I've just built a rigid Norton and put oil in the primary case. I'm a bit surprised that no oil at all is dripping out after a week. Make sure the tin is straight without any dings where the rubber seal fits. Make sure the rubber seal is good and the right size. I've just bough a new one from Andover Norton and it's much too thick for the outer case to go on. I had to use an old one I had in a box, but even that one was too thick. I had to sand it down a bit all the way round with a sanding wheel. Now the outer goes on enough to get the nut started, I silicone the rubber seal to the inner case, then when it's set I put lots of grease around the seal and the inner case. It should slide on and tighten up the nut just enough so the footrest locates without hitting the nut. Hopefully it seals :relaxed: