Mine was not leaking before this weekend. I just replaced it in a hurry. When I did it before I wiped everything scrupulously clean - no bits of grit or anything like that - and cleaned the (new) band. Then greased it and re-assembled. The chaincase on this single is much stiffer than on a Dommie - it is deeper and I think it has thicker gauge steel - so I imagine that's why it was pretty good at retaining oil. Better than the gearbox...
So now I'm fairly sure all I need to do is clean it up better. But it's still a mystery to me why it sits too far out for me to be able to wind the nut up enough to leave a couple of threads exposed - and so get the footpeg on properly. The threaded footpeg supporting tube is (I think) welded to the engine plate so unless some PO had packed the inner chain case out for some reason I cannot work it out. Unless I can find a narrow band - either 3/8" or 1/2" maximum.
Never mind. I'll just have to stay with my narrow nut.