I thought I would share this with others who have irritating oil leaks from the primary case. No matter what I did, I could not get mine completely oil tight. I checked for leaks from the felt seal behind the clutch, the alternator wire grommet and the crankcase/inner primary joint, and all OK, so it had to be from the inner/outer case joint. The standard rubber band is a sloppy fit in the inner chaincase groove. I got some O ring cord of 5.7 mm diameter from a specialist seal supplier (Norton item is approx. 5 mm). The 5.7 mm cord is pressed into the groove dry with moderate thumb pressure, and it should stay put when the outer case is removed. With a smear of silicone gasket on the face of the outer case, so far no leaks (touch wood). It is only necessary to cut the cord with a blade so that both ends are just touching when pressed into the groove, with the gap in the middle at the top, since oil tends to leak only from the bottom. Or you could splice and Loctite it. The outer case tends to distort a little when the sleeve nut is tightened (assuming the inner case is also shimmed properly). The larger diameter cord may be just enough to compensate for this. If anyone wants to try it, you should state the application so you get the correct O ring material. 3 metres should be enough for one seal plus a spare.