CC002 POST-WAR OIL LEAKS/CHAINCASE(1) 

How can I make the chaincase completely oil-tight? - TW 10/6/2000

You can't - it wasn't designed to be. It is open-to-atmosphere around both the engine and gearbox mainshafts.....
However, you can get it nearly oil-tight, whether it is the "band" type or "pin"(screw) type.
Scrupulous preparation of all joint faces, and silicone sealant on the spacer, a new cork gasket with a thin coat of heavy grease for between the case and gearbox, a new cork gasket for the joint on the pin-type and NOT EVER OVER-TIGHTENING the pins (screws). Once you've over-tightened them, the case gets distorted and even their holes seem to get bigger. Then careful use of silicone sealant is unavoidable. But there are other tips which I hope are contributed; and a neoprene seal to replace the felt ring is available - has anyone got experience of it and does it spoil the gearchange....? - TW 10/6/2000

A fellow Thames Valley member, Norman Bailie had the same problem. He discovered that the engine sprocket acted as a metallic oil seal, his engine had excessive clearance between sprocket and crankcase. His solution was to place a bead of silicon on the sprocket, smear the crankcase with light oil so the silicone did not stick to the crankcase, thereby effectively forming a silicone oil seal. It cured the problem. Remember first to check that the problem isn't the main shaft pulling out of the flywheel! 
I tried a neoprene seal when I rebuilt the Venom with new clutch plates it was too thick. Unlike the felt one you cant trim it, however the clutch outer plate just ground it to smithereens. I replaced it with a felt seal soaked in tallow and it worked like a dream. Remember to put rubber beading around the front chain guard, that way any oil flung out of the chaincase runs harmlessly onto the garage floor rather than down your pcc - Dai Gibbison 26/8/2000