B32 1949 Oil leak question?

[Bob Dalziel]

My B32  Gold Star 350cc ,  has a leak between chain case  and gearbox. Has had new sprocket, new oil seal and felt washer between Tin outer chain case and sprocket. Can anybody offer a solution or reason why this keeps happening. I have renewed the seals several times but can't for the life of me stop the leak. It is driving me nuts help please.
Thanks   Bob.