A very late response to an old problem! I also own a 1953 M33 (recent purchase and still getting to grips with the foibles of BSAs and their oil tightness...or not!). I've stripped and rebuilt the gearbox, replacing bearing with modern sealed bearing, retaining the outer side seal. On filling the box with the required one pint and running the engine, oil immediately starts to leak from the output shaft on to the chain. This stops when the oil in the gearbox has dropped to about 2/3 rds of a pint. Being a 1953 plunger model the SRM modified nut doesn't fit. Thickness of oil doesn't seem to make any appreciable difference. Just wondering if there has been any further development with regard to a cure for this issue. Did you ever cure yours Christouring? Either way it looks like the gearbox has to come to bits again!