Classic/Antique Car Repair: Transmission leak in 51 Chevy 1/2 t pickup, extreme pressure gear, cork ring

Question
QUESTION: I have a 51 chev 1/2ton 216 c.i.  It is stock with  three speed and a torque tube. I have not been able to stop the trans from leaking oil at the rear slip yoke collar. It does not leak from the ball socket gasket only from the cork gasket that is around the torque tube. The threaded collar has the washers on both sides of the cork gasket and I have been very careful when I installed the cork ring gasket.  The torque tube is worn and marred but I would think the cork should be successful.  When I say it leaks, it LEAKS!  I lose approx. 1/2 cup or more after every drive short or long.  It is trans oil that leaks and not differential oil. If you could advise me I would be most grateful.

ANSWER: What are you using for transmission fluid? Viscosity and type. Let me know.
Brad

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QUESTION: Brad - the trans fluid I am using is "Extreme Pressure" gear oil SAE 85W-140 meets API GL-5,MT-1,PG-2 specifications. Regards, Keith Atkinson

Answer
85W-140 is awfully thin stuff for that transmission. 85W gear oil is about the equivalent of 30W motor oil and if there is any wear around that seal area it will leak. Back i  the day. yes I was around working on 1951 stuff when it was new, we ran 90W in the winter and 140W in the summer especially if there was signs of wear. Cork was not a very good seal for anything that had any marks or radial movement and that shaft has some radial movement especially when changing speeds. It is the only thing that absorbs and controls the torque applied to the rear wheels. Remember on multi viscosity oils the lower number is it's pour point at a specified temperature and the upper number means that it will absorb the shock of that number viscosity oil. It does not get thicker as it heats up in fact it gets thinner.
Brad