That part is called a Torque Tube Retainer. U-joint ball assembly
The ball seat in the photo is for the Hydra-Matic and Powerglide trans.
The manual trans ball seat is machined into the Tail Housing
Manual trans. uses a 4 bolt hole ball collar.
HM & PG trans use a 6 bolt hole ball collar.
There are several reasons why the Torque Tube Retainers start to leak.
1. Missing O-Ring steel washers, 2 are required Picture
2. Retainer is not threaded on tight.
3. End of Torque Tube is scored and is tearing oil seal.
4. End of Torque Tube O.D. is wore down. Look at slides 35 thru 46
GM film strip.
5. Universal Ball housing I.D. is wore down. Replace
6. Oil seal does not have enough sponge tension to keep the Retainer tight. Rubber oil seals
have better sponge tension than cork.
Tightening the retainer makes the seal bulge, pushing down onto the end of the torque tube, making a seal.
When the rubber or cork oil seals gets old and hard, they loose tension and don't bulge any more, and there
is no longer any back pressure on the threads, which keeps the retainer cap from loosening. Parts illustrationGraphite grease might helpTorque Tube in Living Color
There is also a lot of down pressure at the u-joint ball housing / torque tube
connection from weight, torque and when applying the brakes.
You could give the ball housing a Girdle Picture 1
, Picture 2
, Picture 3
, Picture 4
It clamps around the tail of the U-Joint ball housing and the torque tube.
Its soul purpose was to prevent the torque tube from drooping/sagging downward.
But that's a band aid.