I can speak for a 51 3-Speed on this problem. Between the speedo gear and the U-Joint yolk on the tranny is a metal sleeve. When the u-joint yolk is tightened on the tranny shaft it presses against the sleeve which in turn squeezes the gear agains a shoulder on the tranny shaft holding it in place. I would first loosen and slide the torque tube ball back exposing the u-joint and make sure the u-joint bolt yolk is tight. You may have to dissasemble the u-joint to get a wrench in there. If is is tight I would pull the yolk completely out of the tranny, and insert very thin steel washer between the sleeve and the yolk to give it some more squeeze when I retightened the yolk... Bug
