>It's a '73 Jensen-Healey that, despite two passes at rebuilding, has a
habit of
>jumping out of 4th (50-60mph) that starts not too long after the
>rebuild.
As we learned from an earlier post, the early Jensen-Healey used the Alpine transmission. Jumping out of gear is a problem common to many Alpine transmissions, including mine. Slipping out of 4th is a little unusual, as most of them slip out of 3rd. I am assuming that the shifter plates and springs were replaced with new ones during the rebuild. These serve as detents to hold the trans in gear, and it could slip out of gear if they are worn.
Another cause of this problem is bent shifter forks. The original forks were prone to being bent by overzealous drivers. Sunbeam Specialties has commissioned remanufactured forks that are more robust than the originals. I would certainly try one of the new forks. Another possible fix might be to update to one of the later ZF gearboxes used on later Jensen-Healeys. It might require a new bell housing.
Good luck, and let us know if he gives you the Jag.
Rex Funk