skotlyc said:
.....am I to understand that the Steel Crown gear is eparating from the aluminum hub assembly which is causing the oil leak?......
.....Has anyone ever tried to braze them back together? If one were to drill and place dowel pins between the steel gear and the aluminum hub for radial support, and then braze the two together for axial support would that work?.......
Yes, the steel crown gear has the aluminun seat for the tapered roller bearing pressed into it. Engineers on this site have suggested that BMW did this so heat expansion/contraction of the crown gear assembly would match the heat expansion/contraction of the final drive housing.
And yes, when this press fit is inadequate, the parts begin to slip with respect to each other allowing oil past the interface. As it continues to wear the parts will become looser and unstable with respect to each other. This results in increased risk for bearing damage, and allows oil to past through the interface faster.
No one has ever repaired one that I know of. But nothing is impossible. I once posted here that a post 2005 FD couldn't be converted to an earlier configuration with a hole for the speedometer sensor, but someone on this site proceeded to do just that; so I've learned never to say "never". Welding or brazing aluminum to carbon steel...hmmmm. Drilling and pinning will also weaken components.
The "fit up" of the compents will become increasingly difficult so you should stop riding on it now. You should probably stop riding in any case, bad things could happen as the crown gear assembly becomes increasingly unstable. And if you do attempt repair, I think you'd want an experienced machinist to check and make sure the tapered roller bearing seat (aluminum end) and the crown wheel bearing seat (steel end) are concentric after your repair.
I wouldn't be all that confident in a repair, I'd be wondering about it all the time riding on it. Also make sure you rebuild the drive checking the crownwheel bearing preload. It is likely that things will have changed because of the wear that is going on and the bearing set up should be checked and adjusted as needed.
It would not be impossible to rebuild that drive. You could find a used FD an take the crowngear/pinion set from it. However, it would be much easier to just find a used drive, and check it to make sure it is set up right.