Morris Minor Owners Club

Neil,
If the diff is not noisy I would leave it.
BTW the blue marking compound looks to be good stuff. It shows the pattern very clearly. I think I will try it.
Regards
Declan

2 years on, time to revive this thread.
A halfshaft broke on Saturday. I've got the axle off the car & the diff (a 4.22) out, so that it can be cleaned up properly. However, the diff is very stiff. A quick inspection showed why. These are pieces of the bearing cage from one of the crown wheel bearings.
This is the damaged bearing.

So it looks as though the bearing has failed, causing the diff the become stiff, and the half shaft to fail.

I've read through the workshop manual, section HH7, and it seems a special tool is required to set up the crown wheel & pinion relative positioning when the crown wheel is disturbed, or the bearings are replaced. Is there a way of doing this without the special tool?

What I would really like to do is to put the crown wheel from my spare 3.9 diff onto the crown wheel cage of this diff (a 4.22), then re-install in the 3.9 diff carrier.

Can I do that?

The crown wheel cage of the 3.9 diff is very worn where the planet wheel cup washers sit against the cage body, so much so that it's probably scrap.

To be honest I would ditch the damaged diff and find another serviceable diff.

So is replacing diff bearings and swapping crown wheels a no-go?

I have a good 3.9 crown wheel and pinion on a cage that's worn, and a good 4.22 crown wheel and pinion on a good cage, with a damaged bearing.

If both carriers are of the BTA type, then you should be able to change the crown wheel & pinion and carrier.
All can be done without the special tools, but you do need to know what you are doing especially in relation to the lateral position of the carrier in the casing and the relative fore and aft position of the pinion to the crown wheel.
Personally, I would not even bother to try and repair but try and find a serviceable unit or purchase a reconditioned unit from a reputable trader. Sometimes it is better to sidestep the issue.
Phil

Got the rebuilt diff, now a 3.9:1, back yesterday. Here is the shocking truth:-
The carrier has split apart. No wonder the half shaft broke!

Whether this is cause, or effect, I don't know, but one of the diff bearings had certainly gone. I've been hearing a rhythmic rumbling for a while, and put it down to wheel bearings, but never found anything wrong with them. Never suspected a diff bearing.

That is a first for me. Never before seen a carrier (neither ATA or BTA) split apart like as per the photo above.
The BTA carrier was strengthened in the area of the failure to make sure that the carrier did not fail.
I doubt that the half-shaft caused the carrier to fail but the failed carrier would have a serious effect on the reliability of the half-shaft leading to failure of the half-shaft itself.
As regards overhaul of the failed diff - I would scrap and fit a serviceable S/H unit.

I've done exactly that, next to the broken cage, out of view in the photo, is a rebuilt 3.9:1 diff.
Was certainly an eye-opener for me. I knew diffs could fail, but not like this! I guess it'll make an interesting door stop.