Morris Minor Owners Club

I bought a 3.9 diff some time back and felt that there was a little bit of drag in the unit when turned by hand. All the teeth looked very good so I thought maybe the input bearing was a bit dry. Since it has been used (150 miles or so) it has always been noisy above 40mph. not a particularly high pitch noise, more the sort remeniscent of a non-synchro gear.
I took the diff out today and the oil looked like bisto. Strangely the drag when turning by hand has gone-but then of course the noise is still there.
Could that be rust from a dry and now worn input bearing?

The setting up of a diff unit appears to be a lost art these days. Assuming that you have at least 5 thou" backlash between the crown wheel and pinion the 'drag' is likely to be due to incorrect preload on the bearings. If it whines on either pulling OR overrun that is pretty usual.
If it whines on both of these conditions it needs some attention. You are probably spot on re the drag since it is now easier after some use but may have damaged the overloaded/dry bearing in the process, hence the
mucky oil?

Thats the curious thing. it only whines during engine load and it would appear to be worse at certain road speeds. Although the diff is new to me everthing else was pre-used on my other car so I'm fairly certain it is the diff that is producing the noise. when I changed them yesterday I compared the 'slop' or give in the gears in the diff with it's replacement (an excellent, quiet 4.22) and there was no discernable difference.

That is the problem, it isn't just the amount of backlash but how the Crown wheel and pinion actually mesh together which affects the noise.
The fact that your only sings under load is not exactly rare! I worked my way through about seven diffs before I found one which was quiet and even that one has now started singing above 55mph!

The bearings are not too difficult to change (some specialist tools needed bearing pullers DTI guage etc) I've just made one good diff from two faulty ones to use as a spare.

Mogwai, Do you have the tools then? And where can I source a bearing? I'm reluctant to go back to 4.2 because the 1275 really does suit a 3.9.

I dont have the proper bmc tool but have worked out the size of one of the calibration blocks. I took mesaurements of pinion height etc with a DTI before dismantling so could be replicated on reasembly. as bearings are machined to very tight tolerances its likely the original shims will be ok if it was set up correctly to start with. The bearings can be removed with a bearing splitter.
I was thinking of going along to sadmog tomorrow could explain what ive done better then