Morris Minor Owners Club

I took my first drive today after fitting a 3.9 diff. Unfortunately it whines from 30mph onwards.
I am not too surprised because I bought it knowing it was probably suspect.
My engineering skills are not good enough to tackle it myself, so what are my options? I have got this 3.9 diff, and a spare 4.2 diff.
I would like to end up with a sound, quiet 3.9.
Is there somewhere that still reconditions diffs? How much might it cost?
Any advice much appreciated.
Thanks, John.

I exchanged my 4,22 for a 3,9 version with Heathrow Transmissions - works very well.

Thank you.
I am glad you had a good result. Might be the most expensive option though?

It was not cheap, that is true. I do not remember exactly what I paid so I just had a look in the english ebay - over GBP 400.

Have remade my diff and it’s a bit hard to do . The hard part for me is finding a spider gear housing in good nick... because Metal filings get under the thrust washers and damage the cage.
Also need to set the preload with a spring balance or use the special BMC tool ... which l lucky have.

Also finding good quality bearings is getting harder.

I bet the noise is coming from that cage that holds the small gears.

I was wondering where the noise comes from. The teeth on the crownwheel and pinnion look fine.

Can the 3.9 crownwheel and pinnion be put in the casing from the 4.2 diff; and would that cure the whine?

Could be a combination of worn out bearings and Phosphor bronze thurst washes under small spider gears .... also the shaft that the gears sit on could also be worn....
Or too much play between crown wheel and pinion.... use engineers blue to check.

Cheers Matt....

Could be a combination of worn out bearings and Phosphor bronze thurst washes under small spider gears .... also the shaft that the gears sit on could also be worn....
Or too much play between crown wheel and pinion.... use engineers blue to check.

Cheers Matt....

And yes you can swap Crown wheel and pinion...

Thanks Matt.

Are Heathrow Transmissions the only place that could do this, or are there other places?
There are mixed reviews for H T. I don't know what to think.
I am happy to pay as long as I end up with a strong quiet diff. Does anyone have a recommendation?
Thanks, John.

I’m in East Sussex.

the noise is coming from that cage that holds the small gears

Running in a straight line the whole diff cage rotates as one, there is no relative movement, so no capability to create any whining noise.

Noise will be either the crownwheel and pinion and/or bearings. The typical 'on load' or 'off load' whine will come from the cwp, and likely only resolved with a complete diff overhaul. To do any part of an overhaul requires quite a bit of knowledge and tools.

Thanks for the replies. A lot of this is a bit of a foreign language to me.
I am not sure what to do.
I like the idea of making one good diff from two, if that is possible.

First of all what sort of whine do you have? You said that over 30 mph the 3.9 whines. Is this at all speeds >30 mph and does it whine on or off load, or both?

Also with regards to swapping between different diffs, there are 2 basic types:

• The diff carrier with the oil filler in it

• The diff carrier with no oil filler, and the oil filler in the axle casing

Diff components between diffs with the same carrier are interchangeable, but NOT between diffs with a different carrier.

It whines on load over 30mph. The noise stops if I take my foot off the accelerator. I only took it up to 40 mph because I was in town.
I think the carriers are the same. The oil filler is on the back of the axle casing.

That would suggest the diff needs rebuilding/resetting and that bearings themselves are not the cause of the whine.

Can you check to confirm that neither diff carrier has an oil filler in it?

Swapping between diffs is not likely to help as the crucial set up is between the crownwheel and its pinion, which are a matched pair, and are set up uniquely as a pair, together with the diff carrier and cage. All have their own unique dimensions that have to be accounted for when setting up the diff assembly.

Will do, but it will take a few minutes.
Thanks, John.

Yes, the carriers are the same.

The carrier for the 3.9 is not in very good condition as someone has caught it with a grinder sometime in the past.

John,

That means that you could use the 4.22 carrier as the base for the rebuilt 3.9 ratio, but it will make no difference to what is required to build it - bearings, shims, crushable spacer, thrust washers, seal etc.

Also you cannot just put all the 4.22 parts into the 3.9 carrier without setting it up correctly.

Ok. Thanks.
Are Heathrow Transmissions the best bet?