Morris Minor Owners Club

The web of the BTA carrier where it carries the planet gear pin is much strengthened over the ATA carrier so as to enable more power to be reliably transmitted.

Here's a photo of a reproduction (Turkey) BTA crown wheel (and pinion) on an ATA carrier. It was impossible to set the backlash and was very tight. It is still in the car at the moment but whines above 40 MPH. It is coming out next opportunity. They don't mix.
Regards
Declan

That's interesting Declan, so would this https://www.morrisminorspares.com/rear- ... -1-p831277 be ATA or BTA? Also it looks like you have to match carrier to CWP but they can go in either diff housing?

PG,
That I would like to know too.
Regards
Declan

Declan
The articles and publications referred to in my post above all state (in a nutshell) that the carriers are not interchangeable and that problems can and most likely will arise with the pinion bearings and spacer.

I have a 4.8 diff that I would like to put into the stronger BTA carrier but all enquiries made so far as regards the job point in the direction of the impossible to do dead end.

One of the publications makes the point that machining the carrier to achieve the end is not always possible and the right carrier/crown wheel/pinion should be used.

Phil

Quick update-been to three machine shops today, all reckon that machining the diff cage can only really be done with the correct tool, i.e. the milling or grinding wheel that was used to make it with. That or shape a spherical or dish grinding wheel to match the thrust washers.

It was suggested that I give it a go with a Dremel and small grinding wheel. I think it'll never work on hard steel like that, but if it's scrap otherwise, there's no harm is there?

I haven't totally drawn a blank yet, going to contact a couple of gearbox repair specialists before I give up.

In the meantime, I will focus on the 4.22:1 - changing the planet wheel thrust washers, getting the right size sun wheel thrust washers, adjusting the play to a minimum, and changing the oil seal.

Phil,
When I installed it I wasn't aware of the the ATA/BTA mismatch at the time. I bought a Midget 3.9 CWP (Turkish repro BTA) and thought it would be plain sailing-not to be the case. I had to remove all the shims on the ATA carrier side to get it to fit and even then I was not happy with the backlash. I would have liked to have had 0.008" … 0.010". There was no way and I was 40miles from home and had to install it to get back due to other commitments. The tooth pattern looked OK so in it went-tight as it was. I put it down to Turkish tolerances but I was wrong. I had hoped it would free up with time -again not the case. It would possibly work if the carrier was machined to allow the use of shims. I never got around to it and then health failed so all was put on the back burner.
Regards
Declan

Declan
Thank you for the additional insight.
Phil

Phil,
I stumbled on this this morning which may be of interest. A similar chart to the one you have posted above also appears in the Owen Burton Modification manual on page 46 (wrong place!) just before the drawings start. Below the chart he states-quote

"Note:- ATA and BTA axles use different pinion bearings and redesigned diff housings. Whilst complete units are interchangeable, crown wheel and pinions are NOT."

Regards
Declan

Declan

Many thanks for the info above.

Phil

PoolGuy wrote: ↑Tue Oct 08, 2019 4:50 pm That's interesting Declan, so would this https://www.morrisminorspares.com/rear- ... -1-p831277 be ATA or BTA? Also it looks like you have to match carrier to CWP but they can go in either diff housing?

I have sent a request for information to ESM. I think my last post regarding the Owen Burton list answers your other question.
Regards
Declan

It’ll be interesting to hear what they say. I have a Quaife lsd to fit, and was wondering if it’s affected by the ATA/BTA thing.

The fitting a LSD gets around the problems of the ATA/BTA carrier etc as you are only replacing the crown wheel carrier and sun/planet gears and not the crown wheel and pinion.
You will still have to set the crown wheel relative to the pinion so still not fully straight forward.

philthehill wrote: ↑Thu Oct 10, 2019 4:12 pm The fitting a LSD gets around the problems of the ATA/BTA carrier etc as you are only replacing the crown wheel carrier and sun/planet gears and not the crown wheel and pinion.
You will still have to set the crown wheel relative to the pinion so still not fully straight forward.

I'll be replacing the CWP too, I currently have a 4.55 and I want a 3.9, the CWP that ESM sell isn't identified as ATA/BTA, but Declan may get an answer from them.

To put some additional meat on the question of interchangeability - the pinion bearings are different between early and late diff assemblies.

For example:-
ESM...……….. Late diff assy.
Diff pinion inner Pt No: DIF122
Diff pinion outer Pt No: DIF118

ESM...……….. Early diff assy.
Diff pinion inner Pt No: DIF122A
Diff pinion outer Pt No: DIF118A.

MOSS...……. Early diff assy.
Diff pinion inner Pt No: ATA7328.
Diff pinion outer Pt No: 2A7213.

MOSS...…… Late diff assy.
Diff pinion inner Pt No: ATA7166A.
Diff pinion outer Pt No: BTB440.

The difference in pinion bearings has already been mentioned in the above posts.
I would like to think that ESM will give a positive reply but I would not hold your breath till the required answer comes up.
There are too many stated and qualified opinions that the ATA and BTA component are not interchangeable.
I would love to be proved wrong as I do want to fit the 4.8 diff to my Minor but I am not holding my breath.

https://www.moss-europe.co.uk/shop-by-m ... 58-66.html

Phil

Interesting. The pinion that ESM sell has a step at one end, the one that I have (from a diff of unknown origin) doesn't have the step. The 'ESM late' bearings are for the stepped pinion.

Poolguy,
I don't think the stepped pinion is a problem. The same issue occurs when converting the MG TD to MGA CWP. It is only the inner bearing cone that needs changing to accommodate the step. The MGA has a stepped pinion. The diameter at the step is 1 1/4" whereas the TD has no step and is 1 1/8". Therefore by changing the cone from 3198 to 3188 the issue is solved. Of course the pinion has to be relocated due to the difference in pinion head thickness.
Phil,
Why not try and fit the TD CWP. It is 8/41 (5.125) and would be ideal for your application. I have one here if you want to try it as it will be left over from the MGA diff CWP change that I am currently working on.
Doug Newcomb thinks that the ESM CWP is ATA but he wrote that he will check and confirm. I have explained the issue to him.
Regards
Declan

Declan
Many thanks for the offer and I will bear it in mind if required - but I think that I will eventually fit a LSD carrier to get around any problems.
Phil

Thanks again chaps. I noticed that ESM mention that the early pinion bearings are for the 4.5 cwp. Then noticed that the outer pinion bearing in my ebay diff has a much larger od than the late bearing supplied by ESM. I’ll measure later. The part number on the diff housing is ATA7033.

Back to Mr Evans, I'm still not convinced that the thrust washer is supposed to rotate in the cage, which would mean that the BTA cage is fit for purpose. It looks to me that the thrust is designed for the planet gear to rotate. I would be building the BTA ensuring that there is no play by using the correct sized thrusts.