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MG Midget and Sprite Technical - Diff questions. Various.

I haven't got the tools to strip and rebuild a diff with new bearings, or any experience doing it. So I'm going to get it rebuilt. But before I do, a few questions.

1. I have good 3.9 diff with a damaged diff housing. Can a 3.9 diff carrier with crown wheel and pinion, be built into a 4.2 diff housing?

2. See picture 1.
Took my noisy diff out and found these broken washers floating around loose in the housing. Do these thrust washers often break apart like this? I'm assuming this is why I have so much sloppy backlash at the road wheels. Would this also cause diff whine and a bit of vibration? All gear teeth look ok.

3. See picture 2.
I don't have any propper blue marker, so I used a white board marker pen, and got this pattern.
Is this wear pattern good bad or indifferent? The teeth are polished smooth and don't have any pitting or chips.

Ta in advance.







anamnesis

An expert will answer but in the meantime those "broken washers" are the remains of olive shaped shims that go behind the planet gears (the small ones) I replaced those on my diff and it took out almost all if the serious wheel rim backlash that I had before. Easy to do, involving punching out one roll pin and a little 'Chinese puzzle ' work to reassemble. No other critical settings need to be disturbed to do this. I didnt venture to refurbish anything else at the time as I didnt trust my low level of knowledge.
GuyW

Scary, I've not seen those planet gear thrust washers break up before just the tufnel sun gear washers wear down to nothing.
David Billington

Did replacing those washers reduce noise too Guy?
anamnesis

I cannot remember that specifically. But it got rid of that horrible stop go snatch at lower speeds and in traffic so at the least it FELT smoother and quieter.
GuyW

yes you can build your 3.9 CWP into a 4.2 carrier.
David Smith

Hardy Engineering at Leatherhead are experienced long time Sprite & Midget transmission specialists. Ask for Bill McDonagh. Bill is a Sprite enthusiast owning several.
Alan Anstead

Thanks David.

Really goes against my grain not doing it myself, but without the right kit and experience, and little point getting the kit for something I'll never do again;----

Yep, that's where I'm headed for the rebuild Alan (as per your previous advice 🙂), going to bell them today.
anamnesis

Another vote for Bill, he has rebuilt 2 diffs for me which have been excellent.
Bob Beaumont

And they fit my 1275 clutch release bearings to their customer cars.
Alan Anstead

1. Yes. I did the reverse last winter (4.2 into 3.9 casing).

2. As far as I understood things, worn (or missing!) sun/planet gear washers tend to manifest in a clunks on and off the power. Whining tends to be crownwheel and pinion.

3. Looks OK as far as I can see, what is the backlash like?

Are you replacing all the ball bearings Anam, or just the thust washers etc.?

Biggest hurdle in doing the thrust washers can be getting the locking pin for the cross pin out. After that it is easy.

Good luck,
Malc.
Malcolm

Thanks Malcolm.

Good to know the 4.2 diff I have can be put to good use then. I won't be doing the job myself, I don't have a press, or the type of pullers needed to get the bearings off either the CP or the pinion, or a 140 torque wrench for a new crush spacer, or the experience to do a full diff rebuild. Best I leave the job to Hardy engineering methinks. :)

This is the full story. I have three Diffs.

1). A 4.2 given to me yonks ago as a good diff, that I thought was a 3.9. When I actually looked when I needed it, I saw that it's a 4.2.

2). A new bought 3.9 that came in a BL box, over 20 years ago. I assumed it was brand new. But a little over a year after fitting it, it almost seized solid a few hundred yards from my house. Horrendous noise, and I limped it back home. When I took it out, I couldn't move the crown wheel or the pinion. I took the bearing caps off and pulled out the cage. The cage spins freely in the bearings, and I couldn't(still can't) see any damage to either the CW teeth or the planet wheel teeth. The pinion was locked almost tight. So I took that out and a lump of metal (maybe a bit of broken halfshaft), fell out. As I don't remember ever breaking a halfshaft (43 years same axle -- althoug maybe I've just forgotten), I wondered if the 'NEW' diff was actually rebuilt and wasn't cleaned out enough. anyway, I can't see any damage to the pinion teeth either, and the bearing surfaces look good. But on ONE of the CP bearings, there is blueing, and possible marks on the bearing housing in the diff casing, that suggests it might have been spinning in the casing. Hence my question about putting a 3.9 in to a 4.2 casing.

3). My original 3.9 diff. The is one in the pictures, that I've been using for circa 20 years. This is whining and clonking. The copper washers fell out of this. The backlash between the pinion and c/wheel, is a bit less than 10 thou . The marking on the wheel is 008. I assume this is what the maximum backlash is supposed to be. See picture.

I've already taken the the copper washers out of the 4.2 diff, along with the planet/sun gears as they look pretty good. I'm wondering about putting them in my whining clonking diff to see if it improves it. Nothing to lose there really.

But the 'NEW' diff, I will entrust to Hardy to examine and if need be, build back into the 4.2 diff casing, as I doubt I could do it and get a decent result myself.







anamnesis

Are these washers badly worn, or do they look like this (distinct lip) when new?






anamnesis

The copper washers were in my 4.2 diff.

Copper of fibre? Which is best?


anamnesis

It's bronze not copper and they are the ones to use not tufnol.
David Billington

Thanks David.

Do you know if the smaller cup washers in my earlier post look worn? Are they supposed to have that lip at the edge?

I'm guessing not.
anamnesis

Anybody?

Do new planet wheel cup washers have a lip on them like my worn ones?

Are these too worn for re-use?

anamnesis

Pictures on parts websites would suggest no lip.

But in the few that I have done, I have always reused the cup washers and increased the thickness of the flat (sun gear) thrusts to compensate for the increased backlash between the diff gears. Probably not the exact right way of going about things, but hey ho! A lot of new parts are a junky so I try to re-use as much original as possible.

Malc.

Malcolm

Ok, thanks Malcolm.

anamnesis

I have only rebuilt the sun/planet wheels a couple of times (Hardy does it now) and they look pretty worn to me. I agree with Malcolm to reuse whenever possible as repro is such variable quality. As you say nothing to lose trying them.
Bob Beaumont

Thanks Bob. I decided to buy some new ones. From Wood and Pickett Mini Centre, ebay 100%. £7.74 inc postage.

https://www.ebay.co.uk/itm/272288106789

Along with replacing the thinner fibre washers with the thicker flat bronze washers, if it makes the diff useable, it'll be a good spare to the other one that I'll get Hardy enginneering to rebuild for me.
anamnesis

I have fitted a 3.9 to my Sprite and can highly recommend it. At 50mph the car just purrs along. I rebuilt the 3.9 myself and the hardest job by far is removing the inner bearing from the pinion shaft. I had to get a local engineering shop to do the job using a hefty press. You will also need one of the knife edge pullers to clamp below the bearing. The ones on the crown wheel carrier were easier and I managed with a simple two leg puller but had to grind the ends slightly to fit in the recesses under the bearings. Rebuilding was easy but you really need a 3/4" drive socket to tighten up the pinion nut to the correct pre load. You need a long bar with two holes to bolt to the pinion flange and this will have to have a notch cut out so you can fit the socket on the pinion nut.

Jan T
J Targosz

We find that the cup thrusts generally wear more than the flat ones, and are usually the first to disappear. Once that happens the cage wears, which leaves a ridge where the gears run directly on it.

When this happens the ridge should be dressed back, and there are oversize cup thrusts available.

There isnt a ridge on the cuo thrust washers originally so if there is one then they should be renewed.

Significant wear on the flat washers is much leass common, but the tufnol one can break up and disappear.
Paul Walbran

Thanks Paul.

Am I right in thinking, if you only went in a straight line, the sun and planet gears would never rotate?
anamnesis

That's my understanding, of course it assumes the wheels are both the same size and there is no skidding or slip on either wheel.
Bob Beaumont

Surely the sun gears rotate with the half shafts! In a straight line the planet gears wouldn't rotate as the whole cage rotates so the teeth that are engaged with the sun gear simply transmit the force, from one axle to the other without the planet gear rotating.
GuyW

Intetesting different ways of looking at rotate. Yes sun gears are rotating with the halfshafts, but they are not rotating relative to the cage causing their thrust washers to wear. (And thats before getting philisophical about rotating about the earths axis etc...)

While obviously wear occurs gradually as the gears shuffle during cornering, wheel spin is a big killer. Even worse, large amounts of wheelspin exceed the lubricating capability between the planet gears and shaft. Result is the gears puck up on the shaft, and as they are trying to turn in opposite directions something has to give and one will shatter.
As we found in the very first event our K midget did. Already had the slippy to fit but ran out of time. Thought we could get away with one event, but nope, it didnt. And one of the chunks of broken gear lodged between cw&p, broke that too.
Paul Walbran

I should have added that I meant rotate within the cage. I was trying to understand how the shims and those gears and the pin, can wear so much. Perhaps a city bound car then, wears its cage gears more than a mistky motorway driven car?

Years ago I knew bloke with an rs200 escort who broke multiple planet or sun gears doing loads of wheel spins. But until now I didn't know why. Cheers Paul.
anamnesis

My new cup washers arrived. As said, no lip and a fair bit thicker. So I'll put them in and try the diff before getting my other one rebuilt by Hardys.

anamnesis

If anyone is stuck for good used sun-wheels and planetary gears, I can supply selected sets, chosen after inspection of many sets. Each set (two sun-wheels and two planetaries) has its original cupped bronze washers (but no flat washers). I think a good set should be worth about $120 Australian dollars (approx. $90 US, 75 Euros) plus postage.

I used to convert Morris and Spridget diffs to fit MG TCs. I did about 45 of them. The conversion involved replacing the 10-spline spider gears with 6-spline TC, Morris 10, Y-Type gears, so I removed the 10 spline gear sets and wired them together. Most were from Morris Minors, which are usually not driven very hard, so they are in great condition. I had to use the big flat washers but the small cupped washers were not needed, so they are included in each set.

I believe postage from Australia is not bad. My email address is bobschapel at optusnet dot com dot au.

Cheers from South Australia.
Bob Schapel
R L Schapel

This thread was discussed between 05/07/2021 and 12/07/2021

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