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MG Midget and Sprite Technical - crown wheel run out

One Half shaft broke so I could inspect the diff. It was no nasty job. The inner part of the half shaft came out easy. The diff makes some noise, but I could only hear it after I installed a rose jointed Panard Rod.
The crown wheel run out is 0.15 mm = .006 in. The maximum permissible run out is .05 mm. It is three times more…
The backlash varies from 0.05 to 0.15 mm. It should be no more than 0.28 in. Minimum should be more than .13 mm. So the run out is too much and the backlash is too small on the high parts of the crown wheel.
Do I have to correct the run out? How can I do that?


Flip
Flip Brühl

Flip I wonder if the diff bearing (the one that the diff cage runs in) has been forced out of true alignment in the clamp that holds it. Or if the diff cage is distorted slightly, since the crown wheel is bolted to it.
I don't know for sure if it's possible for such things to happen from the shock of a shaft breaking; it's just speculation. From your description it seems that the rotational axis of the differential assembly (that the crown wheel is fixed to) is moving in an orbit of its own. Barring a wrecked bearing which would be very noisy, a distorted cage or bearing clamp or a misaligned bearing seem plausible to me. But there are others who know the diffs much better than me. It's a very long time since I did anything with a diff - it was my dad's BMW when I was still a teenager.
Greybeard

It may be an idea to remove the diff cage and remove the crown wheel and then replace the diff cage and check the runout of the diff cage on its own to try and gauge if there is an issue there.
David Billington

That sounds like a good idea, but would it be strictly necessary to remove the cage first? Is there enough physical space to unbolt the CW and hold it out of the way to check for cage/bearing runout?
Asking because I honestly don't know.
Greybeard

Manual says:
"max run out is 0.002". Anything greater must be corrected. Detach crown wheel and inspect joint faces for any particles of dirt. When faces are thoroughly cleaned the crown wheel should run true."

So maybe there is dirt in there, or rust particles, or a burred flange.
I would check that first, assuming your measurements are accurate and consistent.
GuyW

Greybeard,

IIRC the crown wheel when unbolted removes towards the pinion to remove it so limited movement to get access for measuring but it may be possible, I haven't tried it. I was thinking how one would measure it and likely with my kit I would make a plate to bridge the diff cage bearing caps to place a magnetic based DTI on to measure the run out as I have that option. The final drive case being an aluminium alloy presents a problem with the A series final drive for easy fixing of the DTI, the last final drive I helped rebuild was on a Morgan and like many Ford units was all iron so fixing a magnetic base was easy.
David Billington

You could undo the crownwheel and turn it half a turn on the carrier and rebolt it and measure again
If the crownwheel has a bit of runout and the carrier as well, it might be just enough to reduce the total runout to the spec----or not
Worth a try---???---it's free
willy
William Revit

The rose jointed Panhard rod is always going to transfer noise, I reckon you'll still hear it whatever you do
willy
William Revit

Thanks David.
Greybeard

Thanks all,

It is the carrier from the 3.9:1 diff that has a run out. Yes, indeed it is not easy to stick the magnetic clamp on the aluminium... But I have a vice where the magnet clamps on. I suddenly realized that I had the original diff somewhere here in the house. After 30 minutes’ research with my better part I found it, a 4.22; the carrier has almost no run out 0.02 mm!
I think I replace the 3.9 unchanged to the car and I am going to rebuild the 4.22 and on a winter day (week) next year I change the 3.9 crown wheel and pigeon to the original 4.22 carrier. A new crown and pigeon is 299 pound…

Flip
Flip Brühl

Hi Flip,

Have you crack tested the carrier? A possible cause of run-out could be that the carrier is cracked. Previous threads have indicated that members of this forum have not had much trouble with cracks but I have had trouble finding diffs with uncracked carriers in South Australia. I have converted about 45 Spridget and Morris diffs to suit MG TCs and TC race cars and have found that a large proportion of the carriers are cracked. (See image)

Cheers
Bob Schapel

R L Schapel

Hi Flip,

The diff carrier changed part way through the life of the Sprite/midget. If both your carriers have the same part number, then swapping the 3.9 cwp into the 4.22 carrier is fine, but if they are different, then it is not possible.

Please see the attached extract from the 1275 Special Tuning Manual for the details.

Is the run-out you measured on the back of the crownwheel when assembled to the carrier, or the mounting face of the carrier itself? It should be measured from the back of the crownwheel.

Before dismissing the existing 3.9 carrier, make sure that there are no burrs or trapped dirt between the crownwheel and the carrier - both the crownwheel and diff carrier faces must be completely clean, and free from burrs, dirt etc. Also rotate the crownwheel around the carrier, as suggested before - sometimes the 2 run-outs can cancel themselves out.

Richard


Richard Wale

Hi Flip,

Oops, I think I may not have interpreted what you are looking to do correctly.

In BMC terms the 'carrier' is item 'B' in the previous picture, and is it that part that changed. The carrier, crownwheel and pinion must stay together, or at least be the correct 'set' of parts.

The diff 'cage' is item 'C' and stayed the same part number - ATA 7086 - throughout. It is used in all the rear-wheel drive A-Series engine cars, and also the later 1500 Midgets.

Richard

Richard Wale

Hi Richard,

Thanks for that clarification. I have always called the revolving bit the "carrier" but your diagram shows that "cage" is the correct term.

It is the cage which seems to be subject to cracks in my experience. The one shown in my image was very badly cracked. Cracks start in a thin, rough section but progress more slowly as they get into thicker metal. I have seen a couple broken right through.

Bob
R L Schapel

This thread was discussed between 18/02/2018 and 23/02/2018

MG Midget and Sprite Technical index

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