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MG Midget and Sprite Technical - Diff rebuild

I am about to rebuild my early type diff ( type with filler plug on side).
Is the bearing spacer a rigid piece and re-usable? I know the later types use crushable spacers.
Dont think the later crushable spacers fit the early diff, need to check but dont have parts to hand.
Fitting new bearings and have measured these - the gap between the inner rings is 10 though wider so if spacer is fixed length I will fit a shim to compensate.
Les Robinson

You can rebuilt the diff without disturbing the pinion or the spacer.
Daniel Stapleton

Hi Les, according to the supplier that rhymes with Toss the early ones had rigid type part number ATA7219 and later collapsible BTA532.

I should think the early type would be re-useable but I couldn't see any shims in the Moss diagrams.

Rob aka MG Moneypit

Hi Rob
I had some shims in the garage that are a perfect fit to take up the 10 thou discrepancy in the gap between bearings - no idea what they are off.
Do you reckon the spacers were sized to fit during build at factory?

Les Robinson

Hi Les,

No problems using shims if needed. The lower ratios (4.5, 4.8 etc) had shims and solid spacer. I can't remember if the 4.2, 3.9 and 3.7 ratios with solid spacers had shims. I did about 40 diffs many years ago but didn't like the crushable spacers so always tried to use solid with shims.

The shims must be adjusted to get the right preload on the pinion bearings (same applies to CW) and both gears have to be adjusted, using "bearing blue" to get the correct mesh. If putting everything back in the same housing it shouldn't be far out but you should check preloads and blue markings if you want to be sure of a silent diff.

Sorry, I will be unavailable for any comments for the next few days. Good luck with the diff.

Bob Schapel
R L Schapel

Butting the two bearings up against each other and measuring the inner gap is a false reading---- as the bearings are not up against each other when fitted in the housing
there is a gap machined into the housing
You will probably find that after replacing the bearings the original spacer will be the correct thickness as originally they were selected to adjust for discrepancies in individual housings. The bearings themselves as you can imagine are highly accurately machined and a simple bearing replacement usually just restores all the preloads etc back to original
BUT good practise is always to check your results as I'm sure you intend to do

Daniel -that's magic

cheers willy

William Revit

Also when dismantling your diff, don't loose the pinion depth setting shim which will be between the inner bearing and the head of the pinion OR under the outer shell of the inside pinion bearing ,depending on who had it apart last
William Revit

The diff is the diff, the final drive is the crown wheel and pinion. The pinion is fitted to the diff carrier but is not the diff. The diff has sun and planet gears and a pair of bearings. The crownwheel fits to the diff but isn't the diff.
Daniel Stapleton

Technically correct!
Dave O'Neill 2

And I thought WE were trying to help?????????????
IF you read Les' question in his posting you will then know he is dismantling the whole thing (diff-pumpkin or whatever you want to call it)
William Revit

I am always helpful and try to help.

Les's original post stated that he wanted to rebuild the diff but suggested he was going to remove the final drive pinion. It was possible that he was confused between rebuilding the diff and fitting a new pinion bearing and oil seal which isn't rebuilding the diff at all.

It wasn't clear why Les was going to remove the pinion if he wanted to rebuild the diff. Also, it's my experience that the diff is more likely to need a rebuild than the pinion bearing is to be replaced.

If Les is removing the pinion for reasons not made clear then he will need to check the backlash between the pinion and the crownwheel - all of which has nothing to do with the diff.

Daniel Stapleton

I don't think it was confused at all.

Les used the term "diff" in a general kind of way to refer to the whole assembly, but it's pretty clear he's talking about the diff pinion.

If he'd just said I'm going to replace the bearings in my pinion, or at each end of my pinion, that would have been confusing. But he didn't.

He said.
"I am about to rebuild my early type diff ( type with filler plug on side). Is the bearing spacer a rigid piece and re-usable? I know the later types use crushable spacers."

The 2nd sentance in that quote qualified the 1st, and made it clear what he was talking about doing. :).

Of course, to someone who's completely unfamiliar with a "diff", and any of it's constituent parts, you might argue that it was ambigous or misleading. But then such a person wouldn't answer the question, or be expected to.

And anyway, the ensuing discussion, made it all clear. And Daniel's intervention, made it perhaps, 'crystal' clear, for those who have no knowledge of "diffs". ;).

Lawrence Slater

Job complete.
Pinion bearings replaced, pinion maintained at original height, bearing pre load set.
Replacement planetary gears and new bearing fitted to diff. Crown wheel to pinion backlash set.
Thanks for everyones help.

Les Robinson

All good then Les - well done
Just out of interest--
Did you have to change the pinion preload at all or did it come up ok with the original shim/spacer
Same question on side bearings
cheers willy
William Revit

I measured the existing and new pinion bearings to compare the relative dimensions of the outer ring abutment face to the inner face of the inner ring. The net result was that the span of the new bearings was 0.010" wider than existing, indicating that a shim would be required to maintan same effective preload.
So I fitted a 0.010" shim but preload was not enough. After trying a few different shim sizes, ended up with a 0.006" shim to give correct preload.
I also measured the height of the pinion gear with both old and new bearings and it was within 0.002" of original position.
All measurements were done using CMM at work so happy they were accurate!
I measured the backlash between crown and pinion gears which was 0.011", however the figure etched on the crownwheel was 0.007". To get the backlash back to the correct figure I adjusted the shim packs to move the crown wheel inboard and then fitted new bearings.
To complete the job I also replaced the sun and planet gears, plus thrust washers and cross pin - courtesy of Bob Schapel in Australia.
Whole diff/final drive unit feels much better. Hope to refit to car tomorrow.
Les Robinson


Useful info in:


M Wood

Parts from Australia--Unreal
and from Bob-- you can't go wrong with him on board

William Revit

This thread was discussed between 03/03/2015 and 08/03/2015

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