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MG Midget and Sprite Technical - Rebuild Diff assy

I am in the proces of rebuilduing my rear axle. Now i want te replace the 1:3.9 crown/pinion with a 1:3.7 one. How to remove the bearing from the pinion (ATA7166A, Item 26 from part cataloque, page 49).
What kind of trust washer should i need (ATA712x, item 27) when i rebuild the diff assy with new crown and pinion. Is it wise to use new bearings of can the old one's do?

Niek Lammerts

hopefully this will be helpful:

Norm Kerr

I found the site that Norm mentioned already. It is very usefull. But i wonder how it is with the new pinion, shimms etc. Nowdays you use a collapsible spacer, but how to adjust the crownwheel/pinion setting, this has to be set to the special value wich the set is made.
Niek Lammerts

Hi Niek,

when I was working on mine I researched a whole ton of web sites and found that while there are many that describe how to set up a new ring and pinion, the one that I found the most clearly written, and had the most easy to understand images for reading the bluing ink on the face of the teeth was this one:

I hope that you will find it useful.

Once you have got everything else set (pinion gear depth, pinion bearing preload, ring gear run out and back lash), then finally check the ring and pinion gear tooth contact pattern, and then do it all over again (and again) until the bluing ink shows that the contact surfaces are meeting in the middle of the tooth (between the root and the top of the tooth).

Getting it right is fussy and time consuming, when working with a new ring and pinion, which is why shops who will do it charge as much as they do, to do it, and why many folks opt to simply buy a used one instead.

It is substantially simpler to rebuild one when re-using the original ring and pinion, as so much of the necessary steps were already done for them and the shims can usually be re-used as-is.

Regarding setting up a new ring and pinion, the good thing is that once you "get" what it is that you are trying to do, it is relatively simple, technically, it just takes time to take it apart and put it all back together again, checking each of the intermediate steps, each time, just to check the bluing ink on the gear teeth to see if it is "good enough" yet, and reminding yourself that if you stop before it is right, you'll get a noisy rear end.

Norm Kerr


Setting up the pinion height in a casing is not only critical but difficult to do it from scratch.

In an ideal world, you would keep the crownwheel and pinion that are matched to the casing together. if you can't do that, the washer (item 27, ATA712*) should be kept with the casing. IE... If putting new CW&P into your old casing, take the washer from the old pinion and use this one with your new pinion.

I have a set of bearings that I have honed so they are not a press fit onto the pinion or diff carrier. I use these for setting up the diff prior to the final build with new bearings. I also have a range of shims which you will find essential for setting the crownwheel position.

If you have not 'built' a diff before, I suggest that you don't do it for the first time with a new and valuable 3.7:1... try it with something more common like a 4.2:1 and see how it feels and sounds...

Or... get someone who has the experience and correct setting up tools to do it for you.

Good luck,

M T Boldry

Sorry, I forgot...

To remove the bearing you need a special clamp and a hydraulic press.

M T Boldry

Thanks for all the good advice. I think i have enough information and will try to find someone in Holland with experience with this matter
Niek Lammerts

Try Koen at Classic Car Clinic in Nijmegen
David Smith

This thread was discussed between 05/06/2011 and 12/06/2011

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