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MG Midget and Sprite Technical - Rebuilding the Differential

Sorry if I appear to be hogging this site but I have only had the Sprite for a few weeks and the novelty hasn't worn off - yet.

I am currently rebuilding a 3.9 diff and have sought advice from the BMC Service Manual, from early and late editions of Haynes and of course YouTube. All tell a different story. Most will be familar with the technique of blueing the pinion and seeing where the marks appear on the crown wheeel. One of the manuals says meshing is adjusted by the shims under the crown wheel bearings. Surely this is wrong. It is the depth of the pinion which determins the meshing, the crown wheel sets the backlash. I seek advice on a method which I propose to use. I will place the small spacer and pinion bearing on the shaft and press it to within 25 thou of it being fully home. I will then assemble the pinion in its housing but without the collapasible spacer and screw the nut up until it is just past finger tight. This should take up play in the bearings but leave the 25 thou gap. I will then assemble the diff and carry out a blueing test. I will be able to carefully tighted the pinion nut until the meshing is OK and the gap between the pinion head and washer will indicate the thickness of any shims that may be needed at final assembly.
The next job will be to set the backlash by manipulating the shims beneath the crown wheel bearings. On YouTube, midget Mania suggests placing shims to the outside of the bearings. This allows them to be adjusted without pulling the bearings off. This appears to be a good idea since even though I have a good puller the jaws do damage the shims in the pockets in the casting. Does anyone have views on this.


Jan T
J Targosz

Sounds a bit dicky to me-
Tooth markings are adjusted by both/either of the shims, pinion or crownwheel, It's the combination of up/down---in/out that gives the correct tooth patter position
If you have the original pinion shim ,refit it and assemble the pinion into the housing without the collapsible spacer and leave the seal out as well
Do the pinion nut up ,it'll be more than finger tight, you need to go just tight enough to get the correct drag on the pinion bearings, then fit the crownwheel up and see what you have for a marking
Bearing blue is crap for this ,I don't really understand why everyone does that, get yourself some nice bright/light coloured (yellow,orange) ochre and mix it with oil to make a paintlike paste,--not too runny
lightly(just cover) both sides of a tooth and wind it down to the pinion , hold the pinion flange for a bit of resistance and wind the marked crownwheel tooth back-forward through the pinion a few times then wind it up for a look/check
If you fit the pinion bearing without a shim you won't be able to get any bearing preload onto it ,it will be in the wrong position and your time will be wasted---and--if you were to try and move the bearing position(without a shim) by tightening the flange nut then you run the risk of damaging the bearing surface
Normally if it's just a bearing replacement job all the shims 'should' be correct as they came out,especially the pinion depth shim, so a check assembly but leaving the collapsible sleeve out should give a good result
I look forward to seeing your tooth pattern markings
using shims on the outside of the c/w carrier bearings is ok but tricky to get in if they are thin adjusting shims--better off staying as is on the inside


Is this diff for yourself or resale
William Revit

Yeah, sounds dicky to me too.

The pinion depth is set by the spacer thickness and the bearings should be pressed fully home on the shaft and not be floating half way on, half way off. Trying to do something funky to draw/push the bearings on to the pinion shaft by tightening the pinion nut sounds a great way to goose the bearings. They are TIGHT on the pinion.

Use the spacer and press the inner pinion bearing fully home and set the bearing pre-load correctly with the crush spacer. Adust the mesh/backlash as best you can by adjusting the carrier shimming left to right. If you can't get it half decent, then worry about changing the pinion depth.

For once I disagree with Willy - I have done two diffs recently by shimming the outside of the carrier bearings. Super easy to set up and both working well so far.

But all kind of irrelevant if you are using the same CW and pinion, you don't specify how extensive your rebuild is...?


Hi Willie and Malc,

Thanks for your prompt replies.

I am totally rebuilding the diff with new bearings, thrusts, bevel shaft etc. As Malc says the bearing behind the pinion is tight and even though I have the tooling to remove it I don't want to keep removing it to adjust the pinion depth and was looking for an easier way to check if I need shims. I was proposing to do as Willie recomends but on the "Dry Run" press the bearing on to the pinion but leave about .025" play. During testing I would tighten the nut progressively, which would push the bearing down, until the meshing was correct. I would then dismantle, measure the "play" and use this to determine a shim pack for a final rebuild.

I also wondered about placing the crown wheel shims to the outside of the bearings and whether they would catch on the bearing carriers. I removed these bearings with a two leg puller with the claws in the recesses in the diff casting. I turned up a mandrel with a centre countersink to fit it the bore of the diff casting to centralise the puller screw and the bearings came off squarely but the claws did chew up the shims.

I have tried various bearing suppliers in Glasgow but none are interested in supplying automotive ones. I have seen ones for sale on line but do want to buy top quality ones so I am still looking.


Jan T
J Targosz

It's this bit I am worried about:

"During testing I would tighten the nut progressively, which would push the bearing down"

I understand the logic behind what you are trying to do, I am just skeptical about it working because I don't think the nut will be man enough to pull the pinion into the bearing. I might be wrong.

If it does work, I certainly wouldn't want to use the bearings afterwards! And would do it without any spacer at all, incase the spacer required is less than what you have.

I bought my shims from here:

Search for "bearing shims" and filter by 72mm OD. The problem is they only go down to 0.1mm (4 thou). I got lucky on the first one I did and they worked fine. The second one I needed some 2 thou, so bought some shim steel and cut my own.

Good luck!


IF- you have the original pinion shim, it should be still correct if it's going in the same housing and using the same pinion
The shim thickness is selected by measuring the pinion depth in the housing and adjusted by the number on the pinion---All being fair, if you're using the same housing and pinion it "should" be right as is-
I certainly wouldn't be pulling the pinion bearing on using the flange nut, it'll wreck the bearings for sure
Malc, -just for you, J. can run his c/w shims on the outside if he wants--lol
I still think, if it still has the original c/w and pinion the original shims should be right first off

What was the original issue with the diff----------
William Revit


I have measured the shims fitted to the diff and also done the calculation as in the manual and they are the correct ones!

I will order some new ones and rebuild the diff as per the original settings.

My Sprite has a 4.2 diff and a mildly tuned engine but the revs are a little high for modern roads. I was given a 3.9 diff but I don't know its history so have decided to rebuild it. My attitude now is if its difficult to dismantle replace everything. Now that its in pieces and out of the parts washer I can see some pick up on the races so the bearings needed changing anyway.

Thanks for the advice.

Jan T
J Targosz

Sounds like you know which end of a spanner is which, even if the methods are a bit unorthodox, so I'm sure you will make a success of it.

Best of luck!


You are not hogging this site - it is great to have new threads and new folk posting them too. Welcome!


PS are you in the Central Belt (of Scotland)? There are a few of us who post on here in such parts and neighbouring, from Prestwick and Troon, Campbeltown and the Lothians. Plus not forgetting more further afield such as Aberdeen, Northern Ireland, lots of bits of England, Netherlands, Switzerland, Thailand, USA, Canada and Taz. More the merrier (and anyone say Hi if I have missed somewhere, such as elsewhere in Europe: anyone from Irish Republic? Anyone from NZ?)

M Wood

You didn't mention Jupiter. Sprites have been seen there too. 😉.


I have been posting on this site for many years but under the TD/TF section and even though my details were incomplete I never bothered to change them. I do indeed live in Central Scotland in Milngavie. My TF was sold a couple of years ago and I have been without a "Classic" until May this year when I managed to buy the Sprite which some of you may have noted at the Morris Leslie Auction. It is in absolutely superb condition apart from a drip from the Peter May oil seal. I am actually looking forward to sorting this when the car is put into hibernation for the winter. In the mean time I am using the TD/TF bodge and have fitted a small catcher tank under the bell housing and my drive and garage floor are totally oil free. Lets see how often I have to empty the tank though!

Jan T
J Targosz

Just completed the rebuild of my spare 3.9 diff. Just a few observations for anyone else thinking about doing the same job. Pulling off the front pinion bearing is a problem and you will need a really good puller and possibly a press to do the job. Removing the diff carrier bearings is easier but the legs on my two pronged puller were to short and I had to take off the outer race and balls for them to fit in. Once done the inner races came off easily. I bought new bevel gear thrusts and the ones for the half shaft gears are fibre, as with a MG B. The new thrusts made a real difference to gear play. I measured the thickness of the carrier bearing shims and they were exactly as per the calculation in the manual. I replaced them with new ones like for like. The only real issue was tightening up the pinion nut. You really need to make up a plate to fix to the diff casing which you can clamp in a sturdy vice. Also you will need a piece of angle iron, at least a metre long, with two holes drilled in which can be bolted to the input flange to stop it turning. The nut needs tightening up to 140 ftlbs, my torque wrench only goes up to 110. I had to use a normal socket wrench with a long piece of pipe over the leaver and progressively tightened the nut until there was a 8lb drag on the flange. The nut needs to be incredibly tight. Now that it is all together it turns over smoothly and I have 8 thou backlash which is well within the limits. The job is doable in a well equipped garage but does need a little engineering expertise. All my parts came from Brown and Gammons. Let's hope the 3.9 helps with cruising and the diff is quiet!

Jan T
J Targosz

Hey Jan, good stuff, did you do a tooth pattern check, .008" is getting fairly wide, is that what was etched on the crownwheel
William Revit

Hi William,

The official BMC manual says a min of .005" and a max of.011". My brother who owned an engineering business specialising in earth moving transmissions reassured me that nine times out of ten original shim packs would be fine. Considering that most often the original ring gear and pinion will be going back, and these will wear, it may be adviseable to move a .002" shim from one side of the carrier to the other. This would have reduced my backlash to .006". It is a messy job removing these bearings and you can not do it without destroying the shims. That's why the Americans suggest fitting shims to the outside of the bearings.


Jan T
J Targosz

All good Jan
I 100% agree with your brother's take on it--It usually pays to run original shims on a used gearset , sometimes if you move them you can introduce noises because the gearset had worn into a sweet spot where it was
I didn't realize the spec was 5-11, fairly large for such a tiny diff----but it is in the middle of the spec as you said
Usually if going to swap shims around, we go in 3 thou steps, 2 though is hardly noticeable when taking tooth markings
I'm sure you've nailed it as is

Tooth marking is the only real way to check
William Revit

Sorry, I have been a bit slack in checking in to this site lately.

If you (or anyone else) needs a set of good "sun-wheel/planetary" gears for a spridget diff, let me know. Many years ago I converted a lot of Morris/Spridget diffs to fit MG TCs. This process included replacing the 10-spline set-up with the 6-spline TC set-up. I always tied the not-required 10 spline gear sets together. As a result, I have a good selection of these diff gears .... everything from well-worn through to excellent condition. I think the best sets are in great condition because they came from low-powered (and gently driven) Morris Minors.

Not sure what I should be asking for a selected good set, but guess it should be about $100 Australian dollars. (Is that reasonable ... $25 per gear?) Let me know what you think.

Happy to supply "average" sets for less than that. (But don't see much point in doing so.)

Cheers from South Australia,
Bob Schapel
R L Schapel

This thread was discussed between 07/06/2021 and 18/06/2021

MG Midget and Sprite Technical index

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