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MG Midget and Sprite Technical - Differential: collapsible spacers

A replacement s/hand diff I have doesn't have the desired pre-loading on the bearings. Previously I have found that with a collapsible spacer it is possible to carefully increase the tightening torque to induce a tiny decrease in spacer length and so build up the pre-load. I want to replace the pinion seal so I have to remove the pinion nut anyway.

Some diffs have spacers and some have shims instead and without pulling out the bearings I can't see how I can tell. The diff is a later one - a 3.9 ratio. Can anyone confirm or otherwise that "all 3.9 diffs had collapsible spacers when they left the factory".
G Williams (Graeme)

As far as I am aware they all had collapsable spacers. The basic design didn't change that much apart from some really early (ie late 50's) 4.22 ratio diffs. i suggest the same procedure as you used before to replace the seal. Break out the scaffold pole!
Bob Beaumont

Can anyone remember the minimum pre-load torque - is it 6 in lbs?
G Williams (Graeme)

Works manual says:
"Tighten pinion nut to 135 -140 lb. ft. The distance piece then collapses to give the correct preload of 11 to 13 lb in."

Is this what you needed?
Guy Weller

THanks Guy. That gives me some confidence in my 6 in lb figure because there was a suggested minimum (which is what I was quoting). It's all a bit "hit and miss" without the proper guage. I remember hanging weights on a length of string wound around the boss of the drive flange.

I think it also gets involved as to whether that includes installed with oil and halfshafts, or on the bench. I wouldn't want to be trying to do that from under the car. It takes a lot of leaning on and would be very easy to over-do it! I have marked the nut position too.
G Williams (Graeme)


You also need to check whether it's with the crown wheel and diff carrier in place. When I helped with the rebuild of my neighbours Morgan back axle he had the torque settings for just the pinion, pinion+crown wheel & diff, and the entire axle assembled.
David Billington

..... should I use nylon string or garden twine?
Actually, can I unbolt the crown wheel and carrier without b*g**ring up any settings?
G Williams (Graeme)

That preload with the crushable distance piece is given for assembling the pinion with the diff assembly in position. There are different (and complex to me) calculations for determining the shims for the crown wheel.

I know when I did some work on my diff some years ago I shied away from any of this complex stuff. I just replaced the cross shaft and copper olives on the planet gears, and fitted a new pinion bearing, oil seal and collapsible collar. All the rest I just cleaned and put back as it was. Seemed to fix the problems I had of whine, excessive backlash at the wheel rim and oil leaking from the pinion seal. My diff bearings were ok though. I think if they are replaced, that is when you need to measure and calculate for new shims too. But if you don't change them, then I don't see the problem.

Someone like Peter B will tell us the proper way of doing it!
Guy Weller

Hi Graeme,

If I have this right, the 2nd diff you have is working fine but the oil seal is leaking at the rear of the diff.

Have you already removed the Pinion nut?

If not, do as you did before. Mark the flange and the nut so you know the relative positions, and also drop a weight off it to see how much weight it takes to rotate the pinion.

Then, when reassembling after replacing the oil seal, do the nut up to the marks, whilst also measuring the weight required to rotate the pinion.

Re-using a previously crushed spacer, it is suggested that the rotational torque on the pinion is 4-6lbs.

Re-read your old thread in the archives. "Diff Pinion Seal" Last post 28 October 2012.
Lawrence Slater

Hi Graeme,

Here is an article from the MG Experience Library that hopefully will be useful:

In answer to the question, "can you remove the ring gear without messing up the other settings?" is Yes, you can, because removing that assembly (remove the two bearing caps) does not change any of the shim settings.

I found that it was necessary to do this, and remove the ring gear from the diff housing (mark it before you remove the 6 bolts, so it goes back on in the same position) in order to access the little pin retaining the cross pin. The drift to push out that little pin needs to be pretty long too, like 3" (most pin drifts are too short for this, so I had to buy a longer one).

None of the MG sources sell that little pin, so I included the information about it in the above linked article.

In answer to your original question, "can you just tighten the collapsible spacer a little bit more if you reuse it?" is No, because its length then becomes too small and by the time you reach the 11 in-lbs targeted bearing drag, you will have over-compressed the bearings, leading to bearing wear.

I am trying to quickly imagine this in my mind, I hope I said that right and didn't get it backwards. In any case, the length of the spacer is as important as the pre-load in the bearings, for the system to work right when it is handling the full torque of the engine/drive train.

Norm Kerr

I DID say it backwards. When the collapsible spacer has been over compressed (like, by trying to re-use one), the result is a too loose bearing (you can still get an 11 in-lbs bearing drag by tightening the nut, but the spacer won't provide the right structural support to the system and when the full torque of the drive line goes through it, the bearing will slop around and wear.

There, I think that's the correct way to say it, the reason why you can't re-use the spacer.

In any case, this is not unique to MG differentials. It is the same thing with all RWD diffs (there are lots of "how to" links on the internet for rebuilding Mustang, Camaro, Corvette, and other RWD differentials, and only the details seem to be different - reading them can provide good cross reference information to help understand what is going on).

Norm Kerr

Hi Norm, I think you were right the first time.

In order to establish the pre-load on the pinion bearings, you have to crush the spacer (collapsable type). Until the spacer is reduced to the correct length, it holds the pinion bearings(tapers) too far away from the pinion races.

As the pinion nut is tightened, it forces the yoke against the crush sleeve(spacer), and pushes the taper bearings into the races. Not enough force from the pinion nut, and the crush sleeve holds the pinion bearings too far away from the pinion races, resulting in not enough pinion pre-load. Too much force will collapse the crush sleeve too far, and apply too much pressure to the pinion bearings against the pinion races, resulting in too much pinion pre-load.

If the pinion nut is loosened on a previously set diff, and then the same spacer re-used, and the nut then done up again to the exact position it was in prior to the nut being loosened, then surely the same pre-load it was previously set to, should result?

If prior to loosening the nut,( - due to slightly worn bearings(tapers) ), there is a little endfloat in the pinion, then tightening the pinion nut a little further will eliminate the endfloat by crushing the spacer a little more. Providing the 11lbs pre-load isn't exceed, the bearings should be ok. But 6lbs is recommened when re-using the same bearings I believe.

That's what we achieved with Graemes 4.2 diff last year, when he renewed his oil leaking oil seal.

Graeme had established that his pre-load was as little as 4lbs I seem to recall, and although there was no obvious endfloat, decided to get closer to the 6lb mark.

It's not very different to adjusting tapers in wheel bearings (cars other than MGBs and converted spridgets), except there is usually no spacer in between, as the nut doesn't have to reach 140lbs torque.

Lawrence Slater

I don't see the problem of just fitting a new collapsible spacer. That was what I did and it was very straightforward. At around 8 it isn't exactly expensive to do it right.
Guy Weller

It's not the cost of the spacer Guy, it's pulling the diff further apart than may be necessary.
I see that if I remove the crown wheel assy I can knock the pinion shaft through with various bits of bearing coming away at the same time. Often these things take a bigger hammer than expected and so shock loads on races increase. Just a little wary about doing it through concern about upsetting the complex spacer/clearance/backlash which is all part of the diff set up.
The manual suggests that if the pinion bearings are changed the diff needs to be set-up from square 1 again, so I am wary about disturbing it all.
G Williams (Graeme)

If you don't replace any of the gears, G, you won't have to start from square 1. MGB and Midgt differentials are both designed so that you can remove the diff cage, and remove the diff pin to pull out the sun & planet gears to replace all of the thrust washers in there (takes an old, sloppy diff and tightens it up to almost like new), without having to re-adjust any of the shims at all.

Over at the MGB side they have a "how to" guide with how to do this without even pulling the diff, but the Midget has no removable rear cover, and its diff requires the ring gear (crown wheel) remove first, in order to push out the little pin that holds the diff pin in place, so, for us, it does take a few more steps than it does for them.

I did it, and re-checked everything when I was done (bluing ink on the teeth and read the tea leaves) and found this was true (no need to re-shim).

If you want to go one further step, and replace the pinion seal, you DO have to replace the collapsable spacer, in order to get its bearing preload correct, though, but that's all.

Norm Kerr

Norm: apologies that this is old ground for many!

I was relating in my mind the arrangement for the bearings on the pinion to be similar to (oh my God!) front wheel bearings. In both cases there is a spacer separating the inner races in such a way that when the tensioning nut is tightened the inner races are pulled together. Because of the design of the bearings, this tightening action pulls the inner and outer races together and pre-loads the bearing.

In the case of the fwb, the spacer is provided in a precalculated length such that (assuming the bearings are made to a precise standard) the gap between the inner races is slightly less than the "relaxed" gap between the outer races when everything is tightened up. The bearing becomes preloaded. Because the spacer length is fixed the preload is not variable and won't change however much load is put on the spindle nut.

Mechanically, I imagine the diff pinion set up to be identical in all but two respects. The bearings are not face adjusted so a precise length for a spacer can't be derived; and to allow for this the spacer collapses in a controlled manner under load until the assembler achieves the appropriate preloading. The preload is entirely down to how much torque is put on the pinion nut. The collapsing spacer gives something to "fight against"

For what ever reason (wear? bedding in? poor initial assembly?) my unit was easy to turn - ie insufficient preload- even though the nut was correctly torqued up. I concluded that in this situation the spacer was still over-long. If I start to load it up again CAREFULLY it will start to collapse and the preload will increase.

Your post earlier suggests this isn't going to happen, but I can't understand why. I would appreciate if you could explain again because I can't see that at the moment but I acknowlege your significant expertise meaning you know what you are talking about!
G Williams (Graeme)

Hi G,

from my study of this, back when I rebuilt my differential, I think that the answer to your question goes like this:

the loads going through the differential are quite large, and the tapered bearings used for the pinion can deal with them, but like you said, they must have that spacer between them, and the total preload on the bearings must not result in more than what is about 11 in-lb of rotational torque, for maximum bearing life. Too loose and they'll slop around and wear. Too tight and they'll burn up.

it was time consuming in mass production to shim so the industry developed a crushable spacer which would compress the right amount, when a certain torque was applied, so that the bearing pack would be compressed correctly and not too much

Early diffs were shimmed, with a solid spacer. But today's diffs are all made this way, because it is quick, cheap and effective.

Re-using a crush spacer should work, in principle, but the big risk is that the difference between about 0 in-lb, and 25 or more is only a few degrees of nut rotation. Way too small of an amount to likely be able to hit a second time, once the spacer has given up its crush.

Since a new spacer is so cheap (less than $20), and once the differential is out to replace the thrust washers and pinion seal it is so easy to also replace the crush spacer, that trying to risk ruining an otherwise good diff doesn't seem to be much worth the risk.
One idea suggested was to add a shim so that the crush spacer gets crushed some more. This might work, but only if you monitor the tightness with an in-lb torque wrench that reads in the 5 ~ 20 range (takes a special, dial type wrench), because since you will be crushing the spacer more than it was originally designed to do, you'll not be able to simply use the 140 ft-lb reading from the big torque wrench when you turn the nut.

It requires that the nut be tightened a little bit, the preload checked with the really small torque wrench, and then you tighten the nut a little bit more and so on. Then, you find that you just went too far and have to buy another crush spacer and try again.

Norm Kerr

oops, I've got to correct something that you said, Graeme:

When you tighten the nut, either on a front wheel bearing or on a rear differential pinion, the torque of that nut should NOT be compressing the two bearings together, it is only compressing the inner races against the spacer.

When everything is the correct dimension in a front wheel hub, then the bearings on a front wheel have 0.002" ~ 0.004" of freeplay after the nut is torqued.
When everything is the correct dimension in the diff, the bearings on a diff pinion are compressed only enough to create 11 in-lbs of drag (they are tapered roller bearings, so they will always have some drag).
This result is created by the careful machining of the housing and the bearings themselves. In the case of the diff pinion, if you tried to re-use a crush spacer and you over compressed the bearings, THEN the nut would, in fact, be crushing the bearings into each other. Using a new crush spacer will prevent that from happening.

Your point that the pinion bearing design is like the front wheel design is completely true, though: the hub spaces the outer races apart (FWB), the diff housing spaces the outer races apart (diff), and in both cases, the spacer holds the inners apart, and when the nut is tightened, creates a very rigid structure for those bearings to live in.

Reference the attached sketch, which shows a front wheel bearing, but is also true for the diff (just replace the ball bearings for tapered roller bearings).

Part of what makes a topic like this so clumsy to talk about is that it should not be done so much with words, it should be done with sketches, but an internet forum makes that difficult to do!


Norm Kerr

Norm: thank you for taking the trouble to write that for me. I now believe we are not talking at cross purposes.

What I think it comes down to is that a new spacer gives the best chance of getting the pre-load correct, whereas an old one is dependent on its history - how much is it compressed to start with?

Hyperthetical: If I put in a new spacer and start to tighten the nut initially there will be no preload. Torque increases on the nut as we start to elastic deform the spacer. We then hit the point of platic flow and the spacer starts to collapse and preload increases. What if we back off the nut now? Would you strip it down and replace the spacer? At that point the conditions are no different to my spacer now. If I start torquing it up I have no preload until I start to elastic deform the spacer. If at that point the preload is too high I am stuffed. However, as I believe is true with mine, at that point there will be no preload (or insignificant). Very careful (emphasised!) increase in torque (monitored by checking the preload and the nut position) will recommence plastic flow until the appropriate level of preload is achieved.

I am not being mean about buying a new spacer - I am trying to avoid upsetting the hole diff assembly with a strip down, probably unecessarily so!

(As you say, words make it difficult. My interpetation of the spacer with fwb is the same as yours in the understanding of "squashing the inner races"!)

G Williams (Graeme)

One potential problem that I would see in re-using the old crushable spacer, but with a shim so that it can be further "re-crushed", is that it assumes that the collapse is linear. I am not sure that it is, or at least that it would continue to be so once crushed correctly the first time round.
There is another issue which might influence thins and that is how clean the thread is on the pinion nut as this will alter the torque reading relative to the collapsing spacer. Easier to get this cleaned and free running if you dismantle the pinion first, - in which case you may as well put in a new collapsible spacer.

Norm, do you know if the nut thread should be lubricated for the given torque reading? It could make quite a difference to the amount of crush?
Guy Weller

Isn't somethings been missed here?

In the front hubs, the spacer enables the strengthening of the spindle (nut done up to 45/50lbs), and the accurate placing of the inner races, relative to the outer races.

On the diff pinion shaft, the spacer sets the preload as Norm just said, but isn't there to strengthen the Pinion shaft. -- Is it?. Presumably for good reasons, the Pinion shaft nut is torqued to circa 140lbs. This is the torque on the nut required to crush the spacer, such that when crushed to the correct length, 11lbs is the resultant pre-load on the taper bearings. Try that without a spacer, and the taper bearings that the pinion shaft runs in, would be instantly ruined. There must have been a good reason for making the spacer so "strong", that it requires circa 140lbs torque on the nut to collapse it. I assume it's to allow the nut to be torqued to circa 140lbs, to prevent the nut unwinding/coming loose, not to give strength to the pinion shaft.

It seems to me, that the only downside to re-using a previously compressed spacer, is that you don't reach 140lbs rotational torque on the nut again, before you reach 11lbs pre-load, and the spacer collapses too far.

But is the nut likely to come off at say a torque of, 100lbs?. I don't think so.

These are taper bearings. Once the spacer( a new one) has been crushed to the correct length, everything is fine. Then the taper bearings begin to wear. If worn enough too much backlash ensues and the pinion can move in and out, at an extreme.

Tapers are adjustable. I can't see anything wrong with adjusting the tapers to take up that wear.

When graeme measured the pre-load on his other diff (4.2 ratio - prior to undoing the nut), the pre-load was less than 4lbs. So the bearings must have worn. Turning the nut a little further (on reassembly after replacing the oil seal), adjusted the tapers, resulted in circa 6lbs, and the diff runs fine.

This chap with a 1978 XJS Jag, did EXACTLY this to take up the wear in his bearings, and remove excess "backlash". See the full story here.

And here's one of his very good drawings.

Lawrence Slater

Hi Graeme,

In answer to your first question: "if you over-compress the spacer, should you replace it with a new one?", is yes. They are designed as a one time use only part.

The reason why is this: your 11 in-lbs of drag on the pinion bearings is just a measure of how much they are squeezed together, but when the full torque of the driveline is going through there, the spacer is also necessary to be the correct length in order for the inner races to be kept the correct distance apart.

To imagine this, think about your spacer instead being a solid spacer with shims. If you did not shim it correctly, the bearings would not last. That is the same as re-using a crush spacer, once it has been over-compressed.

Guy: interesting question about using oil in the threads during this set up procedure. I have never seen that mentioned in any of the differential guides that I read (like I said, pretty much all RWD differentials are built the same way, so pretty much all of their guides say the same things). I did an experiment for myself, where I applied a small amount of light weight oil to the (new) bearings and re-measured the drag. It didn't change the amount of drag, but it did smooth out some points in the 360deg rotation where the drag rose and fell a little bit. In the end, I used the 11 in-lbs bearing drag, rather than the 140 ft-lbs nut torque to set my diff with, because I found that with the spacers I was getting the 140 ft-lbs took me WAY above the 11 in-lbs drag target. I stopped when I reached 11, and the nut torque at that point was around 125 or 130. I suspect the spacers might not be made today with the same care / exact same plastic deformation properties as they were, back in the day of mass production with them.

Lawrence, I think that case, where worn bearings could be tightened up to get a few more miles out, by turning the nut a bit further, could work. Because the bearings are worn, they could benefit from being pushed closer together.
It would not work with new bearings, though. With new bearings, the crush spacer (which holds the inner races apart) must end up the correct height to match with the housing (which holds the outer races apart).

In my own experience, though, when I had tried doing that (I replaced the seal without pulling the diff, and just re-torqued the nut to 140 ft-lbs), the pinion bearings quickly wore and failed ("quickly" = about 10,000 ~ 15,000 miles, it gradually got louder over time, so it might have gone a little longer, but when I pulled the bearings they were clearly dull and pitted). Since my diff was quiet before that, and since the two differential bearings were still like new, I expect that my effort at "cutting corners" had failed me, and when I rebuilt it I chose to do it right (we'll see, in quite a few years, how well it turned out this time!).

Norm Kerr

Hi again,

Lawrence, I will read that attachment carefully tonight, but right away I wanted to post that the Jag sketch is a little misleading and could cause a confusion. The crush spacer the way it is shown in the sketch wouldn't work. I quickly tried to make it more clear with this new sketch.

The crush spacer only contacts the inner races of the two bearings.

The housing only contacts the outer races of the two bearings.

The pinion nut torques the inners against each other, through the crush spacer, against the end of the pinion.


Norm Kerr

Sorry Norm, don't want to labour this to the point where you say "Oh for crying out loud....!" I know Lawrence is of my thinking as we spent time together mulling over my other diff, so we may appear to be ganging up!

Just retorquing the nut to 140 is going to fail because there is nolonger a standard relationship between torque on the nut and spacer compression. Spacers come from a wide variety of sources and can't be relied upon to work to a standard. Just applying tightening torque is no guarentee that the preload is right. A weak spacer and you will have over-egged it!

My pinion nut was bl**dy tight but the preload was non-existent. For what ever reason the load must have backed off probably due to bedding in, wear or whatever, but not because of insufficient nut torque. At that moment the spacer can't be too short (otherwise the preload would be too high) or even "correct" otherwise the load would be correct. It must therefore be too long.

AS I start to tighten the nut I take up the slack, hit the end of the spacer and nut torque rises rapidly. I am not, at this point, squashing the spacer and applying preload to the bearing. Tightening torque is misleading because the correct torque depends on the structural strength of the spacer. So it needs to be monitored entirely by measuring the preload on the bearings.

I am trying hard to see a weakness in my logic but nothing screams at me so far. I'll give it a go and report back. Nothing loast because if the preload goes over the required value then I will have to replace the spacer. There is no going back!
G Williams (Graeme)

I take it that you have already undone the pinion flange nut ? although I cannot remember why. Perhaps to replace the oil seal as this does seem to fail whilst the dif itself may remain serviceable for much longer. Anyway, if the nut has been undone I still cannot see what the problem is in just fitting a new collapsible spacer. That is what I have done (twice, different diffs) and with no difficulties at all that I can remember.

I am not arguing any inherent fault in your logic, although following an "alternative" path from the manufacturer's manual must have some risk. I just don't understand the necessity to devise an alternative method.
Guy Weller

I'm just trying to avoid breaking down the diff any more than I have to. An illogical fear of lost clearances. I'll get over it. I used to be frightenned of Front Wheel Bearings! One fell on me as a child!
G Williams (Graeme)

LOL ! I think you would know about it if a Dif fell on you!!
Guy Weller

Hi Norm, I too will have another look at the Jag chaps drawings, but my feeling is that they are right. I checked them against my stripped diff late last year. It's still in pieces. Crown wheel out, and both sets of bearings removed from the pinion (bearings and races).

The trouble with using a new spacer, if all you are renewing is the oil seal, is that you can no longer use the previously scored marks on the flange and nut. As graeme described, the spacers are likely to be inconsistent, in terms of the exact poundage needed to make them deform to the precise amount required.

So assuming that the diff was otherwise functioning well, if only the oil seal is being replaced, it's actually far easier to scribe the flange and nut, re-use the "old" spacer, and do the nut back up to exactly the same marks.

Not only that, it is EXACTLY what the BMC workshop manual tells you to do, on page 152. It doesn't say use a new spacer, but does say torque to 140lbs. However, I reckon that you won't reach 140lbs before reaching the scribed marks. And that's what we, -- Graeme found last year. I think the 140lbs is a guide, to tell you that the nit is good and tight. For example, if you reached the scribed marks at 30lbs, you have concerns about the nut coming loose.

Lawrence Slater

Actually, I correct myself before anyone else does.

I read that wrong. It's the Propshaft the BMC refers to, not the pinion nut and flange.

However, they do not suggest a new spacer, but do suggest 140lbs, which does suggest a new spacer.

I still think though, that scribing the pinion and flange reaches achieves the same thing, re-using the existing spacer.
Lawrence Slater

We should have read the thread from last year - AND the MGB manual on the diff section, as it gives a clearer picture when replacing the oil seal. Paul (New Zealand) gave us all the answers. I kind of thought all of this was deja vu.

What follows are Pauls posts from the 2012 archived thread. Title : "Diff Pinion Seal"

Posted 24 October 2012 at 21:32:52 UK time
Paul Walbran, New Zealand
Lawrence - Yes, it needs the torque for tightening purposes too. You may have seen in my comments above that after a decade of autotesting behind the K engine we had a collapsible spacer fail, and that led to the nut starting to unwind itself.

The 140 lb-ft derives from the earlier solid spacer version, and is the same figure on the B series.

As per the discussion above, the figure when using a collabsible spacer is "about" 140 lb-ft ... the key thing is to have the right pre-load. Too much pre-load and you'll get failure as per Norm's experience (Norm: was that just the bearings that failed, or the whole lot?)
Too little and you'll end up with unwanted deflection between crown wheel and pinion.

If you get the pre-load below 140 lb-ft, then don't fret about it coming loose unless it's under the 100 lb-ft mark. If less than 100, I would renew the spacer. A bit more work, but ultimately less and cheaper than renewing the bearings and possibly the CW&P as well if the nut undoes itself.

Though having said that, due to the usual time constraints of an imminent event, when we found the K's little problem we ignored the bruised-looking wear pattern on the crown wheel & pinion (none in stock) and just fitted a new spacer with the gears set up to give the best contact pattern we could -promising to do it properly later. It turned out that (amazingly) it ran almost quietly - just a faint murmur. A year later it is still as quiet and this discussion has reminded me we have yet to do that!

Posted 24 October 2012 at 11:59:17 UK time
Paul Walbran, New Zealand
Sorry, I am out of town with intermittent internet access. (But though the access is poor, the back country roads are great!! And free of other traffic ... )

The difference in pre-load figures:

The 12-ish lb-in figure applies when setting a diff up with new bearings etc. It is measure with only the pinion installed - no crown wheel or cage in place. Once the set-up is installed and gets a bit of mileage on, the pre-load gradually reduces. Hence:

The 4-6lb-in figure is what the (MGB) manual lists as a MINIMUM figure if replacing the pinion seal without stripping the diff and replacing bearings etc. From the manual method, this figure includes the seal and the diff cage, as it is referring to an assembled diff in the car.
What the manual actually says is to tighten to the existing preload (that's the mark-the-nut method) but if that pre-load is less than 4-6 in-pb then tighten it till that figure (4-6) is achieved.

On tightening the nut:

The feeling I have (though I hyave never taken measurements to investigate this) is the the highest load to collaps the spacer is that needed to collaps it initially. It feels that way, and it also seems logical as the straighter the side-walls the greater the resistance to buckling, more-or-less.

However ...

The spacer starts to buckle before the bearings are in contact. Once the bearing clearnce is taken up and some pre-load sets in, this will account for some of the effort going into the torque wrench. In an extreme case, if there is no spacer at all you can still tighten the nut to 140 lb-ft eventually. Poor bearings. (Don't ask how I know this bit! I plead old age and memory loss, I'm sure I put that bloody spacer in ... )

So yes, the spacer collapses before 140 lb-ft, but once there is also a bit of resistance from the bearings nipping together then the torque needed to further compress the assembly gets larger. usually, but not always, 140 lb-ft and the requisite torque figure on new bearings come up about together. But not always, as Norm pointed out, so I always do what he said - a bit at a time until the pre-load figure comes right. So yes, you could do that bit without a torque wrench.

On occasion, when in a hurry (imminent event) and being out of stock of the spacer and not having time for another to wing its way halfway round the world, I have recycled the spacer with a thick-ish shim (20 thou-ish) between it and the bearing. While this has proved perfectly OK in practice, my preference would be to fit another spacer if I possibly could. But it's a handy thing to know if you are desparate. (But not to save a few pence!!)

Lawrence - that last question is a good one, I've never been brave enough to find out as it's so much effort to do it all again I experiment only when I have no other option!

Posted 21 October 2012 at 10:27:37 UK time
Paul Walbran, New Zealand
That could work. 100 ft-lb is usually safe enough to not collapse the spacer further whereas 140ft-lb usually will.

As far as I know at least all 1275's had the collabsible spacer.

It is worth checking the pre-load though. We had an issue last year with our K diff that took some while to track down as the symptoms were propshaft out of balance. It was diabolical to the point of scary. Three propshafts erbuilds and re-balances later and no improvement we were still scratching our heads when Arie gave us a tip-off that he'd heard a simliar thing that turned out to be the diff.

And yes, it was pinion bearing preload gone. It appears what happens is the spacer collapsed further under extreme load (in this case a decade of autotesting behind a K engine) which means that the flange nut has nothing firm to grip against any more. So inevitably it loosens off, which if bad enough gives that gross out of balance feel.

We chucked out the collapsible spacer and set it up with a solid one and shims to give better control in this respect. More work but problem solved.

I have come across other hard working diffs where the pinion had loosened off, but the symptoms had always been a sudden development of diff whine rather than no whine and severe vibration.

Posted 21 October 2012 at 02:28:37 UK time
Paul Walbran, New Zealand

The Midget manual instructions are the same as those for the B which had the solid spacer and shims. I have always assumed that this is one aspect they overlooked updating when the collabsible spacer was introduced. Certainly the B manual is referring to an identical set-up to the Midget when it issues the caution note, so it pays to take notice of it.

Following the procedure for the solid spacer - which is that outlined in the Midget manual - is likely to result in further crushing of the collapsible spacer and excessive pre-load on the bearings and thence premature bearing failure as per Norm's experience above.

Regarding the pinion turning freely, it doesn't (or rather shouldn't) The preloading of the bearings results in drag on them when you turn the pinion flange. This is the 4-6 lb-in torque referred to. I have fashined my flange retaining bar so that its weight provides the required torque. However another way of measuring it without the official tools is to wrap a piece of string round the flange and pull on it with a spring balance ... at a radius of about 1.5" 4-6 lb-in translates to 3-4lb pull on the string to get the pinion to move.

The prupose of the pre-load is simple: it better braces the pinion in place against the side thrust from its contact with the crown wheel. With no (or inadequate) preload the pinion would flop about a bit and result in wear and noise.

The MGB manual procedure is quite simple: Measure the torque needed to move the pinion (string and spring balance) before undoing anything, then when replacing do tighten a little at a time until the original trque is reached, or bit more if it was less than 4-6 in-lb.

However, the mark-the-pinion methods others have outline is a simple means of restoring it to its setting before the seal was replaced. What it missed compared with the B manual procedure is checking whether the required minimum preload has been reached. It's worth doing while you are there as over time the preload can reduce due to gradual wear in the bearings.

Posted 20 October 2012 at 20:52:26 UK time
Paul Walbran, New Zealand
It could do - the preload on the pinion is via a collapsible spacer between the two bearings. However, its coverage of changing the oil seal is more in line with that for early A series diffs which had a solid spacer and shims as it doen't comment on the importance of not collapsing the spacer any further and how to avoid that.

So to paraphrase the MGB manual on this:

1. Remove brake drums (to eliminate drag for step 2)
2. Record the torque required to rotate the pinion
3. Remove and refit seal much as you would expect to do
4. Tighten the nut gradually until resistance is felt
5. Rotate the pinion a few times to settle the bearings
6. Measure the torque needed to rotate the pinion
7. Tighten a little more if required
8. Repeat 5,6,7 until a torque reading is achieved which is the GREATER of
(i) what you measured in step 2 OR
(ii)4-6 lb-in

The manual further notes in bold type:

CAUTION: Preload build-up is rapid, tighten the nut with extreme care. If you exceed the target figure the diff must be dismantled and a new spacer fitted.

(But this is the same MGB manual which DOES list 135-140 lb-ft for the banjo wheel bearing nut!)

To stop the flange from turning I made s simple tool from a length of flatbar, with 2 holes to match 2 in the flange. It is necessary to nibble one side between the holes to get clearance for socket access to the pinion nut.

Not sure of the seal size off-hand sorry. I presume the local bearing supplier is very handy?

Lawrence Slater

Thanks for copying that lot Lawrence. I guess that on at least one occasion l need not have used a new spacer then. One time l was fitting new bearings so l think it was essential then. Bit the other one l did was just to replace the oil seal and those copper olive spacers, so l could have used your mark and retighten method after all.

Reading Paul's informative messages it does sound like there is confusion even in the workshop manuals. I also noted the following from Paul:

"On occasion, when in a hurry (imminent event) and being out of stock of the spacer and not having time for another to wing its way halfway round the world, I have recycled the spacer with a thick-ish shim (20 thou-ish) between it and the bearing. While this has proved perfectly OK in practice, my preference would be to fit another spacer if I possibly could. But it's a handy thing to know if you are desparate. (But not to save a few pence!!)"
Guy Weller

Yay! It looks like we are all in agreement now.

I agree with Paul's method, when trying to just replace the seal, on the car: set the pinion nut not by what its original position was, and not by 140ft lbs, but by returning the nut to the same bearing preload as it had when you started (remove the drum brakes first, measure the drag with a suitable system, then remove and replace nut and seal, and tighten the nut to the original bearing preload amount).

His "pre-load" method ought to minimize the risks that the other two methods have, of not getting that one, key thing right, leading to early pinion bearing failure.


PS: Paul, in answer to your question, yes, the only thing that failed on my diff was the two pinion bearings, everything else was still in good shape. The whine noise which was gradually increasing must have been coming from the ring and pinion getting gradually out of alignment as the pinion bearings wore. If I had let it go too long, I expect the gear teeth would have begun to wear.
Norm Kerr

Here's the FULL MGB description. Note the last in bold type. It clearly says a new crush spacer is not needed.

However, if an original torque figure (pre-load) in excess of 6lbs is exceeded, then in that case, a new crush spacer, MUST be used.

And so harmony and tranqulity descend on the combatants for another night(day if your in the 'wrong' time zone. ). :)

Lawrence Slater

THAT'S IT! Someone posted that before. That's the source of the New Philosophy Movement in diff maintenance. Might still be cr*p though!

Tomorrow I shall mainly be tightening my pinion nut.
G Williams (Graeme)

This thread was discussed between 04/04/2013 and 07/04/2013

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