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MG Midget and Sprite Technical - diff leaking oil?

Hi there

I can see at the bottom of the car oil near where the incoming axle (prop shaft) enters the diff. I gues this indicates the diff is leaking (throws oil around).

What to do?
a.o. arnold

they all leak a little bit, so the key question is whether it is leaking so much that it is difficult to keep it properly filled

my advice is to monitor the oil level for a season and then decide if it is a big leak or not (and clean off the area and watch for how quickly it becomes oily again)

one thing you can do right away is check to make sure that the breather is not clogged (it is a "mushroom" shaped thing on the top of the RH side of the axle housing - the little cap pops off and then the stem is unscrewed by sliding a small screw driver through the hole - it is plastic so be gentle).

to replace the pinion seal can be done, but if the torque is improperly done it can lead to differential pinion bearing failure, so it is best done on a workbench, rather than done-in-place

here is a link to an article that discusses rear axle repairs like this:

I hope that it is helpful,
Norm Kerr

Thx Norm, sure helpful. I can keep the oil in and will check the breather.

As my car is supercharged the diff has tough job. It appears to me it has (too) much slack, some times the car is 'bucking' in the diff it seems... any ideas?
a.o. arnold

Not sure that is correct Norm, in fact I would suggest the reverse is true and very few leak.

However as you say if they do leak then check the breather first and then I would simply replace the front (nose) oil seal. Remove big nut etc etc. :)

They all have a lot of slack in them I would suggest? so it is difficult to evaluate the condition by words alone but what do you mean by "bucking" Arnold? If I understand that correctly I would be more than convinced you mean "tramping"? This is caused by the rear springs twisting and winding up. Not uncommon and an easy fix by fitting anti tramp bars.
Robert (Bob) Midget Turbo

sorry, I am dutch.... the thing is like go -no go, go, no go., especially at the lower rpm's / cruising at 'idle'
a.o. arnold

is this just not the engine running smoothly? suggesting a very weak mixture?
Robert (Bob) Midget Turbo


I must dissagree... they DON'T all leak and if yours does then it's time to replace the pinion seal. They go hard with age and will never repair themselves.

Replace the seal and 'polish' the seal surface on the pinion.

A A Arnold, I think a better description of this bucking may help us..... Your problem could be in many places...

Brakes binding or contaminated... is your diff overfull with oil...?

Finally, is your breather clear?

M T Boldry

Bob and Mark,
you are right, I really overstated what I had meant to say!

Instead, I should have said, "some oil film in this area is not unusual on older differentials, and unless it is enough to register that the seal itself is cracked /failed completely, it can be lived with for some time as long as you keep an eye on the oil level in the diff."


Replacing the seal while the diff is still in place, and then getting the pinion bearing preload wrong, can lead to more harm than leaving a slow leak in place until ready to remove the diff and replace it on a bench so that the important pinion bearings can be confirmed to be preloaded correctly.

that is not to say that it can not be done in place, just that the tricky bits must be watched in order to have a chance of it coming out right (and if it is not right, it may take many miles to show up).


Norm Kerr

Hi guys

Well, my english is limited, I would call it 'bucking'. Only while cruising at idle / low rpm's and it happens especially by slowing down. When accelerating a bit it is gone. My earlier did not this, I am pretty sure it is caused by the diff.

Also: oil is in it, as mentioed I can see oil at the bottem of the car, so it appears as been thrown out at higher speeds. The diff does not leak when the car is not moving.

a.o. arnold

Let's see if there is free play somewhere in your drive line:

Please check how much freeplay you can feel, like this,

Hold the driveshaft with one hand, to make sure it does not turn, and rotate the differential input flange with the other hand. There should be 0 freeplay. If there is, then the u joint is worn. (replace the U joints)

Next, rotate the pinion flange back and forth while the parking brake is on so that the wheels can not turn. There should be no more than about 6mm of total rotational play. More than this indicates that either your differential is worn, or the half shafts are worn where they spline into the differential, or both. (replace the differential with a better one, or rebuild the one you've got, and confirm that the half shaft splines are not worn where they insert into the diff itself).

The third location of possible freeplay is between wire wheels and their axle hubs. If you have wire wheels, loosen the nut while the parking brake is set and confirm that you have no more than about 3mm of radial play at the wheel rim. More than this indicates that the wire wheel splines are worn. (replace the wheels and the half shaft hubs).

One other possible freeplay source is the transmission. This is probably accompanied by increasing gear noise over time.

Hopefully this is helpful,

Norm Kerr

Regarding the wire wheel check:

I said "radial" play, but I should have said, "rotational"!

Norm "all thumbs" Kerr

Norm Kerr

I checked the rotational play while the car lifted and on handbrake. It was about 1 cm...

I checked the breather: clean
a.o. arnold

Where is the play? Between prop shaft and the input flange on the diff, or inside the diff?

Hi Andrew

As far as I can see in the diff or in the gearbox
a.o. arnold

I was aware that that my diff was leaking slightly, but I wasn't sure by how much until last weekend at Peter Burgess's workshop. The car was on the rolling road for about half an hour during which time it was revved to 5,500 rpm four or five times. When it came off I noticed an approximately 2 inch pool of oil on the steel deck of the RR where the diff had been positioned. From that I guess my front seal needs changing.(I will check the breather first).
There seems to be a difference of opinion on here as to how easy the job is. I've never done one on a Spridget, but I did do it on MK1 Ford Transit and I found it quite difficult to get the preload right, so I'm torn between doing it or to just keep topping up the oil.
Any ideas and advice gratefully accepted.

b higginson

a.o. The "bucking" you mention could be caused by having a too weak carb return spring and when travelling over uneven roads your foot is being made to move about slightly by the movement of the car over undulations in the road causing the thottle to also move. It may not be this, but it's worth checking.

b higginson

Hi Bernie,

changing the front seal is easy to do, but like you said, getting the preload right is not possible to be sure about unless you do it on a workbench and measure it.

Theoretically, if you mark the nut location, and count the number of turns, and you re-install the input flange on the same pinion tooth (so your nut location mark will be any use), and then re-tighten the nut to just the same as it came from, then you ought to be pretty close.

But, what I found on the workbench, with a 6 ~ 15 in-lb torque wrench checking the pinion preload, the difference between zero, and way over the 12 in-lb target is only a few degrees of nut rotation. The tolerance in the manual is only +/- 1 in-lb!

Granted, that tolerance is meant for production parts that are expected to last for the life of the car, rather than an enthusiast's car that might only be driven a few 1000 miles each year, and who might tolerate rather more differential noise than an OEM customer would have, so you might be fine.

Let's say, it has been done (replacing the seal in place), and some have had no problems from doing it. In my case, I tried it and toasted my pinion bearings in about 20k miles (the noise began much earlier, but it took a while to decide that was what it was). My experience is surely worst case, as I had torqued the nut to 140 ft-lbs because I did not think of the "mark the nut" idea until later!

Norm "noisy rear end" Kerr
Norm Kerr

Arnold, the behaviour mentioned sounds like what I have experienced with a very worn uni joint in a tailshaft (not in the Midget, but regardless), but I guess maybe it could also be caused by excessive backlash (play) in the diff gears? I don't know, you would have to go off others experience. I would have thought the diff would be VERY noisy though if the gears had that much play. Determine where the play is and you will find your problem.

The problem I had was 'clonking' when getting on and off the accelerator as the play due to the worn uni joints took up. It caused what could be described as a 'bucking' through the car as you say.

Thanks Norm. I'll keep topping it up until winter, then I'll maybe take it out and do it on the bench. I'd better go and check it right now as we hsve a run tomorrow (Saturday).

b higginson

This thread was discussed between 10/06/2011 and 17/06/2011

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