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MG MGA - Diff oil leak?

I have been driving around for a while without the battery cover in place, and noticed yesterday that the backs of both seats have a very fine film of oil on them. The only place nearby that appears to be losing oil is the front end of the diff housing, just behind the rear propshaft u-joint. It doesn't drip when the car is standing after a decent run, but I can't see any other 'culprit'.

Is there any other possible source that anyone can think of? And, if I'm right, is it easy enough to replace the oil seal and dust cover on the diff?

Many thanks
Robert Sinclair

Replacing the front oil seal is straightforward. To unscrew the nut all you need is a 1.125" socket and a long bar with a 5/16" hole in the end to screw to the flange. This will give you sufficient leverage as the torque to unscrew the nut is high at 140ftlbs. I unscrewed mine a couple of days ago!...................Mike
m.j. moore

I made a flange holding rod to hold it steady while the nut was tightened. I found it quite a trick getting the new seal in place with the diff in situ. Successful, yes, but took a number of attempts lying out under the car.


Steve Gyles


I also had to replace the front differential seal due to a small leak. As is so often the case a little problem lead to the discovery of bigger issues. So here are a few suggestions to consider before tearing into the job. The pinion bearing preload should be checked as part of the work. In my case the pinion nut was not property torqued and the pinion shims were trashed. Unless you have some means of holding the differential flange as (Mike suggests or ), you will need to set the car back on its wheels to tighten the pinion nut. If the shims need adjustment or replacement you will have to repeatedly jack up the car to retest the pinion bearing preload.

First check to be sure that the diff breather is clear and working. It is on top of the axle, fuel pump side. Also be sure that the differential was not overfilled. The oil level should be about below the filler hole, . Check your u-joints.

Before loosening the pinion nut, check the pinion bearing preload to get a feel for before and after measurements. The road wheels should be removed and the brake adjusters backed off before measurement. If new shims are needed they are fitted by trial and error which involves a lot of up and down work.

Be safe, I found that getting the pinion nut to 140 lbs not an easy task without some means to hold the differential flange. Relying on the road wheels and parking brake was barely sufficient to get to the proper torque. I did all of the final torque work from above with all four wheels on the ground. Hopefully you will not have to mess with the pinion preload.

Happy New Year,



With the battery cover removed the air flow onto the seat backs can come from anywhere under the car. I would suspect it is also likely coming from the front. Any oil dripping from the engine/gearbox could well be to blame.
N McGurk

Yes, I agree with Neil.
I would certainly place newspaper under the car after a drive and then look for a drip.

My first MGA (1972) had a steady stream of oil right along the chassis, covering everything, and ensuring a rust free car, but it was difficult to pinpoint the leak.
The newsprint soon showed multiple leak sites!
Good luck, Barry
Barry Gannon

The air pressure under the car is certainly greater than in the cockpit. On one occasion I removed the tunnel carpeting and the gearbox filler bung. When under way, the air blasted out of the hole and into the cockpit. It was a contaminated oily smelling blast. Not all that pleasant. The bonus was that the engine temperature dropped about 5 degrees F. I would agree with Neil that the oily deposit is from the same source as he described. Probably all over the hood as well. You may have noticed your engine also running a little cooler with the battery cover removed.

Steve Gyles

Thanks for the comments.

It never occurred to me that it might be coming from the engine area, but that might explain why the oily film is quite fine (certainly finer than the EP90 in the axle). I know I've got a couple of leaks up front, so I think I'll tackle those first and see what happens.

Robert Sinclair

This thread was discussed between 31/12/2011 and 01/01/2012

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