Transmission seal that leaks after my 1948 Mark IV sits for a long time

Any suggestions short of replacing the seal - I’ll probably be getting there. The seal is fine if I drive the car regularly. But once it sits for awhile it leaks. Any additives that would freshen up the seal?

Do you mean the rear output shaft seal? Is it the original leather or a replacement rubber seal?
Mark V gearbox 011

Magic trans sealer elixirs may or may not work on a very small leak, but I never had any luck with them. But it won’t make it any worse.
This seal can be replaced without pulling the trans, you just have to get to that big flange and remove it, then there are seal puller tools of various designs. I have one that looks like a battle axe for an elf. Tap the new seal in gently, either using a big socket as a driver, or going around with a small hammer.

Thanks for your help. But it is the front seal.
Was hoping there would be a seal rejuvenator additive that someone has had some success with.


Sometimes tranny front seals leak when the fluid level is too high. I had a Willys jeep which leaked when filled to specified volume, this worried me for about six months. Once I let it drop to non-leaking level, no worries ever after in ten years of ownership.

Well, then it has to come out. But the good news is it’s an easy fix after that; you don’t have to take the whole trans apart, and there is a modern rubber lip seal substitute.
Mark V gearbox 013

Hi Rob,
I’m just about to embark on my first bolt safety wiring job, (XK Camshafts) and now look at any photo of safety wiring with interest…
Is the bolt at one-o’clock correct, as it looks like it could turn anti-clockwise and the wire would loosen…

The bolts are all tight. The wire is not intended to hold them tight, and yes I could have threaded the wire through so as to pull them all in the tightening direction, but I don’t think it’s all that critical in this application. It prevents them from loosening more than a quarter turn anyway, though I don’t know why they would.

Thanks for the answer; that makes sense.


Sorry, not really. Except perhaps make sure you use oil in the gearbox that is not too thin. I know some say that modern synthetic oils are superior but IME the mineral oils (in this case I would and do use SAE30 Engine oil, Castrol mineral oil in the MKV gearbox) are the only ones that make the organic seals (leather, paper) swell a bit.

But yes, you need to use it more often, the seals will leak more once they get dry.

I have a friend with a 1986 XJ6 and BW66, it is fine if driven at least 1-2 times per week, but if you allow it to sit for several weeks, it will leak at least 1/2 liter or more of the ATF. YMMV.


I agree with Jon’s comment in that it is standard engineering practice to wind the wire in the direction that prevents excessive loosening of the screw. So I endeavour to do this every time. I have just helped a colleague, who has limited experience, to do his box and explained this principle.

I also agree with Rob in that the wire does not prevent loosening, and it is hard to imagine how they would come loose after correct tightening.

The purpose for wire locking is to prevent a loose bolt falling into a place where it could do damage, or to prevent a screw, that should not be fully tightened, from unscrewing from its adjustment position. These two situations exist in these boxes - the seal retainer shown, the selector forks, and the selector rod detent grub screws.

I wonder if there is much, or any, wire locking in today’s vehicles. I doubt it because of the labour time required compared to just lock washers or thread locking fluid.

I noted too, that the 1.5 box I just finished has tab washers under the bolts connecting the bell housing to the box casing. The washers have a tail that bends down into a shallow blind hole beside the screw. I mention this in case you don’t have these washers or don’t know the reason for the little hole. The previous owner of the donor box omitted these. They are another belt and braces security device to prevent a loose bolt from dropping into the flywheel space and quickly punching through the bell housing.


Just a few comments about what appears to be a gradual leak from the bell housing while at rest.

Check whether it is gearbox oil or engine oil - they are the same oil and the the rear crankshaft seal can leak and drip out of the same bell housing opening. You can distinguish between the two by smell, provided they have been in service for a while.

A consistent leak at rest from the front gearbox seal cannot occur because the static oil level is well below this site, only being to the level of the layshaft. If the front seal is leaking, what is showing on the floor is the residual in the bell housing after a drive. Also, if it is leaking, it will be doing so whilst driving but you cannot witness that. The MG fraternity is frustrated by a traditional leak from the rear crank seal of the T series engines and often rig up a discrete tray attached to the housing bolts. It allows them to pass inspection but needs emptying from time to time. Maybe something similar could be done to test for the extent of the leak and to confirm its source.