Gearbox oil leaks on 1948 MK IV

My 1 1/2 litre gearbox was working OK and with no oil leaks, when I then drained the oil ready for a full service on the car. I forgot about it (senior moment) for several weeks and only just today filled it back up. It has now got 2 small oil leaks from around bolts at the bottom of the Extension Box housing - one is a flange bolt and the other the bolt (item 2 on the parts drawing: Part No 37444) holding the Locking Plate for the Counter and Reverse Shafts.

It does not seem to be leaking from the main gasket between the flanges of the Gearbox housing and Extension Box, but rather up and out of the treads behind the heads of the 2 offending bolts.

Question: Should there be a separate seal/gasket for the Locking Plate (nothing is shown on the Parts List, and I can’t physically see anything there), and/or is it viable to torque up these two offending bolts (or indeed all flange bolts) to see if that stops the leaks? Alternatively I could re-drain the gearbox, remove these two bolts, put some gasket seal around the threads and pop them back in. Anyone any experience of solving this type of problem (rather than taking the Extension Box apart fully to replace the gasket - which could be done I believe with the gearbox itself still left in place)?

Has anyone sourced a supplier for these 1 1/2 litre gearbox gaskets (Parts: 39773 front; 37555 Extension box; and 36542 for the rear end cover).

Coincidentally, I just did exactly that yesterday.
No, there is no seal on the locking plate nor the locking screw nor the smaller of the two shafts which is reverse.
You can try tightening the screws, since you already have oil in it.
If that doesn’t stop the leak, then drain the oil. Wait a couple of days to be sure it is pretty much all out. Remove the offending screws. Clean off the oil with paper towel or something absorbent. You want it clean and dry to get sealant to stick. Use a Q tip to smear sealant around the shafts and under the lockwashers. You don’t need it to go down the hole, just around the washer, although a little down the hole is ok too.

Gaskets for the JH SH boxes are sold by XK120 parts vendors, which would have worked for my 2.5 but I don’t know about 1.5, but I made my own with brown wrapping paper, pencil and Exacto knife.

But I would not try to do the #4 main tailcase gasket while in the car.

Having got the car onto my workshop lift and getting a better look, I can see that oil is seeping down to the 2 nuts via the locking plate, but actually coming from the externally protruding shafts of the countershaft and reverse spindle (parts 27 and 28).

How are these shafts sealed? I can slightly rotate one shaft end (the one towards the top of the photo and slightly smaller shaft diameter I think) by a very small amount, so maybe the locking plate is not fully engaged properly.

Do I see Rob on your photo that you have sealant on these shaft ends?

Yes, I did put sealant around both shafts. The larger one has a felt seal inside the tail case, which can’t be changed from the outside, and in your case is probably gunked up with old oil and dirt and not doing much good anymore. The smaller shaft has no seal at all inside.

The locking plate does not capture the shafts tightly, it just prevents them turning and working themselves out. The locking screw hole goes all the way in so is another potential leak.

I would drain the oil, clean the area, smear sealant all over, and tighten the screws. In theory not much oil will seep past thread in screws if they are tight.

The small leak is indeed from around the smaller diameter shaft end, perhaps not surprising then if there is no seal. It drips a couple of drops a day - not ideal but then perhaps not worth loosing sleep over!! I will try draining the oil, cleaning/drying it, and putting on some sealant around to see if that helps - if not it will go on my “to do list” for when I next have the gearbox out.

I hope you are not seeking to disengage the Jaguar chassis corrosion protection scheme!

Peter :wink:

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Clearly this then is a common problem with the gearbox to “live with”. Either a Jaguar design oversight perhaps or indeed as you say an added corrosion protection device bonus give away with every car. I like a nice clean garage floor - so it looks like an early Christmas present of a drip tray.

Doubt that, given the universal tendency of older cars to leak.

Till the Japanese showed us all how to make leakfree engines and gearboxes, no one much worried about the odd drip or ten!

And they would not consider rising oil prices!!

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