JH Transmission Restoration

Hello all, new to the forums and Jaguar stuff but not new to automobiles or airplanes. I just recently picked up an early XK120 transmission for, (what I thought), was a pretty good price. It is in pieces but is complete and will need a few parts that I’ll be on the lookout for. Serial number stamp is JH2854.

The gentleman that I bought it from was very knowledgeable about it and gave some insight but if anyone has anymore info on it I’d love to learn more; I plan on doing this restoration myself and would like the finished product to be a complete, aesthetically pleasing, and working example. I’m not sure if I’ll keep it for a shop piece or sell it, I haven’t decided. I have found the gasket set, crush washers, shift lever, shift knob, bearings, seals, pivot rebuild kid, etc. for it. I am trying to source a dipstick for it as this one is broken.

Has anyone done a concours or close to concours cleaning and restoration of theirs? I’m curious what people are using to get the original aluminum finish for the top plate? I’ve heard Purple Power works pretty good but can leave a residue and I’d like to avoid stripper if possible.

Here’s the top plate just resting on the housing, all of the internals are in two other boxes.

And here’s a photo of the top plate serial number stamp

Welcome Josh.
The JH trans was used in '49-52 XK120, '48 Mark IV, '49-51 Mark V and '51-52 Mark VII saloons.
First, you must remove that red paint. Gloss black is the only color.
One possible aluminum cleaner is Star Brite.
Look for wear in the counter shaft and the needle bearings.
As long as it is apart, you might want to do a modification that the factory did in 1954 to all these gearboxes, add a stop pin in the second synchro sleeve, as described here.

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Thanks for that information Rob! The needle bearings are toast on it so those will have to be replaced as well. I appreciate the comment on color too, I liked the red but want it to be as period as possible so black it is.

Is the drain bolt typically brass on these? Mines in good condition so I’d like to use it if its factory spec.

I’ve started reading through that modification to the gearbox and I think that’s something I’ll do with this one when the time comes, just makes sense.

That is an amazingly well-written description, Rob! You are truly a golden asset to this website.

Yes, always brass. I don’t remember the thread pitch but it is British and straight with a sealing washer, not tapered. All other screw threads on the JH are British Standard Fine, not UNF nor metric.

I’ve read that the owners manual has some pretty detailed information in regards to the transmission, would this be the one I’m looking for?


Also, would mine be a synchro transmission or non-synchro?

No, that’s the owner’s handbook, tells you how to adjust the seat, where to put in the gas.
You want the XK120/MarkVII Service Manual.
I read back and realize you don’t have a car, just a trans.
You can download it here.
You just need the gearbox section, and not even all of that, so I would just print the pages relating to assembly.

Non-synchro 1st gear…iffy synchros for rest of gears!

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Ah, yes, that reminds me, on the second gear wheel synchro cone, you may find the longitudinal grooves are full of debris worn off from the internal cone on the second synchro sleeve from decades of speed shifting, so clean out those grooves. They will work fine when they are clean like new.