Thermo vs Standard Jet Bearing Kit for rebuild on XK120

My 1953 XK120 with H6 SUs needs a rebuild of the jets. lists two different jet bearing rebuild kits- one for H-type Thermo Carbs and the other for H-type Carbs. Looking at the photos of the kits, the Thermo Carb kit looks to be consistent with what is currently on my car. Can anyone confirm that XK120s generally had the Thermo H6 Carb? They also offer a “Superdry” Jet bearing seal kit option- any experience with this?


The XK 120 definitely had the “Thermo” H6 carburetors ex factory. The jets of the two versions you refer to are different, so use the needles as prescribed for the Thermo version.

Bob K.

Definitely Thermo- was able clean up rear carb jet assembly. The adjuster brass nut/cap was stuck tight to the brass housing- will replace along with new O-rings and washers.

Apparently the non-Thermo would be the manual choke version, such as was used on H8 carbs and MG, TR, AH et al.
The Superdry seals are intended to combat the problem of ethanol degradation on the previous version with cork seals.

Decided to completely replace the jet assemblies and needles. Really happy with the SU parts from the Burlen Group:

Here’s the video of the engine running:

Resurrecting this older thread - is that kit in the photo the Superdry kit, or the standard item?
Each time I take the jet cover nut off the bottom of my 140’s H6 SUs, there is petrol in them - not a lot but enough to drip out and take the paint off my (RHD) steering column. The carbs were fully rebuilt by David Lonsdale before fitting, who I think used the original style of rebuild kit.
So - just wondering if these minor leaks can be stopped with the Superdry parts, really.


I had the same problem for a long time. I then replaced the fibre End-cap washer SU AUC 3159 by a rubber O-ring of the same diameter and all my troubles disappeared immediately. Even tightening the End-cap only finger-tight (no spanner) seems enough to stop the leak.
Give it a try…

Bob K.

Yes this was the Superdry kit- it is better than the original, but I wouldn’t call it “super dry”.
Good luck

Thanks both - in the interests of simplicity, I’ll take Bob’s route and see if I can find a suitable O-ring. Nitrile, I guess… I read somewhere on another thread that the large cork washer AUC2118 was the source of the problem, but I suppose if the end cap can’t leak that achieves the same end result.

Bob, I’ve ordered a small selection from Polymax (an excellent supplier), mostly in Viton but one size only available in Nitrile. Only a few pence each.

I’m not overly keen on a kit that replaces most of the parts in the original - it’s not that I think it won’t work, it’s more that the further I go from stock the easier it is to forget what you’ve done, especially with the big 7-0 just around the corner!
I’ll report back on sizes.


I passed that big 7-0 about 5 years ago and I’m convinced that the more we dive under our cars, the better our brains will keep working.
I read somewhere that keeping the blood pumping through our brains, will at least slowdown the process. So that’s why I don’t have a car hoist and you will find me laying under the Jag, at least for as long as my back will allow that, but that’s another point … :wink:

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Yes, I’ve always had an inspection pit in my garages and they do keep me fit!
Yesterday I was changing the oil and brake fluid in my Cooper S - I must have climbed up and down that pit ladder 30 times! Mostly because I kept dropping the spanner I was using to bleed the clutch slave cylinder, which is hidden down the back of the engine between the gearbox and the bulkhead, and is completely inaccessible from below. Minis really are horrible to work on, there’s no space for anything!*

  • The one exception is the carbs - the 140s are horrible to get to, the Mini’s are easy.

An update:

I decided to go with the Superdry kits as I don’t really want to go into this again too soon. The kit is very nicely made, and contains new ‘rubber’ (not specified, but Burlen usually know what they’re doing) seals in place of the upper seal and lower cork seal. The lower is made to a cross-section that seats very effectively in the bevelled nut.
The only part not in the kit (which, if the kit works, it shouldn’t need anyway) is a rubber replacement for the small thin washer that sits in the jet end cap. Following Bob’s advice, I have bought some Viton seals to do this job just for completeness. Photos attached of the seals compared, one installed and the packaging to show what they are. They’re a snug fit inside the thread but are not so wide as to interfere with the jet adjuster screw. At just a few pence each, it’s good insurance.


Now the only question is: has the “incontinence” disappeared? Keep us posted…

Bob K.

Yes, and no!

The front carb is now absolutely dry with the jet adjuster cover off. The rear is not, still dripping. But the ‘Bob’ fix with a Viton O-ring in the jet adjuster cover has fixed that!


That’s fine.
I still think that the thread tolerance of Adjusting Screw AUC 3158 in the Jet Bearing doesn’t prevent fuel dripping down. The accumulated fuel in the End Cap AUC 3160 will move upwards via the thread of the End Cap and the solution SU had chosen with fibre End Cap Washer AUC 3159, is (apparently) insufficient to keep the fuel in the End Cap.

Bob K.

Yes, Bob, I’ve studied the diagrams and looked at the carbs and the source of the leak on the rear carb is the jet adjusting screw thread. It has to be - assuming the float level is set correctly, fuel should not reach the top jet bearing so should not be at a height where the large seal (previously cork) could leak it out. As these are thermo carbs, with a different jet and jet adjusting nut arrangement to the more common manual choke H carbs, there is no seal below the jet assembly apart from the thin fibre washer in the end cap. So the Viton seal in there is doing its job, whereas the fibre one will only work if you’re very lucky! The Burlen technical pages for this type of carb state that fuel leaking down the threads is OK, so SU were obviously relying on this end cap seal too.
On balance I reckon the Superdry kit was worth fitting, as it means all the threads etc. are brand new and not worn - but it’s a shame they still supply a fibre end cap seal when it would be easy to supply a Viton one.

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