Thermostats for XK

Does anyone know of a source for the correct thermostat for XK140? Barratt’s currently sell one advertised as correct for XK, but the sleeve is too narrow and moves in reverse - i.e. when cold, it covers the bypass passage, but opens it as the engine gets hotter, which is the opposite action to that Jaguar intended.

I found them on Ebay

Thanks Peder - no new ones being made, then?

Hi Roger…have you see this from Bob…Steve Thermostats for Jaguar XK 120, 140 and 150 | Jaguar XK 140 FHC Part & Assembly Information Moss and Rimmer have “New”

Ha - you just beat me to it with your edit, Steve!

Not cheap, but correct, and very important with regard to keeping the temperatures down. With the bypass left open, hot water is going to recirculate quite easily, letting the engine run hot.
As ever with these cars, the total bill is so high, what’s another 90quid? That is a self-generating argument, as time goes by…

Only problem is that its a remanufactured item…so is it correct?..size and actual operating temp. .you can probably get 2 or 3 NOS for the price…i picked one up at Stonleigh for £10…Steve

Hi Roger…just to add some info to this thread…Wax Stats are offered by many of the usuals when you look up thermostats for XK,s …this is a great drawing with lots detail that will that anyone can use if they measure their bypass slot to work out if its suitable for them…Steve

Steve, point taken on the repro accuracy - which applies to all repro parts these days, buyer beware!

For info to go with the Barratt drawings, on my 140 intake the bypass slot starts at 9mm from the front face of the casting and finishes at 18/19mm from the front face. Therefore the bypass slot is uncovered by 8-9mm when the engine’s cold, and uncovered by 3-4mm on the other side of the sleeve when the engine’s hot. Not perfect, but getting there, I suppose.

One of the new Moss sleeved thermostats arrived today, and I’ve just done a comparison with a NOS TF2 bought on ebay, using a saucepan and a mercury thermometer.
The new one is labelled 160˚F, (71˚C), which is pretty much where it starts to open.
The original TF2 started to open at around 78-80˚, which is correct for a TF2 - it was a bit ‘jumpy’ but as it’s been in a box for some decades that’s probably reasonable. Both started from pretty much the same position - i.e. bypass slot fully uncovered - and both stopped at the same point - bypass slot fully closed. Both thermostats had moved back to the ‘cold’ position by 74˚ on the way back down.
For me, this is ideal as I believe that if any of the slot should be run uncovered once up to temp, Jaguar would have designed it that way. The only way to achieve this is to use a bellows thermostat that pushes the sleeve back towards the face, because there simply isn’t room for the sleeve if a push-type is used and a compromise results at both full open and full closed - i.e., neither position is fully achieved, as far as the bypass-blanking sleeve is concerned.
It is important to note that Barratt’s instruction sheet states very clearly that if the bypass slot is closer than 11mm to the thermostat face, a blanking sleeve may be needed. This is indeed the case with my original inlet manifold’s slot, which is set 9mm down from the thermostat face, so ideally some kind of sleeving would help.
Of course, as Steve says, who knows how long these will last in service, and how accurate will they remain? The new Moss unit is ‘Made in China’, but these days what isn’t? It is also the case that there is some excellent engineering and manufacturing done in China, we* just choose to pay for the stuff made down to a price.

  • well, some of our suppliers do…

Hi Roger…im no expert on the bellows stats but my understanding is that if they fail then they do so in the closed position…and iv heard that this has been the cause of more than one engine seizing up becaus the driver didnt notice the temp guage…I know that in the E type world there have been similar problems with thermostats and the slot depth you mention…a modified thermostat houseing and modified wax stat sleeve have been used…waxstats fail in the open direction…I havent finnished my rebuild yet but the thermostat is something i will get to later…Steve

Not always: I recall more than a few failed in the closed position.

I did say im no expert…and there are always exeptions…so is it more typical for a bellows to fail closed and a waxstat to fail open…looking at the waxstat drawing above its closed by spring pressure…so i might have this totally the wrong way around…Steve…ps this is a modification i was refering to above

I’ve certainly had a Waxstat or two fail in the closed position back in the day - clouds of coolant and a pinned gauge! Obviously not in a Jag, as this is my first.
The problem with trying a mod for an XK is that the slot that needs to be modified is in the inlet manifold, not the thermostat housing. I don’t know anything about E-types - are they not the same design? Maybe that extra-deep machined groove positions the thermostat further in… but if that’s the case, I’d have thought the sleeve would still not be able to be in both positions accurately. Not sure, I’d have to see it, I think.