Thermostats for the XK120: a study

Nick, yesterday I stated that the sleeve on the Smiths X.85025 thermostat (as well as other period thermostats) does not have the original .375 inch sleeve travel that the XK120 Service Manual indicates for the original Smiths X.43655 thermostat. I said that I measured the sleeve travel of my NOS Smiths X.85025 at only .325 inches. This was a hot water measurement, which is difficult to photograph.

An easier way to determine the sleeve travel of any of these thermostats is to measure the distance between the moving bellows and the built in “hard stop.” All of the bellows & sleeve thermostats have some type of hard stop that limits sleeve travel.

The Smiths hard stop is a fixed brass bridge running the width of the thermostat body. AC, Remax, and QH used a similar type of hard stop. In the photos below, I show that the hard stop of the Smiths thermostat limits sleeve travel to .326 inches - roughly what I measured in hot water.

The first photo shows a “feeler gauge” I cut from sheet metal. The feeler gauge is the exact width of the distance between the Smiths movable bellows and the brass bridge, which acts as the hard stop.

The second photo shows the measurement of the feeler gauge, which is a distance of .326 inches. I challenge you to provide photographs showing how you determined your Smiths thermostat sleeve travel at 9.5 mm to 10 mm (at least .375 inches).

The distance the sleeve travels is critical to whether the bypass port fully closes when the main valve of the thermostat fully opens. Last week I posted photos on the E-Type forum showing the Smiths thermostat inside an XK120 thermostat housing. At the time I assumed the Smiths X.85025 has .375 inch sleeve travel just like the original Smiths X.43655. Assuming a .375 inch sleeve travel, I stated that the XK120 bypass port does not fully close. Now that we know the later bellows thermostats have even less sleeve travel, the problem for the XK120 is that much worse.

There may be some hope for the Moss thermostat because of the design of its hard stops. The Moss hard stops are two brass tangs that would lend themselves to shortening. However, as you admit, the sleeve travel of the Moss is only .3 inches. What is unknown and untested is whether shortening the hard stops by .1 - .2 inches for the necessary increase in sleeve travel would damage the thermostat.

Nice work and empirical proofs.,.I like that and totally accept what you have shown. I measured using inside caliper tongs of my digital calipers, and did check several times for confirmation,I do have photos tho not as clear, as the boiling pot fogged the lens. I will re-do with the measurement visible. Se we do have a variance in travel…but the stops as you say should be the limit factor. (I wish we could decide: mm or inch) the stops act and stop on an inner part of the sleeve where it is at least 2mm more than the ede of the sleeve. But the Remax when full open was 10mm, I see in your photo that you did measure with the feeler inward of the actual edge of the sleeve… I simply measured the top ede of the sleeve to flane top when cold…and then when fully open, sleeve upward as much as would at near boil water. The sleeve moved up…9.5 to 10mm…an easy measurment to make…no feeler needed., I will check on a Smiths and AC that I have. WHY would Jaguar, for many years, have a housing and thermostat design that is inefficient, and does not close the bypass? Why…with the similar thermostats specified (other than temp) ., would the XK140 bypass be in a different position from the housing edge than on the 120 ? .Rmx%20bellows%20in%20pot%20open%20therm%20bell%20sl%20open%20in%20pot|666x500 have hope for the Moss where the stops could possibly be revised. Can the stop be accessed to modify it…maybe a dremel…it would have to be all the way across it. By the way I went for a drive today in 55 F temps.,.I have an XKs 160 thermostat (only because they did some work and installed it) and the car ran at about 65C. The air temp has to be quite low for any thermostat in an XK120 to ever cycle back to closed, I suspect they open and stay open. Then it is all up to the radiator (if the waterpump and internals of the cooling system are ok.)

XK120:The bypass on the 120 housing is 10mm round, and is located within the housing, at its top 5/16 (.3215”, 8mm) below the housing flat surface, so with a diameter of 10mm it is at the low point 18mm below the flat mount. We can say the hole is from 8mm top, to 18mm low and this is what the sleeve has to cover.

I measured all the static dimensions of the Smiths 85025/80, and the Remax NT100/68 **eye-**Dennnntiical (identical), and will soon do same with a few others: …goal is.to find if all are the same in fitment, sleeve, stops, every exterior dimension. . Next…the hot water pot: to find, though spec temps may vary, do they all open with the same travel of the bellows, do they stop before or at the “stops” , what is the open distance, poppet to body, and sleeve to the flange which is the important measurement: to see if the sleeve covers/blocks off the bypass. Since I have a 120 housing, I can test the entire assembly in hot water and visually see if the bypass port is closed, So far, in the two types tested, yes it is, I do expect to find some differences due to the 60 year age ( of parts…not me.,.) so back to cook-up some thermostats on the stove.,Stay tuned. Nick Nick (PS…I almost cooked the electronics in my Harbor Freight $9.99 (they work very well) caliper…in the hot water…heat quickly from metal to the housing…DON DO DAT)

For Lee…and Mike: My tests on a NOS Smiths 85025/80, showed the sleeve at the low-cold point is 16.6mm below the inside of the top flange. When fully open the sleeve is 6.5 mm below the inside of the top flange, thus the sleeve moved up 10.1mm: The thermostat reacts very quickly to cool and close to radiator: as soon as out of the hot water the sleeve starts to drop and the poppet starts to close.
Nick
just re measured, verified…on the Smiths that I have…sleeve lip to underside of flange is 16.6, the stop acts upon a top of bellows surface that is inside of the sleeve edge and this surface is a 1-2 mm inside the edge of the sleeve. (4mm on an AC TF which also has a 2mm lower stop…so the same travel) At full open the sleeve moved upward at 9.5 to 10mm.

Thanks for measuring…this 'stat would work on all 140 manifolds I have seen, including my manifold, but only IF it had reached the stops (.18" to mounting surface). As it is, it would work on any 140 that had the slot at least .26" below the mounting surface, which may include the “Mills” foundry parts, but probably not on all of the “West Yorkshire” castings as they appear to be different in this regard. I would advise everyone to measure the slot depth on their particular manifold to ensure proper fitment.

I have tested 1 Smith’s, and 2 Remax (all look the same),and a AC TF1) . So…we know the XK120 bypass on the housing is round, a 10mm opening. (FYI far end the exit to the bypass hose is 13mm ID). The bypass round hole is at its top 5/16 (.3215, 8mm) below the housing flat surface (which as an indent for the flange of 1.1mm) , thus 18mm from the housing flat at the hole lower edge. This is what the sleeve needs to cover. This location and hole size is important as the thermostat sleeve needs to cover/close it when warm, but have it open and not obstructed when cold. The thermostat sleeve is 16.3 in its height (Same for the Smiths 16.3mm) (BK cites 15mm…no matter) and 48.3mm sleeve width (diameter). When closed it is 15mm below the upper thermostat flat. The stops on the Remax and Smiths are nearly identical, the AC TF is different, but so is the dish of the bellows top to the sleeve to edge, so…it comes out the same. The Remax when full open was 9.5 to 10mm, Smiths and AC are the same.
I also have in front of me, the AC TF-1. It is marked 72C on top of the flange, with AC manufacture code 2235, “AC Made in England” on the base. The later blue box/white horiz stripe (some have yellow) has AC Delco Davison of general Motors Ltd. Dunstable England. The dimensions closely match the Smiths: flange 54mm, 48.3 sleeve diam, sleeve height 16mm, the stop is apprx 9 mm below the inside of the flange, the Smiths a mm or 2 less. However, On the AC there is a 2mm greater distance inside the sleeve, from sleeve edge to the bellows top: it is 4mm and it is approx 6mm from sleeve edge to the stop) (it is more dished than the Smiths-Remax) which is what acts upon the stop (if it reaches the stop on full open it is a total of approx 10mm). The AC is more dished from sleeve edge to the inner bellows top, whereas the Smiths and Remax are not but the stop is itself different so it comes out the same:…9.5mm to 10mm of possible opening travel.
You have found, for the XK140 manifolds you have seen so far, that the sleeve covers the bypass ONLY IF it had reached the stops (.18" to mounting surface). As it is, it would work on any 140 that had the slot at least .26" below the mounting surface, which may include the “Mills” foundry parts, but probably not on all of the “West Yorkshire” castings as they appear to be different in this regard. I would advise everyone to measure the slot depth on their particular manifold to ensure proper fitment.
I hope we can have some more XK140 thermostat housing measurement…housing flat to the bypass slot, with the dimension of the slot itself. Maybe some of this in posts prior…have to look, Thanks, Nick

Yes…Smith’s 85025…the nos one you tested.