Thermostat for XK120:
Jaguar in SB167.and SB198.lists some thermostat numbers, and states that 70C is normal operating temp.(I think most would find that optimistic…as 75C may be the usual seen unless a very cool day…and 85C to 90C on hot days in traffic. ) It is advised that if an early thermostat with a C3731 60C temp is used there could be some spit back (with coolant at 55C) thru carbs on very cold days when the coolant may never reach the 70C op temp, so to fit the C3731/1 Smiths X43570/5 which is 73C. This became the X85025/72. These SB are the only place I have found a Factory reference to the Smiths part brand and part #s…but there it is, alon with the 70C as “normal”. SB 97 discusses thermostats for the MK7, as does SB108. There is reference to the British Thermostat C5015/1 ONLY for Mark 7, but also to C3731/1 which is also an XK thermostat. Interesting, in SB112 is that the start carb cut off is at 35C, noting that a wired switch can be installed to control it.
Here is what I “think” I know, after some tests and measurements of two XK120 thermostats. (I say “think” pending your confirmations or errata. )
The XK120 thermostat housing: I used/measured a re-pop XK120 thermostat housing, new, from Welch. It may be slightly different than an original. Rob Reilly found very slightly different measurements. The inside main passage ID, inside of the machined lip indent is 50mm., at the indent outer edge has ID is 54mm, the machined indent at the lip 1.1 mm deep, the thermostat flange lip is 1.1, so both thermostats slip in and then are flush with the mount surface. Compare the 50mm to the 54mm and it gives the machined indent as 4mm wide. The thermostat flange lip is 53.9mm so the thermostats fit closely, with tiny clearance. The bypass on this 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, thus 18mm at the hole lower edge. This is what the sleeve needs to cover to close the bypass port to the engine. 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. As we will see neither thermostat avoided cold obstruction perfectly but one was better. FYI, the main large top hose passage on the thermostat housing, is ID 29mm and OD 37.7mm.
Now for the sleeve-bellows Smiths type thermostat: I tested a Smiths 85025/80 and a Remax manufactured Made in England code P154 68C…( I believe made for Quintin Hazell by Remax) The QH number would be QT100…this one is stamped NT100 68C P154. In appearance and all measurements they are nearly identical. The AC has a minor difference that makes no difference. The Smiths and Remax seem identical and are quite possibly the same actual manufacturer The thermostat flange lip fits the housing indent perfectly with less than 1mm clearance. When in place the sleeve is visible thru the bypass: notable is that it blocks about 20% of the bypass opening. The thermostat sleeve is 16mm in its height (Same for the Smiths 16.3mm) (BK cites 15mm…no matter, there is and must be clearance between sleeve and the housing so the sleeve can move freely, this means a very small amount of coolant can pass to the bypass hole but it is a restricted path when the thermostat poppet is at full open so most coolant will follow the least resistance path to the radiator.) and 48.3mm sleeve width (diameter). When closed the sleeve upper edge iis 15-16mm below the upper thermostat flat. The stop (cross bar under the flange on the Smiths and on the nearly identical Remax are slightly different than the AC: The stop act upon and stop inside of the sleeve, on the bellows top. The AC has a lower stop, but also a lower bellows top, thus the same available travel. When closed there is no coolant passage thru the thermostat to radiator. It is open to bypass only—with some volume blockage but the bypass hole is small anyway. The Jaguar Factory Service Manual provides specs: starts to open 60-63C, full open at 80C and 3/8inch (9.5mm lift).
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, n 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 9.5-10mm). The AC is much more dished from sleeve edge to the inner bellows top, whereas the Smiths and Remax are not but the stop is itself different (lower) so it comes out the same with 9.5mm to 10mm of possible opening travel. The 9.5mm lift is the important part as that is how far the sleeve moves up to close the bypass port. You can use any of the bellows sleeve ‘stats with the various temperatures. XK120s will mostly run at coolant temps that will have the thermostat full open all the time, the exception being the “winter” 86C thermostat. The others are 68C to 73C, and one at 80C which is still a quite acceptable operating temp.
NOTE these check points: All the thermostats install into the housing with the sleeve inside the housing. The thermostat must be installed, positioned so that the one of the two side flats does NOT block the bypass hole: if they are in the way the flat can block the bypass hole partially or completely. At the same time the piddle or air hole should be at 12 o’clock. Be sure that the sleeve has clearance to the housing, and also to the inner bellows (I had one with a sleeve that was cockeyed, so was against the inner bellows which could inhibit its travel. ) These are important install notes for the any thermostat. The side flat type of blockage is a lesser issue with the C3731/1 or 09-0160 as the side flats are not on the outer edges. I heated the thermostat installed in the housing with the whole assembly in the pot. When the thermostat opens, the top poppet within the lid extends to open about 9.5 to 10mm to allow coolant to radiator; at the same time the sleeve rises and DOES block off and close the bypass completely. There remains a space from sleeve to the upper thermostat flange of 5.8mm when the thermostat is full open, likely at the stop. There is a piddle valve, flopping around in the tiny hole at the top edge of the thermostat: …it “piddles” all the time, thermostat hot or cold. This aids in removal of air from the system when filling cold. The poppet travels to its open position in the same direction as coolant flow, closes against the flow. The Smiths type sleeve moves up, being quite low when cold, it has little bypass blockage when the thermostat is closed. It closes the bypass completely as it should when the thermostat is open. It is likely that if the thermostat failed, maybe the alcohol had leaked out, it would not open, thus fail closed.So the bellows/sleeve thermostat does what it is supposed to do which is no surprise, as Jaguar specified this thermostat. The Smiths 85025/80 and the Remax NT100/68, and AC TF1 have identical dimension measurements. . 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.
Now for the XKs 09-0160 thermostat: a different design,(actually 2, or more versions exist) it has the sleeve mounted, so that when cold it is at the top, right under the flange, not at the bottom. It has a cone top in the 3 bar version; and a rounded top in the 1 bar version, but neither are flat as is the Smiths type. The brass rounded center top, single top bar version does have a very small hole in the top plate (flange) so that air could pass during a fill when cold and closed. I did not find any such hole in the silver, cone top, 3 bar, closed bottom version which could be a problem with this version, in trapping air in a drain and refill. Would any air in the system, once thermostat fully opens, then be moved with the coolant flow into the radiator space? Both of this type are smaller overall in mass, but the top flange also fits exactly in the 54mm slot. The top mounted sleeve is 12mm in height, (less than the 16mm of the Smiths type) , and 49mm wide (diameter), 1mm more than the Smiths, so fits in the 50mm diameter housing with 1/2 of 1mm clearance around it which when closed to bypass would allow a very very small amount of coolant to the bypass. This thermostat, from the underside of the top flange lip, to the lower part of the top positioned sleeve is 16mm, so cold, with the thermostat closed, and bypass open, blockage from the sleeve occurs. But,noticeable, even more than than the Smiths type where the sleeve is at the low end and out of the way, this top mounted sleeve blocks about 35% of the bypass, while at the same time the path to radiator is totally closed. Cold, closed to radiator, open to bypass, you can see a lot of the sleeve thru the bypass port. When heated the thermostat opens as it should, the poppet opens about 9mm (the Smiths type was 10mm on my test, tho 9.5 is the spec. ( One can expect variance in 60 year old thermostats…if they work at all, so do test before install) and at the same time this sleeve moves DOWN to close the bypass which it does completely. The bypass is closed, and port to radiator open. So it works: minus the issue of the bypass to engine opening being more than half obstructed at a time when the thermostat is closed and directing all coolant to the bypass: Coolant can flow around the thermostat base, so maybe this matters, maybe not, as long as enough coolant is flowing thru the port to the block to prevent super-hot spots. So, the later model, C3731/1 -09-0160 works: the sleeve moves DOWN to block the bypass. The sleeve blocks about 35% of the bypass at a time when it should not. It has no piddle valve but DOES have small air hole in the top. If the thermostat failed, it would also likely fail closed to radiator, but it depends on how/way it fails.
And now for the new entry: the currently manufactured Moss 434 156 bellows sleeve thermostat: The Moss Europe bellows / sleeve thermostat’s flange diameter, and sleeve diameter meet the Jaguar spec already discussed. The Moss sleeve is 16.256mm wide, wide to mean its height in the terms we have been discussing. We know the relevant Smiths sleeve is 16mm in height: looking good, a very small bit wider (more height) certainly no harm done, likely an aid. Now as to sleeve travel, the Moss sleeve travels 8.382mm of sleeve travel. We know the Smiths has 9.5mm to 10mm: , I am concerned about there being 1.118mm less travel and still completely covering the bypass port-hole. It does ain 1.256mm back in its height. Does that mean it will cover the bypass port despite less travel? We shall see. Close is enough in horseshoes and hand-grenades. Lee said his test of a Smiths with 8.5 travel did not close the bypass, (my opinion is that his test Smiths thermostat did not fully function thus the 8.25mm observed instead of the 9.5mm that is known to be the Smiths spec.) The Moss with less travel but more sleeve height may just do it,but I have to withhold stating that the Moss bellows/sleeve is applicable to the Jaguar cars until actually tested in the relevant housings. Does it fit in the housing? , Yes. Will it open flow to radiator? Yes. But will it close the flow to bypass sufficiently? I hope so. It may be that the sleeve stops could be modified to allow more travel. A $123.99 part that is new, is likely better than a hard to find $70 NOS that is 60 years old, and I won’t be the one to whine. It may be one of the less costly things I would do in maintenance of a XK120.
About the job of the thermostat: it’s job is to allow the engine to warm the coolant to optimal engine operating temp: good for lubrication, fuel ratios, combustion, etc. Then when coolant becomes too hot, the magic little thermostat works to cool the coolant by allowing the radiator to do its job. The thermostat should cycle: closed to radiator at start up, then as coolant warms opens to radiator, then cycles closed to radiator while open to engine , and on and on. The coolant “should” stay close to the temperature of the spec of the thermostat. If you have a 72C then op temp should be close to that: if you have an 82C then close to that. A higher temp thermostat in and of itself, does not make the coolant overheat, but it does take away some margin that may be needed due to other cooling system issues. That said, Dick Maury says, “if a car is running hot, the fix is NOT a change to a lower temperature thermostat. The thermostat only controls the minimum operating temperature of the engine. If the car is overheating beyond the rating of the thermostat, the excess heat is beyond any thermostat control: the thermostat is open, job done. Only the radiator can now do the cooling. Changing to a lower temperature will only cause the engine to run cooler when warming up. Once the thermostat is open, it is open. Because these thermostats often fail closed, is why a sudden overheat is likely a failed thermostat. For 140 150 folks your thermostat placement is in a different place:…if the bypass location dimensions and placement are similar to the 120 housing, then all should be as for the 120,
The XK radiator cap must be 4lb, maximum, do not use the 7lb or 10lb thinking you can now operate at higher coolant temps without boiling: this will cause the thermostat to fail, and they may fail closed, with no flow to the radiator which is very quickly not good, as this is how head gaskets and core plugs fail. Note also that the original red seal within the radiator filler neck is important with original type cap which does not have its own seal within (a later cap may have the rubber seal within the cap.) Be sure there is a “witness”mark on the cap rubber seal.
Finally we are likely to end up stressed, when engine heat production exceeds the capacity of the radiator to cool the coolant. The thermostat is wide open doing all it can, the radiator is doing all it can, and the coolant temperature continues to rise. That is when we realize that we are in our beloved vintage automobile, with a vintage cooling system, likely a lot of vintage crud within it, while the vintage temperature needle keeps moving toward the vintage peg.+
Credit due to many forum posters including Bob Knijnenburg Dick Maury, Rob Reilly, Roger Payne, Mike Balch, Michael Frank (+and MGA Guru…for the vintage wisdom) , and no doubt many others. Thanks. Most pleased for review and errata comments.
Many road test reports done in the day skirt the issue of cooling / over heating, but a few do report the issue as common, known to Jaguar, and wonder why it was not fixed in the XK120 models. Most said it was OK on the road…but if you are caught in traffic in summer…“just pull off the road, brother, you’ve had it.” Been there, done that ! Jauar only briefly mentions overheating in particularly hot countries, and discusses coolant loss of 1.7 pt from full , as normal…See SB121 and SB142 where by an improvement in the water pump impeller was made, from 4 vanes to 5 vanes. There seemed to be a known head gasket problem as a different head gasket was part of the fix, and was installed at the Factory, from W7199 for 1 fix, and from W7207 onward for both fixes.
A truckers tip: a small aftermarket windshield washer pump/with water reservoir can be installed hidden inside the fender well, a pump on off sw under dash, run two little rubber hoses with spray nozzles from pump to front sides of the radiator…when the temp creeps up to hi…hit the switch for a few seconds spray of water on the radiator front. (if an aux electric fan is installed even better when you switch that on as well) the temp will drop:…a few of these cycles may get you thru the traffic to clear sailing.
See separate post for “WHICH thermostat to use”
Nick
credit to: Bob Knijnenburg see http://www.bobine.nl/jaguar/02-engine/thermostats-for-jaguar-xk-120-140-and-150/
and FYI this discussion on the web:
http://www.mgcarz.com/thermostats.html
at left, a Smiths type bellows/sleeve, 
and on the riht, the XKs bypass stat 090160 with sleeve at the upper end.
same as this, new in box, and I have 2 of the Remax NOS. T
