Thermostat AC TF4 bellows sleeve

this is a NOS winter thermostat…AC TF4, 86C .the TF4 bellows/sleeve 86C: TF4 is a “winter” thermostat…as it maintains 86C.minimum operate temp, …

.that said,…if the cooling system is otherwise in fine shape…ya can run all day and nite at 86 to90C…this fits any application that would use the C3731, C3731/1, C7105, C12867 bellows/sleeve type thermostat…ie XK120, 140, 150…and E S1 early… TF1 and TF3 are around 73C, The Smiths will be an X85025/xx…See the post on Thermostats for the XK120 for details., All Must be used with only a 4lb radiator cap as well. These are on Ebay UK occasionally. Nick

Possibly… but do you really want to drive it in the winter?

drive it anytime !! the operating temp will be a bit higher with the 83C full open thermostat, …but it is the correct type, fully functional with a correct type bellows/sleeve thermostat which once fully open will maintain cooling with the 10C or so increase. While this does mean less heat transfer from engine parts to the warmer coolant, there is an offset as a higher coolant temp thru the radiator means slightly more rapid cooling of the coolant. (the diff from coolant temp to ambient air temp is greater,) . A car with an otherwise clean cooling system will then run at about 85C-90C or so. Less leeway for traffic or really hot days…but otherwise OK. The winter thermostat recommended by Jaguar for the XK140, the C7105 had a temp of 83 to 86C. so similar. Yes…it’d be nice to have the 68 to 72C bellows/sleeve thermostat…if one can be found. BTW…Peter, at this same source( “bitsn_pieces” on Ebay UK) has no more of the QH. this one sold quickly after my post to a USA buyer, .but does have the AC bellows TF4…which is the equivalent…still the winter 83C thermostat. Nick
(which COOLS faster…very hot coffee…or coffee with cream that is less hot?)

and a good thread on thermostats recently on the E Type Forum…be sure to click on the links to take you to the “paper”…and click at the end of the paper…for the bellows part…the first section is later E type so you have to click “bellows” at the end. Also…click on the prior thread link… BTW…I spent a lot of time on it…now have a “summary” of what I felt was the most important of the posts about thermostats and how they work. Happy to send to anyone that wants it…it is very lengthy…excerpts basically not edited.

When the roads were dry…in went the winter front, and OUT went the Jag!

I drove my prior 53 OTS up into the mountains in full winter. about 50 miles…crisp, cold…but road mostly clear…a few slick spots…top down, with my skis sticking up out of the cockpit…spent the day skiing…wish I had a photo,.Heater on full blast…did work…and as we know…plenty of heat comes in anyway,…but still a nice parka and gloves on… Nick

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I never drove Tweets much, in cold: MnD did, on a regular basis. With its ginormous NASCAR radiator, and even with a 160 t-stat, it needed a winter front to keep the engine at a decent operating temp (~90C).

Once I did the Fiero fan.motor, it wouldn’t drive you out of the car, at an ambient 30F (-1C) but was certainly adequate.

speaking of “normal operating temp” .(car…not the human) …is that spec listed oficially by Jauar in some manual? I am looking…but by the various thermostats fitted…it is obviously between 63 lowest and 86C (maybe a 90C thermostat as listed? in the service manual) . 75C to 83C seems about “normal”. ?? Nick

Poor kitty… disturbing her nice cozy snooze…


Getting him out, exercising its muscles!!!

Dang fat fingers…fixed!

this is the hard to find TF1 bellows/sleeve thermostat…

this one by QH…the number QT100/72 thus a 72C open temp equivalent to the Smiths with Jaguar part no C3731/1 which was std for XK120 with heater,. If/when you can find one…
(later I add this) : the 120 and 140 thermostats “should be” interchangeable, although the fitment is in a different location. Jaguar Journal Sept and Nov 2016 has the article on thermostats by Bob Knijnenburg, which is available on line…bobine…cited earlier. Per this study, The bellows sleeve thermostats cited have top outer diameter of 54mm, sleeve diameter of 48mm, and sleeve height of 15mm. The 140 thermostat part number REMAINED C3731/1, Smiths X43570/5 and later just a part no change to the suffix to /28. the sleeve moves upward with the opening expansion of the bellows. The raised sleeve raises about 10mm and thus closes the bypass, (" bypass" means bypassing the radiator so coolant is a direct flow through the engine in order to warm to op temp quicker) and as the bypass closes…the flow to the radiator opens. (There is some minimal flow thru the closed thermostat to the radiator flow path to aid in a more rapid opening.) The thermostat should cycle repeatedly to re-warm…the radiator cooled coolant,.
Any vendor now…can list a thermostat with the Jaguar part number…but supply whatever thermostat they choose…often not necessarily the correct one. Others list a later type poppet/cone thermostat (no bellows, no sleeve) I suppose thinking…good enough. It is not. By the way…a failed, no op, bellows/sleeve thermostat will usually fail CLOSED…very minor radiator cooling…so serious overheating can occur rapidly. Just the way it is,

Hi Nick,
Can You share the link to the auction? Either it’s gone or I cannot find it…


none I can find are currently on ebay…the last two were sold…,.one to me…one to someone in USA…quickly after my post…so try a web search…for Etypeparts…last I saw they had a TF4…80C,…and a web search for bellows thermostat. I am attempting to learn if the MossEurope remake of a bellows/sleeve stat will fit an XK…the remake they list does not list for Jaguar XK…Meanwhile the discussion continues on the E type Forum…Nick

I’m curious to know whether the Moss remake of the original bellows thermostat would fit as well. It would be interesting to take a pair of vernier calipers and see how close it measures out compared to an original – which I don’t have. But maybe the question is moot as Moss may have already ruled it out for one reason or another.

Christopher…exactly…I just posted same on the E Type forum…Michael provided the Moss dimensions…I do have a NOS AC…and a therm housin for 120…have to retrieve them from deep in my spares boxes…but will…and will measure. We need of course the OD diameters…and sleeve heiht…and where the sleeve is located, and how much it travels…Moss EU told me…they just did not know about Jauar XK application…(but they should !) XKs Unlimited said they would refer to their restoration mechanical staff…will see…DO look at the Etype forum…

With Moss now having XK’s Unlimited under their umbrella you’d think a bellows thermostat would be among their offerings. I hope so. I started reading the thermostat thread on the E forum but got sidetracked by Coolcat’s treatise on same – there went most of my lunch hour – so will come back to it. Now I at least know to never make the mistake of putting on a higher pressure radiator cap.

If you are looking for a correct cap for your 120 rad… I think I still have a couple NOS… PM me if you want one

The Thermostat thread on the E Forum is more of a quilt now. but lotsa info…a few check points for the XKs: use only a …4lb cap max; don’t run without a thermostat…or with an inoperative thermostat. The cooling margin for the xk120 is small, only a bit better for the 140 due to the improvements made. The innards of the cooling system must be clean. And…NOS thermostats that fit/work can be found.68C, 73C, and .even if 80C or 86C…will work. The thermostat available now from Moss Europe is now the subject of our tests to see if it can be used in the Jaguar XK. This is not yet known. Moss of course issued some specs to have this bellows sleeve thermostat manufactured, then found a place that did it…whether China or India. It is listed as for the Triumph, Heally , MGA, etc but not Jaguar XK. I wonder…if…XKs, Welch, Broad, Barrett…it would send a NOS of the type the XK120 uses, 70C, …and say…Make this…there’s a lot of Jaguar’s that use it…from Saloon, all the XKs and the Series 1 E. I am prodding Moss Eu to do just that.

Meanwhile I have done a lot of research on what is correct thermostat for the XK, and what to look for, and why.

See the post by me on this forum “Thermostats for the XK120” for a lot more…a work in progress, Who knew…that the XK thermostat would be such a quagmire. Use only a sleeve type thermostat. The current X3731/1 while not “original type”, and have a top mounted sleeve…and they do work in the XKs. NOS bellows sleeve can be found on ebay.,.about $50USD. Be sure to read the article as there are other “check points” as to type, tests, and how to install. or…PM to me.


If you now have a correct original NOS XK120 Radiator Cap available, would you please send it to me in exchange for the one you sold me a couple years ago claiming to be correct NOS, but wasnt!


Hello Roger I remember sending it… the one I currently have is the same as the one I sent… Forgive me I forget… what was incorrect about the one I sent you?