Part 2: Thermostats that you can fit to an XK120/140/150 and early Series 1 E Type that use the bellows-sleeve type thermostat. (All must use only the 4lb max radiator cap). The Jaguar Factory Service Manual provides specs: starts to open 60-63C, full open at 80C and 3/8inch (9.5mm lift). 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 listed bellows sleeve ‘stats with the various temperatures.
Smiths The Smiths coding system changed in early 1958 whereby the X.43xxx numbers were replaced by X.85xxx numbers, some with and others without the X prefix; Note: there were more Smiths thermostat-temperature versions manufactured than the ones Jaguar had chosen. They are sometimes close to the nominal value Jaguar opted for. Examples: 85025/70 instead of 85025/74. With the only difference bein the temperature spec, they can be used, but must be bellows-sleeve type.
Do not use: any non sleeve type: Do not use: Smiths X85032 X, 85035, X.85024/74, X.85025/74).series, (Service Bulletin 235/239 refers to Mark 7, not the XKs, and revises only late Mark 7 use of the above # s in place of the std C3731/1, again, ONLY applies to later Mark 7. Do Not use any “British Thermostat” fitted to early Mark7 only. Do NOT use the AC TC series which have no sleeve,
Although Jaguar fitted various temp thermostats thru the models, that really does not matter. Any of the NOS different numbers that are applicable to the XKs “fit”. So far all exterior dimensions have measured to be the same across all manufacturers and numbers. All work the same, except for the opening temp and full open temperatures. Once open, all else being equal, the car will operate at / near the thermostat’s open temp spec.
Jaguar part codes: Jaguar used part # C.3731 and at the factory fitted only a Smith’s thermostat bellows type. Later part number C3731/1 is just a different opening temperature. It is the same exact fitment part, no exterior difference. Current vendors that use the Jaguar C3731/1 numbers do NOT supply new old stock Smiths. They supply whatever current, and different type thermostat that they select that they feel fits and works, and just use Jaguar’s part # to identify that it is “a thermostat”. Jaguar revised the thermostat part number thru the XK140/150, (such as Jaguar code C7105 (83° C, or C.12867) but the only difference is the temperature spec. The part fitment in the housing is the same and all are interchangeable within the 120/140/150, and early E.
Use any of these:
Smiths:earlypart #, laterpart # but essentially the same Jaguar part#
X.43655 opens 60 to 63°C , open 80°C. 85026/60 C3731
X.43570/5 opening 73°C (163° F). X85025/72 (1 de C diff) C3731/1
X43605/3 83C same as X85025/86 3 de diff C7105
X.43570/28 73C same as X85025/74 1 de diff C12867
X.43570/16 79 same as X85025/80 C12867
AC: use any of the TF: TF1, TF2, TF3, F4…, But NOT the TC series which has bellows but no sleeve. Each TF also has a manufacturing code (next to the temperature stamped on the top flange to refer to as an additional check.
• TF-1 72° C 2235 mfg # code
• TF-2 80° C 2233, 2664 and later 2310
• TF-3 68° C 2202
• TF-4 86° C 2307, there exist TF4 incorrectly stamped 68° C , which should have been 86° C, The production code 2307 will still prove it is the correct TF4 86C.
Lucas (Motostock) In the 1980s some of Smiths had been sold to Lucas, which started selling Smiths thermostats, although they were most likely made by AC, Lucas LF1 72° C (160 F) correct bellows/sleeve, and Lucas LF2 80° C (176 F) winter, correct bellows / sleeve
Quinton Brivec / Quinton Hazell (QT on the part #) Quinton Hazell QT 100/200 series, QT 100/70, QT 100/74, and the QT 200/80 and QT 200/86: both winter temps
Remax much like the Quinton Hazell, the part # was similar: QH refers to QT100, Remax: one example is the NT100. 68C P154
SNG Barrett: C3731/1, One top cross bar and, top mounted sleeve and a visible spring. The flange has a small hole. (the top mounted sleeve moves downward, not up as does the Smiths, AC, QH, Remax. Possibly the same as the current XKs C3731/1 090160.
XKs current C3731/1 (the XKs 090160,) is a 160C: not marked as such on the one I have, 160C is on the cold side, but as said, once open—thermostats are open. The car will likely run warmer with the stat open all the time, except in very cold air. It has one top cross bar and, top mounted sleeve and a visible spring. The flange has a small hole. Note that an older XKs version,( likely a sleeve added RobertShaw), was sold as 09-0160 Superstat that has 3 cross top bars over the flange, a brass enclosed bottom, but does not have a piddle valve or tiny air hole in the flange: It fits, it works, has the sleeve, however, if it is to be used, a small hole should be drilled where possible and functional; Some of this older type are still around, so see the current one, one top bar, as an example. The hole is an air bleed hole to allow air to escape to radiator when filling the cooling system after a drain. Smiths type have the piddle valve.
Moss Europe bellows/sleeve thermostat, part 434-156: this is the exception to the “all the same”, as this is not old stock, but a new manufacture, listed for Triumph, Austin Healy, but not Jaguar. What we have now is an original “Smiths type bellows/sleeve thermostat” being manufactured currently. It fits the housing, but remains to be seen if it closes the bypass when open to radiator: the bottom positioned sleeve moves upward. It travels less to full open at 8.382mm than did the Smiths at 9.5mm, but the sleeve itself has 1.256 more height to compensate and maybe thus close off the bypass. I have no knowledge of an actual test in Jaguar housings as of yet: Does it close the flow to bypass sufficiently? I hope so. It may be that the sleeve stops can be filed to be shorter if necessary. A $123.99 part that is new, is IMHO better than a hard to find $70 NOS that is 60 years old.
The thermostat installs in the XK120 housing, bellows/sleeve into the housing. You can opt to a test of the entire assembly in hot water to prove if the bypass fully closes, or you may test only the thermostat to be sure the sleeve moves at least the spec 9.5 to 10mm upward. With 60 year old parts, do the test before install.
Check points: The thermostat body inserts into the housing; be sure that the side flats that connect bottom to top are positioned so as to not block off the bypass port. The piddle valve or small hole in the top flange should be at 12 O-clock (positioned so that air can escape). Check upon fitting that the sleeve does not bind on the housing; there should be very tiny clearance so that the sleeve can freely move. It is a good idea to test any new (or old when servicing) thermostat in a pot of water to be sure it operates, and opens fully, 9.5mm travel,( except for the Moss at approx. 8.3mm travel which remains to be tested in a housing.)
Use only a 4PSI radiator cap which has a long reach so that the cap internal will contact the radiator neck. Be sure the cap contacts the radiator top and leaves a “witness” mark on the rubber seal. An original cap does not have a rubber seal ring within the cap so the detached seal is necessary in the radiator neck. A later cap will have the rubber seal ring within the cap itself, so if used, no rubber seal in the neck. 50/50 anti-freeze has a boil point of approx. 223F, a 4lb cap is 3lb more, at sea level, The boil point decreases about 3 de F for each 1000 ft. of elevation. At 50-50, the freeze point is -34F. Most anti-freeze specs found on the container have a boil spec if 265 but this is (in fine * print) with a 15PSI cap. Boil with 50-50 is 226 F with our 4lb cap at sea level. A 70-30 provides -84F and 276F (spec is with 15lb cap which we do not use) but may cool less efficiently. Most have 5 year coolant life, some are now more. Anti-freeze left too long creates some gooey junk…nuff said. The 4lb cap is a safeguard and will release too much pressure which will destroy the thermostat and likely a head gasket or core plug: even a block can crack with excessive hot spot heat and / or steam pressure. Most thermostats start to open 10C to 25C lower than the full open temp spec. If you test one in a pot of hot water, the sleeve should open fully at/near the spec temp, fully means the sleeve has moved 9.5mm -10.1mm. There is a “stop” which prevents it from moving much further. My tests on a Smiths 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: it did not reach the “stops”. The “stop” is 4.6mm below the inside of the top flange. 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.
For the Moss, where the stop does stop the sleeve at 8.38mm yet untested is whether modifying the stop would allow the sleeve travel to move any further without internal damage and possible failure.
Comments/errata to Nick Email:firstname.lastname@example.org
See separate post" Thermostats for the XK120".