Where Can I Find Early Thermostat C3731/1

I have not been able to find the proper thermostat for the first series 3.4 head on my MK 1 car. The usual place to buy stuff lists the correct part number C3731/1 but followed by a *. This is an aftermarket part that I bought. It may be OK but it does not look very well built to me. For example, when cold, the sliding skirt covers up about 1/2 of the silt that routs the fluid back to the engine. Can someone suggest where I can get the correct part or a better version of it?

Here in UK, Ken Jenkins.
He specialises in such parts.
Donā€™t get one off ebay. Most of the suppliers on there have no idea and will give you the wrong one.
Refer to previous post on this subject, there are hundreds going back years. The same mistakes keep on cropping up time and time again.

see my article in archives in the XK120 forum on thermostatsā€¦use the search : Thermostats for the XK"ā€¦in the XK120-150 forumā€¦in short some vendors use that Jaguar C3731/1 as their part number for thermostats that are not thatā€¦that means not as the original bellows-sleeve Smithsā€¦there are other brands that are the same as the Smithsā€¦Remaz, Quinton-Hā€¦that are the same sleeve bellows, same sleeve dimensionsā€¦they come in various opening temps from 68, 72, 76, 86ā€¦so do always check which temp. 72 or 76 being the better to run at recommended operationg temp. they can be fouind NOS on ebayā€¦check pics to see that not too corrodedā€¦new in box is better. Moss now makes oneā€¦under their Classic Gold labelā€¦it was made for Healeyā€¦I found the upper lid needed just a tiny bit of file dressing to fit welll in the housing indent. about $100 USDā€¦but on ebay the NOS range is about $45 to $80ā€¦and I have bought several in that rangeā€¦Another new statā€¦the super stat sold has the sleeve at the topā€¦and it moves downā€¦in some housings it accomplishes the sameā€¦and closes the bypassā€¦I do not know how the housing is in your carā€¦but if the spec is for the C3731/1ā€¦fit should be the same. Nick

I spent months accumulating information on Jaguar thermostats for E-types.The information applies here as well. If the car requires a sleeve type thermostat, and it has a higher pressure cap, it requires a wax thermostat, not a bellows. The only choice is the 3731/1 that is sold for the E-Type 4.2. However, if the car has a lower pressure cap, then it may be able to accept a bellows style thermostat. If the manifold was designed for the wax thermostat, the bellows is not a valid retrofit and may not work correctly.

The 3731/1 is a cut down Land Rover application, it barely closes the bypass. I think theyā€™re all 160F setpoints, as well. You can read all about it in my write up on the 4.2 system. You can see what the original (and unobtainable) thermostat looked like, but donā€™t hold your breath waiting for one to turn up on E Bay:


The fact is that closing the bypass may not be very important. The vast majority of the flow will follow the path of least resistance and go through the main passage, even if the bypass passage is wide open.

Partially agree, partially donā€™t agreeā€¦
Yes, water/coolant will flow along the path of ā€˜least resistanceā€™ā€¦
But the path of ā€˜least resistanceā€™ is surely the short/direct path to the water-pump via the wide open bypass passage, rather than through the tortuous, long and back pressure path via the radiator ???

The whole reason why the bellows style thermostat was used, is when water is cold, the bypass is open, so rapid warm up of water bypassing the radiator, but as water gets hotter, radiator path opens up and bypass is closed off, forcing water along the more difficult path via the cooling effect of the radiator


Newtonā€™s first law of motion is at work here. The coolant is moving in a straight line, with considerable speed. It will only make a turn into the bypass passage if the resistance to flow causes dynamic pressure to transform into a local area of high static pressure. This is what happens when the poppet is closed. The unknown is how well the system was engineered, but if it was designed well, then most of the coolant will make it past the the thermostat and into the radiator. There are a lot of these engines that have survived reasonably well by using Chevy single poppet thermostats, and Jaguar themselves once specified this style of thermostat as a service part.

In any event, there are no wax sleeved thermostats in the market, other than the chopped down 3731/1. Bellows thermostats generally donā€™t work in manifolds designed for the wax version.

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For my engine head, there is a straight hollow coolant casting that is attached to ports in the inlet manifold which passes the coolant from the head to it. There is round chamber at the front of this casting which contains the thermostat. There is a slit at the front of the chamber that returns the coolant to the engine (bypassing the radiator). This slit is on line with the coolant flow. When cold, the thermostat skirt uncovers this slit, when hot the skirt covers the slit. When cold, the coolant, after passing straight through the slit, makes a downward 90 deg turn to travel to the water pump. When hot, the coolant must make a 90 deg turn to pass upward through the thermostat and then
another 90 deg turn to travel to the radiator.

All XK engines prior to about 1968 had the ā€œslitā€ style bypass. If your car is a '58 as your handle suggests, then it originally had a bellows thermostat, not a wax thermostat. The bellows are filled with liquid alcohol, which expands as it evaporates. The boiling point of the alcohol determines the opening point. Since the bellows is dependent on the phase transition of alcohol, the system pressure is relevant to its operation. If the pressure is too high, the boiling point of the alcohol in the bellows becomes elevated. For this reason, cars with bellows thermostats usually have 4 lb caps, and never higher than 7 lbs.

The good news is that if your car is designed for a bellows thermostat, they are being reproduced by Moss. So you can solve your thermostat problems for a price. There are usually lots of old stock examples on E-Bay as well.

The wax version of the ā€œslitā€ bypass was only employed for a few years in the mid sixties, so S1 4.2 E-Types and perhaps some contemporary engines. Thereā€™s really no perfect modern alternative for the NLA wax version. Here are photos of the correct wax thermostat for these later cars. Iā€™d venture to guess that nobody has seen one of these in half a century:

And this is how it opens:

for the sleeve bellows typeā€¦see my prior postā€¦scroll to aboveā€¦Moss Classic Gold is 434-156ā€¦looks like temporarily out of stockā€¦many NOS similar on ebayā€¦one nowā€¦the winter 86F for $35ā€¦you can find the 70-72 Fā€¦Smiths, or Remax or QHā€¦butā€¦must be the correct onesā€¦I listed all the part # in the XK 120 forumā€¦archivesā€¦Thermostats for the XKā€¦Nick

Dear Nick,

Thanks a lot for all your help.

I am now 74th in line to get the back ordered Moss thermostat.

They claim they should have them by the end of November.

However, in view of all those ships circling outside the ports and the present and continuing SNAFU at all levels of government, perhaps it will
be some time into 2025 before Moss will have them.

Thanks again,

Lowell Brown

I have the one I used when I was writing my report. Itā€™s never been in a car, just a pot of hot water. If you want it, You can have it for half the price of new.

nice offer from Michaelā€¦and there are usually some NOS on ebay europe or uk. On some themostat housings, I found that the Moss Classic Gold top lid needed a tiny bit of ā€œdressingā€ around the edge to fit in the housing recess. Soā€¦check it. Nick

Thanks! Letā€™s do it. Where are you located? How much is 1/2 price? Could I pay you with PayPal to your email?

Lowell Brown

PM Sent. Check your messages.

The best you can get is a FAIL SAFE thermostat, it will
stay open when engine over heats,
Just in case if the radiator over heats.


NO! First of all, there are no fail safe thermostats with a bypass sleeve. But for any application, the fail safe idea is a non-starter. The manufacturerā€™s pitch is that when engines overheat, thermostats fail closed. This is only true if the engine becomes so hot that the seal melts, allowing the wax to extrude. But by that time, your engine would be toast. A fail safe will lock open mostly in false alarm situations. And you wonā€™t know that it locks, the only tell is that warm ups take longer. The lock canā€™t be reset, so youā€™ll end up replacing a lot of perfectly fine thermostats.

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please click the little pencil at bottom of the erroneous ā€œfail safeā€ postā€¦and click deleteā€¦thanks, Nick

Michael Frank is a wonderful, generous person. He really helped me out by selling me the proper Mk 1 thermostat at a very generous price, and then he went to all the trouble of mailing it to me in a wonderfully cushioned package.


Nah. Iā€™m just old, I have to declutter before I go. I wouldnā€™t want my heirs to remember me as a pack rat.