Question on thermostats

Would anyone happen to know when the saloon cars switched from slot bypass thermostats to dual poppet thermostats? For E-Types, this happened with S2, so around 1968. I’ve been assuming that it happened at the same time for saloons, but I need to know for sure. For reference:

Dual poppet:

Slot bypass:


Smiths side

any particular reason for the Q? or just curious

Trying to get a manufacturer to correct their catalog. I know the E-Type applications, but I may as well deal with the saloon listings at the same time.

my xj6 s2 had 2 listing here in Aus/ one for the OEM 1/2 blocked side & other for the double head, i checked again today an now it’s just the double head…trying to get a maker to change something!!! think you might have better luck getting your God to give you next weeks winning lotto numbers

The problem is that there has been a consolidation in the industry. The now dominant manufacturer in the US has all the listings wrong. Are you saying that the XJ6 S2 has a slot bypass?

it didn’t have the full bowl on the bottom but a 1/2 brass plate on one side to divert the flow during warm up

By any chance, could you point me to a picture?

trying to find one but no luck yet, jaguar sedans in Australia mainly late 60’s 70’s were assembled in new Zealand, there were a couple of differences to the UK market, can’t name them all but do remember having to replace the thermo/ local auto supplier had the by pass type for it or the double header, i took the double header. not the bypass type not even listed.

it probably was discontinued 'cause if you didn’t pay attention to how you fitted it, then the purpose was useless.

This is C28370, which I believe is the housing for S1/S2 Xj6. It takes a standard dual poppet thermostat. Is there a different housing?

can’t confirm that, i have that standard thermo/ in but i also fitted a recovery tank near the battery that continuously cycles the water

I’m pretty confident that anything 1968 or later would have a dual poppet thermostat, early ones around 75C and later around 80C. What I probably need to know is where the transition was for the Marks, 420, 420G.

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Michael, C28370 housing is listed in the parts catalog for all S1 4.2 XJ’s and they all use the same standard dual poppet thermostat, c33012. I’ve never seen a slotted thermostat in any XJ.

While I was working on adapting a Mk10 4.2 triple carb manifold to one if my XJ’s I found 3 distinct versions of the 4.2 Mk10 thermostat housing. It’s hard to pin down the exact year when the slotted bypass ends but the '66 Mk10 I’m working on - which is just before the 420G changes started - has the slotted style.


My 3.8S has a slot in the housing leading to the bypass hose and I used this type thermostat which is intended to block the slot off when the engine is warm and thermo open but I was never convinced it really sealed it off very well. So I fit a restrictor inside the bypass hose which I guess works at least sort of.

Welsh thermo

That particular thermostat was a gross error. Someone had the idea of cutting down a Land Rover thermostat, something that sort of works. But they cut down the cap without also cutting the sleeve.

I’m getting the understanding that the saloons followed the same development timeline as the E-Type. So maybe I should ask in a different way:

Does anyone have an example of a slot bypass after 1967, or of a poppet prior to 1968?

I’m in the process of trying to cool a 62 MK II 3.4 and located a “no foot” thermostat in place. Obviously, this leaves full flow to the bypass and the radiator was also remarkably cool. I too was skeptical of the “sleeve” thermostats and am contemplating the hose restriction.
Rather than a complete blockage I intend to drill a small hole through, maybe 3/16, so there’s some flow. Are there any ideas out there around this subject or are we left with trial an error?

Many thanks

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I fitted a restrictor in the bypass hose with about a 1/4 inch hole in it. Not able to do a definitive test on how this works since i also have a thermostat controlled pusher fan in use as well. No cooling system tests now as weather here in Oregon has turned cool. No idea where you are located and currently trying to cool your Mark II. You might want to update your Jag-Lovers info to show your location on the globe.

Many thanks, John and my apologies regarding my out of date details. I’m located in Southern California where temperatures are cooling somewhat also. I took delivery of the car a couple of weeks ago and the following day was 95 and the day after 85. Driving on both days resulted in temperatures on the gauge around 90-100. The car has an aluminum radiator and a pusher fan also. It also has A/C with it’s condenser in front of the radiator.

You seem to have done all the normal upgrades. For what it’s worth, my 3.8S with stock radiator, pusher fan, no AC, will in ambients of 90F+, run about one needle’s width below the mark on the stock gauge that is halfway between the 70C and 110C marks which would be just under 90C.

Before you jump to solutions, verify the accuracy of the gauge reading. I’m finding that sensors are poorly calibrated. You can use a plastic temperature strip beside the sender boss to get accurate readings.