I've discussed this thermostat in early chapters...it's a dual poppet, wax motor thermostat. The main poppet is pressed onto the wax motor, and moves backwards, against the direction of flow. The bypass poppet "floats" on the opposite end of the wax motor, and has a light spring that allows some variation in casting depth. 

When the system is cool, the bypass is wide open and the main poppet is closed. As the engine warms, the  main poppet opens and the bypass poppet closes. In between cold and hot, both the main poppet and the bypass are partially open. In this way, a continuous flow of coolant circulates around the block at all times. The bypass isn't completely sealed unless the thermostat is at its max opening height. (If start of open is 80C, then max open will be around 90C.)

Architecture

The unique complication of the Jaguar V12 is that each bank is essentially separate, and has its own thermostat. Both sides of the system are fed by a common pump, and they share a common bypass tube.  The gauge sensor resides on the right and senses the right bank only, so it's possible for the left bank to be failed completely closed. If this happens, the gauge will still read almost normal temperature. The shared bypass tube is a partial fail safe for a failed thermostat, because coolant will continue to flow through the system and half the flow will still go through the radiator. So the engine can still be driven under light loads. The important warning here is that any increase in running temperature should cause the thermostats to be suspect, and they should be replaced in pairs. 
 
 
 

Right Housing	Left Housing

There are two thermostat housings on a V12, and they are symmetric. Water enters each housing orthogonal to the thermostat, creating balanced force on both valves. This reduces the stress on the wax motor, and should prolong its life and limits the lifting effect of the flow. The bypass passages drain downwards into the common bypass tube. Only the right side is drilled for a sensor, but there is a boss on the left which could be used for a second sensor. 

Choosing a new Thermostat

As discussed, the S3 thermostat is nothing special. Many, many cars have used this pattern through the years, and so there are any number of choices still available over the counter. The only hitch is that the thermostats that are actually listed for the S3 E-Type aren't necessarily correct, so you need to know what to ask for. My research was done in the US, so folks living elsewhere may find these selections unavailable. But I also supply specs in case you have to use a different brand. I can't vouch for quality, nor can I offer negative reviews. All I can attest is suitability.

First of all, some specifications. The thermostat used is the same 82C Waxstat as the late S2. The thermostat flange is 54mm in diameter. The depth of the bypass lip  below the flange is 1.607 inches (about 41mm). My guess is that the cast depth started out at 1.6 inches, and a bit was taken off to provide a smooth sealing surface. This measurement is from the bottom of the thermostat flange. Since most of the thermostats in this pattern have an 8mm travel, if the thermostat measures 34-35mm cold, you should be fine. 

The diameter of the main poppet on the original thermostat was 1.060". Larger poppets increase flow volume, but reduce velocity. This would be good for higher RPM operation. Smaller poppets have the opposite effect, and are better for street use. 

The bypass valve is 1.125" . The exact diameter matters little as long as it's large enough to control the bypass port.  The bypass port is much smaller, .750". The stock S3 thermostat was designed to open at 82C (180F). It's acceptable to use the earlier S2 74C thermostat if you desire a cooler engine. When changing to a different thermostat, thought should be given to the set point of the otter switch.
 

There are many thermostats that will work for this application. In the following table, those in Green are recommended, those in Yellow are acceptable, those in Red are not recommended. Since the S2 thermostat housing has an integral air bleed, it's not necessary to have an air bleed or jiggle valve in the thermostat itself. However, as noted in the table, Gates 33747S has a reversed jiggle valve, which will somewhat extend warm up time. Several of these thermostats are badge engineered, and are identical products. For example, Gates/Stant products appear to be identical products. Waxstat/Unipart/QH are also identical products. Tridon thermostats are manufactured by Motorad, but appear to have somewhat different specs. As a group, the Motorad thermostats seem least likely to have proper bypass control. Either the travel is too short or too long. Wahler and Borg Warner are identical products. They just barely extend to the bypass port. 
 

Q&A:

Corporate cousins? Stant and Gates have been owned by the same entity in recent history. Their thermostats have different part numbers, but their comparable products are indistinguishable. Every thermostat I tested from these companies provided 1-2mm of bypass spring compression at full open. 

Single source? Comparable products from Tridon and Motorad are indistinguishable, and likely come from the same source. None of the products from these companies tested with the correct bypass height at full open. Each thermostat was 3mm short cold. 

What is a high flow thermostat? A high flow thermostat will have a poppet size larger than stock. High flow helps in high RPM operation, but not ideal for stop and go driving.

What is a SuperStat? A SuperStat (trademark of Gates/Stant) uses a weir-valve rather than a plain poppet. Weir means "wall" or "dam". A weir-valve has a cylinder around the rim of it's poppet that restricts flow at low lift. This would just delay opening, except that the weir has a series of V-shaped notches, so that flow increases progressively as the engine warms. This is intended to help the thermostat find a stable equilibrium state in cooler, low flow situations. When the thermostat is fully open, the weir valve will perform like a standard thermostat. The standard version of these thermostats have a taller venturi, but the flow path diameter appears to be identical. I have no facilities to test whether the design actually improves temperature stability, however it seems to be a reasonable approach. Superstats are usually nicely polished, which mostly has bling value on the sales desk. Here's a comparison of open thermostats:
 

Standard Thermostat	Superstat

What is a fail-safe thermostat? A fail-safe thermostat has a trap catch that will lock the thermostat permanently open if a trigger temperature is reached. The thought is that the thermostat might fail closed if the car overheats, but this is unlikely. The fail-safe thermostat doesn't solve any real world problem, but it does guarantee that you will need a new thermostat if the engine gets too hot. None of the available fail-safe thermostats are a good fit for the S2 E-Type.

Should I fabricate an insert to extend the the height of the bypass port? It's possible to use a "short" thermostats if an insert is used in the bypass port. But it's better to use one of the recommended thermostats. Too much height will cause excessive spring compression and will cause the thermostat to fail prematurely.

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