must be said here: automobile manufacturers don't make thermostats. They
don't design them either. They are at the mercy of those that do. We've
seen how the cooling system of the S1 3.8 was designed around the
standard Smith's bellows thermostat. How the S1 4.2 improved on that with
a Western Thomson wax version of the sleeved bypass. But by the time of
the Series 2, the thermostat industry was producing the sort of designs
that would be standard for the next half century. The Waxstat thermostat
used by Jaguar in the S2 and S3 E-Type and later models is essentially
the same architecture used in many cars both of that era and later:
Porsche 944, Pontiac Firebird, Mercury Grand Marquis, Saab 9-3, AMC Concord,
Jeep CJ-5, and many others used more or less similar thermostats of approximately
the same size and design. Whether Jaguar should have 'pride of first' can
be debated, but from Series 2 onwards, there's nothing especially special
about Jaguar thermostats.
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.
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.)
2 housing is fairly simple. Water flows straight through the intake manifold,
and exits through the thermostat. The bypass passage is oriented exactly
180 degrees opposed to flow, and exits through an "ear" in the casting.
This is connected by a hose to the the water pump inlet, which it meets
at right angles. It's almost a textbook design, except....
Jaguar did so many things right, but every E-Type cooling system has some sort of inexplicable oddity. This housing is a good example. A jiggle pin is pressed into the mating surface of the casting. The only purpose of the jiggle pin is to allow air to bleed out of the thermostat housing during initial fill. Once the system is charged with coolant, it serves no other purpose. This jiggle pin mates to a nipple in the thermostat cover, which leads to a small hose, which ultimately terminates in the right upper tank of the radiator. Just about every thermostat has some provision for air bleed. Either a jiggle pin, ball valve, or a tiny hole. Even if the thermostat supplier didn't do this, Jaguar themselves could have simply drilled a small hole in the flange and achieved exactly the same effect. Instead, the pointlessly redundant jiggle pin added cost and complexity to the casting, and required yet another hose for an engine that already had twenty potentially leaky hose connections. But no worries, they added a bottle of Bars-leak to every new cooling system.
Choosing a new Thermostat
As discussed, the S2 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 S2 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 flange is 54mm in diameter. This is probably the most common size in use today. 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.
valve is 1.125" . The exact diameter matters little unless it's so large
as to restrict flow. The actual bypass port is much smaller, .750".
The stock S2 thermostat was designed to open at 75C (168F), but 82C (180F)
was recommended for "winter use" and should be fine year round. Late Series
2 cars (starting around June 1969) all came with 82C thermostats for emissions
reasons. When changing to a hotter thermostat, thought should be given
to the set point of the otter switch.
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.
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, Unipart, Quinton Hazel 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.
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 venture, 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:
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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