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Thermostat - Fail Open or Closed


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A recent incident with my 1920's car has raised a question regarding thermostat design. I had adapted a NOS thermostat from an unknown make and it failed in the closed position causing a very rapid rise in engine temperature because the thermostat isolated the engine cooling from the radiator. Fortunately no damage occurred as the car was only travelling slowly and I stopped immediately.

However it raises a question, Why do so may thermostats (or at least the ones I have looked at) have a spring holding them closed and they open against the spring when heated. With this design if they fail they fail closed with potentially very serious consequences.

Surely a better design would be for the thermostat to have the spring holding them open so that they close on heating against the spring. That way if they fail the worst that happens is the engine is overcooled.

I am a now looking for this type of thermostat - one that would fail open.

I asked Restoration Supply Co whether the thermostats they sell fail open or closed but their answer indicates they did not understand the question.

Can anyone comment on the Restoration supply thermostats or suggest any other units.

The thermostat needs to open around 160 deg F as the cooling system is not pressurised so modern ones are not suitable.

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I don't know of any thermostats the go to open during failure. A lot of modern thermostats are available in 160 degree.They might not be in stock but can be ordered. Tell us what car you have and someone might be able to recommend a thermostat that will work.

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That is what I have found but I cannot understand why it is so. Some years ago the thermostat in my modern car failed closed cooking the engine, it was being driven by a relative who did not realise the problem and it caused a cracked head and blown gasket. In my view the thermostat valve should be held open by the spring and closed on heating so that it fails open. However if such devices are not available then I will take my chances with a new thermostat as you suggested.

The car is a 1929 Packard 633, it had a thermostat originally and it is not difficult to adapt a modern one to fit the housing. An even easier solution is to use one of the thermostats that Restoration Supply Co offer that simply clamp into the radiator hose.

The car also has thermostatically controlled radiator shutters that work but they open too early allowing the car to run a bit cool (140 deg F)

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Thank you Wildcat465, I will contact MotoRad to see if they have a thermostat suitable for about 160 deg F. The cavity in the head where the thermostat is mounted is quite large so it should be possible to adapt one. Exactly what I am looking for - thanks again.

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If the engine overheats I would much rather that, if the thermostat fails, it fails open. With "normal" thermostats when they fail closed they isolate the engine from the radiator causing extremely high engine temperatures instantaneously often too quickly to avoid damage. The worst that will happen with the FailSafe unit is that it will be wide open and with no control the engine will overcool. This is a vastly preferable failure mode to a cooked engine. Also the radiators on these old cars are marginal in capacity so overcooling is not a short term issue. In my case with the failed thermostat removed the car was running around 140 deg.

I am currently in contact with the vendor, they have 160 deg fail safe units, we are looking for one that can be adapted to suit.

The vendor's web site states that the FailSafe units will lock open if the car overheats. As these old cars are inclined to overheat under adverse conditions I have asked whether the locked thermostat can be removed and reset.

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