Triumph TR3 drawing

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How a Triumph skirted thermostat works

The Triumph TR2 through 4A engines are designed to have water circulating through the cylinder head at all times to prevent localized hot sports and head cracking. There is a passage that allows coolant to bypass the radiator when the thermostat is closed and circulates the coolant within the engine.

The thermostat housing has three passage ways. One is the coolant inlet from the cylinder head which is always open (flow is radiator bottom, to block, to head then radiator top).  Another is the outlet to the radiator which the thermostat keeps closed while the engine is cold and opens when the engine gets up to operating temperature.  The third passage is the outlet to the bypass passage which bypasses the radiator.  The thermostat keeps this passage open when the engine is cold and closes it when the engine gets up to operating temperature.  A skirted thermostat can be thought of as a two way valve that opens one exit passage in the thermostat housing while closing the other exit passage.

Like all common thermostats, a skirted thermostat has the standard temperature operated plate that opens and closes controlling coolant flow to the radiator.  In addition it has a cylinder or skirt that moves when the thermostat opens and closes. This moving cylinder or "skirt" is the coolant flow valve that regulates the flow of coolant through the radiator bypass pathway.  Bottom line is that a skirted thermostat allows coolant to flow at all temperatures and sends coolant either through the radiator or bypasses the radiator depending upon the engine temperature.

A common style thermostat will only open and close the passage to the radiator and ignores the bypass passage.  This means coolant is always flowing through the bypass and not getting cooled in the radiator. The engine runs hotter and more readily overheats. Some people have tried blocking off the bypass to keep the engine from overheating when using a standard skirtless thermostat.

If a skirtless thermostat is used and the bypass blocked off there is very little to no coolant flow through the engine until the thermostat opens.  Since heat cannot be quickly conducted away from the cylinders, hot spots develop. These can be hot enough to cause local boiling of the coolant into steam. The steam forces coolant back away from the hot spots letting them get even hotter.  The steam can force the coolant out of the head and reach into the thermostat housing where it quickly heats the thermostat. The super hot steam forces the thermostat to fully open immediately, allowing the steam to pass into the cold coolant of the upper hose where it rapidly condenses.  The now wide open thermostat allows cold coolant in the radiator to be rapidly pumped into the engine with consequent rapid cooling of the head and block. This commonly causes a cracked head and occasionally a cracked block.

Even if the coolant does not get hot enough to force steam into the thermostat housing there will be local hot spots within the head that causes uneven metal expansion followed by rapid cooling then the thermostat opens. This will also cause head cracks over time.

To make things more interesting, the temperature gauge sender is located at the front of the engine in the thermostat housing. If most of the water bypasses the radiator, by going through the bypass, temperatures at the rear of the head can be significantly higher than your temperature gauge indicates.

If you insist on using a skirtless thermostat what you want to do is partially block off the flow of coolant through the bypass allowing only a small amount of coolant to bypass the radiator. A bypass passage plug of some kind with a 3/8ths inch diameter hole for cool & moderate climates (like Seattle or San Francisco), 1/4  inch for hot climates (like Phoenix) will usually do the trick.

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