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Thermostat Basics
Modern thermostats do a great job of regulating coolant temperatures in today's engines. Although they can get gunked up and stick, it is rare to see them fail in this manner. They  are actually quite simple in their design, yet perfectly functional for the task at hand.  Even better, they are inexpensive and generally easy to replace (and worthwhile to do so)  any time your cooling system is opened up. For emissions purposes, optimum engine temperatures are approximately 212° F (100° C),  regulated by the opening and closing of the thermostat to cycle the coolant. While many  vehicles use stock thermostats rated from 180° to 195°, some even come with a 170°  version right from the factory. In any case, the thermostat is an important part of the  cooling system necessary to regulate engine temperature. This diagram illustrates the components of a thermostat, in both the  cold (closed), and hot (open) positions. The "heat motor" is actually a  metal cylinder and "valve" (item #7) filled with wax that will melt at a  calibrated temperature. When the wax heats up and melts, it expands  and squeezes the piston, or "Thrust pin" (Item #2). Since the piston is  fixed and cannot move up, the heat motor in turn is forced down, or to  the open position. The coolant will then flow past it until the  temperature of the wax eventually drops, causing it to harden and contract. This then  closes the thermostat against the force of the main spring and the cycle continues. Expanding on that slightly, modern cooling systems will accommodate  thermostats that are for the "hot side" or "cold side", depending on the design  of the vehicle. A hot side thermostat is simple enough: the thermostat is  mounted on the engine (i.e., the hot side), and when the coolant gets hot, it  opens and allows it to flow out to the radiator, which dissipates the heat.  A cold side thermostat, on the other hand, is mounted off the engine in a  separate housing. The coolant will circulate through it continually from a  bypass line, and the thermostat will open when it reaches its specific  temperature. This allows the lower temperature coolant to then flow in  from the radiator, towards the engine (indirectly letting the hot coolant  out as well), blocking off the bypass. These systems are meant to allow for  a faster warm-up, while also regulating the coolant temperature more closely by way of  the remote thermostat. While tending to be a bit more complex in layout, they still  essentially do the same thing: transfer hot coolant through the radiator where it will  have its temperature reduced. Again, a thermostat is not complicated, but in order to function properly, it must be an  exact fit for its location. It also has to operate in the manner dictated by the flow of the  coolant (installation of a hot-side thermostat in a cold-side system will not accomplish  this). My thermostats are perfectly suited to this task, and are meant to work with your  Ford hot- or cold-side system to give precise coolant flow accompanied by a reduction in  engine temperature.
Thermostat Basics