Series Land Rovers
Coolant Flow & Heat, or why you use the skirted thermostats in Land Rovers by Dave Lowe
The Series III and earlier thermostat housings have an oval opening in the side, which leads to the bypass hose. The purpose of this is to prevent the pump from "dead heading" or churning when the thermostat is closed. In other words when the thermostat is closed to the radiator there would be no flow and therefore no circulation through the engine, which would be most unhealthy. To prevent this happening, the oval opening allows water to by-pass the radiator and circulate water throughout the engine alone.
So, we have a pump that circulates water continually through the engine regardless of the thermostat position. When the engine is cold all the flow goes through the by-pass opening and through the engine because the skirt is raised. As the temperature rises the thermostat begins to open to the radiator but as it does so the skirt descends and proportionately closes off the by-pass opening until the flow is solely to the radiator. If you use the "flat" single acting thermostat the by-pass opening remains open continuously regardless of engine temperature. The pump of course will pump the same amount but the flow will take the line of least resistance which is through the by-pass rather than the long way around through the radiator, and under high ambient conditions will cause the engine to run hotter than it should. The temperature sender of course is at the front of the engine and does not give you the temperature at the no.4 end of the block. You will therefore have a condition where the temperature on the gauge is reading a bit on the high side but the temperature at the back is significantly higher. I consider this to be one of the causes of the manifold cracking and I`ve cracked a few before I found out about the operation of the double acting thermostat. I used to use the flat type because I could get them for 195.F.
On engines from the 110 onwards the thermostat housing was changed and a flat single acting thermostat is standard. Instead of the large oval opening it has a hole about 3/8" in diameter instead. This is calculated to provide sufficient flow when the thermostat is closed but has a higher resistance than the radiator circuit, consequently the line of least resistance when the thermostat is open is through the radiator not the by-pass.