Justin, On just about every engine that I know of there is a hole below the=20 thermostat. When the thermostat is closed coolant is forced by the pump=20 through this hole. This hole is called the bypass hole because coolant=20 will bypass the radiator and reenter the engine. I said that if the hole=20 did not exist that there would be localized overheating in the cylinder=20 heads around the exhaust valves and the spark plugs. The combustion=20 chambers are the hottest parts of the engines. If coolant is not=20 circulating through the engine the heat will build up in the cylinder=20 head/s to dangerous levels before the thermostat can open. Also if there is no circulation through the engine the only way=20 that heat can get to the thermostat is by conduction through the coolant=20 and through the metal of the engine block and cylinder head. This is a=20 very slow process and the engine will over heat before the thermostat=20 can open. If you have a thermostat that doesn't have a small bleeder=20 hole in it you can get an air bubble that will insulate the thermostat=20 and make it very difficult to bleed the cooling system. You can ruin an=20 engine if you aren't careful and you overheat the engine. I know of people whom have done this. When I had rebuilt an engine=20 I would leave the thermostat out and the thermostat cover off. I would=20 then fill the radiator up until there was coolant showing inside the=20 thermostat housing. I would then install the thermostat. I would=20 continue filling the cooling system until I had a small amount of=20 coolant coming out of the thermostat cover. I would then install the=20 upper radiator hose and finish filling the cooling system. I have=20 rebuilt thousands of engines in my career and over a thousand rotary=20 engines. I have never had a problem bleeding cooling systems. In fact a lot of engines have a reverse flow design. The coolant=20 enters the cylinder head first and then it flows into the block. The=20 thermostat is mounted at the bottom of the water pump. These engines are=20 very hard to purge and it was pretty easy to get an engine hot enough to=20 warp the cylinder head and ruin the engine. I actually know of people=20 whom have done this including some professional technicians. The=20 benefits of running the coolant backwards is that since the cylinder=20 head is the hottest part of the engine it is easier to maintain a=20 balance of heat flow through the engine because the cool coolant is=20 cooling the hottest part of the engine first and then it flows into the=20 block which isn't exposed to as much heat. This makes the engine run at=20 a more even temperature. The important thing here is to use a thermostat with a bleeder hole=20 or a tickler and to be sure to install the thermostat with the bleeder=20 hole at the top of the thermostat if the thermostat mounts in a vertical=20 direction. Now the other part of this equation is how big is the bypass hole,=20 how large is the radiator, how large is the load on the engine, and how=20 hot is it. The bigger the bypass hole, the smaller the radiator, the=20 heavier the load, and the hotter it is the more likely it is that the=20 engine will over heat if there is no thermostat in it. Like I said=20 earlier most Detroit iron was immune to this. They had small bypass=20 holes and large radiators. Most European vehicles weren't. Some Japanese=20 vehicles weren't. So it depends. Your teacher had no background in the laws of thermodynamics. This=20 is an old wise tale that has been passing around ever since the first=20 internal combustion engines came out. It is repeated by people who do=20 not understand how heat flows and that if you run a cooler fluid across=20 a hotter surface the faster the coolant travels the more heat that will=20 be removed from the hotter surface. You were correct in having the=20 feeling that your teacher didn't understand how it worked. I still run into people who believe that if you leave the=20 thermostat out the engine overheats because it is moving too fast to=20 remove the heat. Again what is happening is that enough coolant is=20 flowing through the bypass hole that the temperature builds up and the=20 engine overheats. Thanks, rich! On 11/24/2014 5:45 AM, Justin Richards wrote: > I am happy to accept that what you say is true. But I would like to > understand the underlying theory. > > I have only limited experience with old Holdens and old Fords. > > >From memory the flow of coolant left the bottom of the radiator after it > had been cooled and made its way to the pump. After the pump it made its > way to the block. And lastly thru the thermostat to the top of the radiat= or. > > I think (I am most likley wrong) it had only one path from the pump to th= e > block so it was unable to circulate coolant other than thru the block and > back into the radiator. > > As you say there have been many makes and models of engines and perhaps > they allow coolant to circulate freely while the thermostat is in its > closed state but from memory this is not what I have seen. > > In any case I am unable to understand how it would overheat if the return > flow to the radiator was less impeded and would welcome a more detailed > explanation. > > I did like the explanation with regards to air pockets forming in the Mia= ta > that was previously discussed but I finding it hard to apply this general= ly. > > My motor mech teacher taught that the coolant didnt have time to collect > the heat from the block if the flow was too fast and I always had > difficulty accepting this. > > But I have since pondered why he said this and thought there must be hidd= en > truth. > > So in conclusion, if the coolant can circulate thru the block while in th= e > closed state or if air/steam pockets form while in the removed state then= I > must accept that engines may overheat if the thermostat is removed. But I > dont think this applies generally. > > Please correct inform me either way. > > Curiously Justin > > PS closed state refers to closed with a bypass hole. > On 24/11/2014 6:11 PM, "Richard R. Pope" wrote: > > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .