Many thanks for taking the time to reply John in a bid to enhance my understanding, which in Morgan terms is based upon my mid 80`s +8 and which differs from that fitted to your Morgan.
I am somewhat confused to think that although the expansion tank is exposed to whatever pressure might be applied within the system as the engine warms up causing the airspace within the tank to reduce and thus compressing the air up to a possible max as determined by the blow off point of the rad cap... That this increase in air pressure may not play any part in the return of the coolant to the rad, regardless of whatever height the tank might be at....?
Again with limited understanding I thought syphoning required some measure of initiation, and if the pressure cap is in place on the expansion tank providing sealing to to the system from atmosphere, it would seem syphon may be unlikely to occur..?
Again using nothing but my limited logic... I can imagine that there may be a possibility of some siphon occurring due to the effect of gravity on the coolant within the tube connecting the rad to the expansion tank, assuming the run of the tube is such that the end of the tube connected to the rad is higher than the end of the same tube connected to the expansion tank.... However for the syphon action to begin, it seems logical that the process might only begin if the pressure cap was removed or failed to seal the expansion tank and thus the whole cooling system as it is designed to....?
Even when cold and no pressure in the system as I understand it, with the pressure cap in place on the expansion tank and thus sealing it, I fail to understand as to how syphoning might occur.. I guess if one has fitted an expansion tank where the level within it can not be viewed without removal of the pressure cap, then dependant on the height of the tank mounting relative to the coolant level in the rad, some degree of syphoning could then occur..?
As for the tank it`s self, I suspect all that matters relative to it`s construction is that it can safely operate at internal pressures in excess of that which the rad cap is set to blow off at. The tanks internal volume is such that it can contain the amount of coolant expected to be forced into it as the engine rises to operating temperature. Also that the tank is designed to be capable of maintaining the amount of coolant at normal levels when cold. I also suspect that the positioning of the tube that links the tank to the rad is such that it may be near the bottom of the tank. Also that when the engine is up to normal running temps that there is enough airspace maintained above the coolant, and perhaps finally that the siting of the connection for the blow off tube is such that it directs whatever is expelled, in such a way as to avoid danger or damage to people or equipment..
While rambling on perhaps it may also be worth mentioning that the tube and clips connecting the rad to the expansion tank will be designed to operate at both the max temperatures and pressures expected to be created within the system...
As ever I am keen to learn where I might have got it wrong...
