Agreed Rog. In essence the permanent narrow bore bypass flow from cylinder head(s) squirts onto the temp sensing side of thermostat facing this flow in a mixing chamber. The other side of stat receives cooled coolant via rad bottom hose. The heater return also squirts into mixing chamber, I'm guessing causing turbulence to mix flows which then feed to water pump inlet. Relative flows determined by hose sizes so rad gets priority once stat opens.
I understand in the case of less powerful engines with a larger after market radiator, once stat opens the engine over cools so it closes again. The greater hysterises can then allow coolant temps to rise above normal, on say a motorway run, taking some time before stat finally opens again when temps drop. I believe the solution was a lower opening temp stat. One with a greater temp range between starting and fully open would have maybe been a better option.
