If it's not boiling coolant out past the radiator cap, it's not overheating. I assume the 0.9 cap is a metric measurement, .9 BAR which converts to 13 PSI. A 1941 cooling system should probably have a "zero pressure" cap, but one that gives a maximum pressure in the cooling system of 4 PSI will probably work OK. The cap you have might present a danger to the radiator core, and/or the heater core if the system ever pressurizes far enough to bypass the cap. Every pound of pressure on coolant raises the boiling point 3 degrees, so a 4 lb. cap would allow the coolant to reach 224 degrees before it boils. A .9 BAR cap will allow coolant to reach 251 degrees F. before it boils, and that much pressure will destroy your radiator.
Jerry
