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A 'sealed' system is a more modern system. The system is designed to keep the radiator itself completely full, without air in it. Older styles where you simply had an overflow system that would not allow excess coolant to be sucked back into the radiator had the top couple of inches of the radiator exposed to air and allowing for expansion increased the rate of corrosion inside the radiator. An expansion tank is the best way to go! Any radiator can be converted to have an expansion tank. You need to have the proper radiator cap that will allow coolant to be drawn back into the radiator when it cools, and you need to have the tube between the radiator and expansion tank submerged in coolant so that it can draw the excess coolant back into the radiator. The expansion tank needs to be vented so that it is not pressurized and can allow the coolant to flow back and forth.