Problem with the old screw cap is you dont get any pressure so the coolant will boil at a lower temperature, you will lose more coolant and need to regularly top it up. One way is to connect the overflow pipe to another container (header tank) that has a pressure cap just like on many modern cars. The header tank will have its own overflow pipe to atmosphere under the car somewhere. Alternatively, take the rad to a radiator repairer and get a pressure cap neck fitted. Third option - get a later model Prefect or Pop rad as these had pressure caps. Good luck, Dave ps is your Pop in "The Garage" ?
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