A lot of the older radiators were "honeycomb" design, with the coolant passages formed by thin zigzag strips of copper soldered together to form a sort of maze for the coolant to flow through. These were not supposed to be pressurized to more than 7 pounds due to the danger of separating the solder joints. Later model tubular radiators could take much more pressure, up to 12 to 16 lbs or more without damage. If you can see oval-shaped tubes when you look into the top tank of your radiator, you can run the higher-pressure cap. Every pound of pressure on a cooling system raises the boiling point 3 degrees, so a 15-lb. cap will let the coolant hit 250 degrees or more before it boils. A 7-lb. cap would raise the boiling point 21 degrees at sea level, less at higher altitudes.
Jerry
