Packard Forum Index » Pre-War (1899-1942)

The 1939 systems were non-pressurized with the radiator having very large cans that cannot tale any pressure, They might get by with a 4lb cap but no more than that! I once tried a higher pressure one and watched the tanks grow before quickly taking the cap off before any serious damage happened.

Most of the unpressurised radiators have the overflow tube below the level of the cap seal seat. So chainging the radiator cap won't many any difference, unless you sealed off the overflow tube.

Don't seal off the overflow tube!
The cap should have a gasket to prevent water loss at that point. The overflow tube inlet is located near the top of the tank to maintain water level.

With the car sitting level and the engine off and cool, it should be possible to fill the radiator completely full to the level of the cap. At that point some water should run out the overflow tube until the water level is about 1/2 to 3/4 inch below the filler neck.

Refer to the Literature archive 1939-'39 shop manual for cooling system info.