In working on our old cars, we all know the importance of a clean and efficiently-working cooling system. When a car has sat for a long period of time, one of the first things we do is to go through the entire cooling system, rodding out the radiator, checking the water distribution tube, removing the freeze plugs, cleaning out the block, making sure the water pump is working and the thermostat opens at the right degree; many times overlooking the obvious, the radiator cap. If it didn't appear to be leaking, I usually didn't change it. Recently while reading through some Packard and Buick service letters, I read that 80% of overheating problems experienced in their shops were caused by faulty radiator caps. I then checked the caps on all of our cars and noticed a difference in how the caps were made. Some were short, some were long, while some had pressure systems and some did not, and I wanted to know more about why the systems are different.

        The basic type of cooling systems are liquid and air, pressurized and non-pressure, and open and closed. The open, an older kind of system, was a radiator with an overflow pipe which expelled coolant when hot and pulled air into the system when the system cooled down, reducing cooling efficiency. The closed system uses a special radiator cap and overflow reservoir tank, making what used to be the overflow into a connection between the radiator and the bottom of the reservoir, thus eliminating air in the system and providing better cooling. We will cover more on the open and closed systems in a future article.

        The early cars had non-pressure systems, in which the water circulated via a water pump without pressure, boiling at 212 degrees Fahrenheit. In the late '30s, many car manufacturers began using pressure systems. They found they could raise the boiling temperature by increasing pressure in the system. For each pound of pressure exerted on the systems coolant, the coolants static boiling point is raised by approximately 3 degrees F, ie: a 4# cap boils at 220 degrees F (at sea level), 7# at 230 degrees F, 9# at 234 degrees F, and a 13# cap at 243 degrees F, etc. The higher pressure also transfers heat from the cylinder heads in a more efficient way.

        During this time, Harrison Radiators were used by GM and other car manufacturers along with AC caps. There were 3 cap designs: The Internal Valve Type, named because of the internal location of the pressure relief valve, and was used with a flat bottom filler neck requiring a fibre gasket to seal against both pressure and leakage. (Figure 1) The internal valve controlled the maximum pressure, but was dependent upon a tight seal at the gasket joint. The original installation was for passenger cars intended to control the loss of coolant due to surge aeration and after-boil expansion. During WWII, a larger model of this same design was developed for use on military trucks and tanks.

        This pushed the limits of the design, showing its weakness which was gasket damage while filling the radiator. To remedy this, the filler neck was turned down, creating a flange around the hole. (Figure 2) Even with the flange, there still was leakage caused by a dried out or damaged gasket.

        This led to the development of the External Valve Type cap. (Figure 3) This used a beaded surface in the flange of the filler neck to seal against both pressure and cap leakage. In this design, the valve is faced with a soft Neoprene washer which compensates for small irregularities in the sealing surface, eliminating the need for a fibre gasket. This type of cap provided for a more reliable control of rated pressure and eliminated the possibility of gasket leakage.

           After the war, the carshoods became lower, creating a problem with the taller radiator necks. A maximum neck height was proposed and adopted as standard to permit maximum radiator height in the reduced space. The cap and neck were thus revised to fit this minimum height (Figure 4). During this time, the tubular core radiator was developed to replace the cellular core type, to withstand the higher pressures. In order to prevent the installation of a high pressure cap onto a low pressure radiator, A series of caps and necks were designed with the widths of their locking tangs corresponding with the slot widths in the filler necks. The proper cap for use with a radiator can be selected by measuring the width of the slot in the filler neck as follows: 1/2" slot width = 4# cap; 5/8" slot width = 7# cap; 3/4" slot width = 9# cap; 7/8" slot width = 13 # cap. (Figure 5)

                Even with the above precautions, it is still possible to put a worn cap of the long external valve type into a short-designed filler neck. By doing this, the compressed length of the long cap completely fills the space in the short neck, keeping the valve from opening to relieve pressure by expansion and vaporization, resulting in serious damage to the radiator core and tank. Newer caps with sleeves enclosing the spring, help to prevent the spring from being compressed enough to fit in a short neck. It would be something to check when you buy a NOS or a NORS cap.

        In conclusion, some heating problems may be eliminated by using the correct cap, and when an oversized radiator core is installed, check with your radiator shop to see if going to a higher-than-OEM recommended cap would be beneficial. Keeping the engine temperature low can add years to the life of your engine.

        See you next month.  Keep 'em driving!