Radiator Caps Restoration

One of the earliest pioneers of engine cooling was Carl Benz who, in the mid-1880s, devised what is called the Thermo-Siphoning system of cooling. No water pump was used, as the system utilized the principal that hot water, being less dense than cool water, will rise in a non-vented cooling system. This hot water will cool as it rises through the radiator and then gravity-feeds back into the engine. Primitive, but effective, for the low-performance engines of the time.


What many people don't realize is that the radiator cap is one of the most important parts of your cooling system. A faulty spring in the cap, or using the wrong cap, can result in an overheating problem even on a new or re-cored radiator. Water normally boils at 212-degrees Fahrenheit at sea level, but using the correct radiator cap can substantially raise that boiling point in your cooling system. Each pound of cap pressure raises the boiling point 3.25-degrees X cap pressure. So, if you're running a 15-lb. cap, then you've effectively raised the boiling point of your system 48.75-degrees or up to 260.75-degrees. Most Sixties and Seventies performance cars' normal operating temperature was between 160-180-degrees. Some systems were also equipped with relief valves at the cap to allow coolant to escape or to be retrieved from overflow tanks.


If you don't already know, the best way to find out what cap your car requires is to check the cap application charts at your local auto parts store. There were several types of caps used during the musclecar years and reproductions are generally available. If not, then your only choice will be an over-the-counter replacement with the correct pressure designation for your vehicle.

Closed-System Cap
This is a late-Sixties/early-Seventies cap used on most GM muscle cars. It is a 15-lb. pressure cap for a closed system. At this point, the caution "Never Open Hot" was imprinted into the cap. This cap also had the instructions to "check fall and spring"—like you would really do that—and it was manufactured from stainless steel to prevent it from rusting. The closed-system cap features "short" ears for easier removal and replacement. Obviously the warning is in reference to the pressure built up in the system during operation and the fact that the hot coolant can literally explode out of the radiator and cause severe injury.

Pressure-Release Cap
The cap shown here is a lever-style, pressure-release cap that can be purchased over-the-counter at most auto-parts stores. It's a 13-lb. cap for today's cooling systems that utilize an overflow and recovery system. The coolant is topped off through the recovery bottle and not directly into the radiator. The warning "DO NOT OPEN WHEN HOT" leaves a lot to the imagination—but don't press your luck. The best advice is to NOT open the cap at all when the radiator is still hot—wait until it cools to the touch. Many restorations are equipped with this style cap for daily driving and the restoration cap used only for show.