Check out my vintage overflow tank!! Super Rare!!!

macadoo1961 - That tank looks like it is out of a later model Dart. I have one that looks like it out of a 1973 Dart 318 car mounted in mine.

Dodgenut64 - Basically, the radiator is designed to cool the coolant until the thermostat opens allowing the coolant to circulate in the engine and push the warmer coolant from the engine into the radiator to cool. The thermostat then closes and holds that cooler fluid in the engine to allow it to soak up the heat from the engine. The other coolant is now in the radiator and the radiator is cooling it. Then the system repeats.

When the coolant gets hot enough the pressure will displace the now "extra" coolant into the overflow tank. When you turn the car off the coolant cools and causes a vacuum that pulls the displaced coolant back into the radiator system. If you look at a factory tank you will see that is has a cold line that you top up the tank to and a hot line. In operation the coolant level in the tank will displace from the radiator and increase the volume in the tank towards the hot line and then go back when it cools. Keeps the radiator system full to optimum level and allows it to handle very hot situations without loosing coolant level.

Factory tanks will also have an overflow ability to the tank in case the system overheats. For example, the tank both macadoo and I have has a made on tube as part of the tank. The tube extends from the top and down the side of the tank and is open to the ground. If the car overheats and fills the tank the excess fluid and steam will rise up the tank until it reaches the tube hole and then it will vent out the tube. The other thing this tube does is allow air to be sucked into the overflow tank so that in normal operation the coolant can be sucked back into the radiator. Think of it like you are drinking a soda from a plastic bottle. If you place your mouth over the top of the bottle (don't get dirty out there!) and drink the soda keeping the top sealed in your mouth, the bottle will compress due to the suction. On the other hand, if you put a straw in the the open soda bottle and suck up the soda it flows freely and does not compress the bottle. This is because the open top is allowing air in the bottle to replace the now sucked out soda.

MORE INFO THAN YOU EVER WANTED EH?

If you look at those generic box type overflow tanks at the parts store you will see that if they do not have the vent tube on the tank that they generally have a small hole in the cap or something to do the same thing.

So, hopefully in the gibberish I've just written I've answered your question.

I know many in the classic Mini world fit tanks to the car as the early cars had none. Simply had a tube off the radiator to the ground. On a hot day and under load conditions the coolant warms up enough to displace onto the ground. In this instance the coolant is just lost and when the system cools it just sucks air into the radiator. The result is that the radiator now can be a bit low causing the system to perhaps not be able to cool like it should due to low level.

So, if you are designing a tank for the car you need to decide what you want it to do. Simply drilling a hole in the top of a container and putting a hose to it at the top works if you just want it to be a catch can to stop coolant from going on the ground.

If you want a system that works like the factory to allow that coolant to go back into the radiator then you will need to design the system a bit differently. Simple way to do it is either have the tube off the radiator attach at the bottom of the tank or have it attach to the top with an internal tube inside the tank that places the opening at the bottom of the tank. This keeps the tube always covered by the coolant and allows it to suck it back into the system. The next thing you need to do is place a provision for venting to the tank. Probably the cleanest way, but certainly not the only way, would be to mount a 90 degree fitting on the side of the tank up top (out of the coolant and in the air at the top of the tank) and then run a line down that would allow overheat coolant to flow out and also this would be the vent that allows air to be sucked into the tank when the coolant goes back into the radiator.

Hope that makes sense.

Wow do I have to work on not being so wordy. Must be in another of my blathering stupers this AM.

Cheers