66 Coolant comes out of overflow once warmed up

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Just how much comes out? You might simply be overfilling it.

Generally, the radiator is full when the surge tank is somewhere between the bottom of the tank and about halfway. If you add coolant, drive the car once -- and allow it to get to full temperature. Then park it. More than likely you'll get some spill. Maybe, possibly, a tiny amount the second time you drive it, depending on how warm it was the first time. After that, leave it alone -- there should be no more spills.
 
KandyOrangeZ,

I believe that model has the expansion tank mounted as part of the thermostat. If you fill the tank all the way to the top, there is nowhere for the coolant to go but out the out the overflow once the engine heats up. Most US cars didn't have a coolant recovery tank until 1969. This allows space for the expanding coolant to be temporarily stored. When you stop the engine and the coolant begins to cool, it will be sucked back into the expansion tank. You will need a modern radiator cap for this the function.

My 56 has a large expansion tank on the top of the radiator. If I keep the coolant level down about halfway. There is room for expansion usually. A few weeks ago I stopped at a Dairy Queen to get a sandwich and an ice water before the car show. There were about 5 cars ahead of me and the car show was just a block away. Well my 56 puked all over the parking lot. The coolant on the ground was getting more attention that my car.

I believe we both need a recovery tank so we don't get embarrassed again.

Good luck,
Steve
 
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Assume you do not overfill the radiator? Also assume there is an expansion tank connected to the radiator? Are there bubbles in the coolant fluid caused by a leaking gasket that allows exhaust gases to get into the cooling system which might cause overpressurization . Does the radiator coolant cap fit tightly? If none of these items, you probably have a blockage/ restriction in the new aluminum radiator. Could also be a thermostat that is not opening fully.
 
Put the model year in the subject on future posts
Added.
model-year.jpg
 
There is a factory overflow tank . I haven't dived to far into this issue. Bought the bird last year and only start it once every few weeks or so. Everytime i let it run , this happens
 
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