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Good thermostats should have a little one-way valve in them to bleed the air out of the engine. Its possible that after the coolant cools down it takes up less space and it could leave a little air pocket behind the closed thermostat. The air acts as insulator between the coolant and the thermostat. The air would take longer to reach a temperature that would open the thermostat than if coolant was directly against it. Once the air gets hot enough, the thermostat opens, coolant starts flowing and the hot coolant in the engine can go on its way to the radiator and your temperature drops down into the normal range. I would try a new thermostat and make sure you get one with the little bleed hold and that the hole is placed at the top when the thermostat is installed.

I'he had 13 early Mustangs along with the Early Ford flathead cars. There is no overflow tank its built into the upper tank of the radiator as just extra space for expansion. You never want to check the coolant level cold and fill it to the top of the radiator neck. Its just going to expand and puke out on the ground. Leave the cap off. Let the engine warm up until the thermostat opens. YOu can tell by putting your hand on the upper hose it will get real hot real fast as soon as thermostat opens. With the engine running fill the radiator up to about an inch from the top and you should be good. Don't worry about the level in there when its cold other than it should be above the tubes.

You will see engines that do not loose any coolant until they are shutoff That is because the coolant continues to absorb the heat from the block and the coolant temperature increases. It might expand enough to go out the overflow. If you keep topping it off it will keep expanding and nuking out. Let it find its happy level in the upper tank and don't woory that its not filled to the top.

If you do decide to put an overflow tank on you need a differ style radiator cap. As the engine cools down and the coolant shrinks it creates a vacuum that sucks the coolant out of the overflow tank and back into the engine, The cap has to have a little valve that allows the coolant to flow back into the block. The regular cap does not have it.
 

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Hey Jsbenami,
Got any pics of the tank installed? I've got a 66 (not sure of your year) and trying to decide between a 17" and 15" tank, as they both show on the site as applicable to 65 thru 73..

Unsure if 17" is overall (tubes/cap)or just tank size..

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Kind of...never purposely photographed it and the car is kept 35 miles away during winter.

Part is 8A080-2A from NPD. 17” my car is a 68’ w/302 and an aftermarket aluminum rad.

This thing never even close to overfills. But I use it as a catch, not to store a serious volume of extra coolant. No issues so far though early on I did have to put a S all clamp on the bottom hose to seal things up perfect and tight.
 

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