After having a little issue getting coolant pressure on my new 496 fire up, and being initially confounded by it...and solving it rather easily, I thought I might make this little commentary.

I used a old rad on the test stand (the original one from my GP), and rather than take the cap of the Champion aluminum rad in the car (the original cap was gone), I thought I'd buy a new one from CARQUEST. So I went down and asked for a "Rad Cap". Now the counterman knows me, and knows I'm putting a engine together, so he goes to the back, come back and hands me a shiny new "Rad Cap". I don't question him, thinking a cap is a cap.

I install it, run the engine. Hmm. Temps just don't seem to be really rising like they should. The upper hose is soft, but hot, and the water is obviously flowing. Heat is blowing back out of the rad. After a while, there is still no pressure. I figure airlock. so I let it cool and open the cap. The rad is still full. I close it and try again. And again, no pressure.

A few days later, and after reading up about the different cap types, I take the one off my Champion rad and compare it to the new CARQUEST cap. And low and behold...the new cap is what is known as a late 60's Chrysler cap. The type that does not build pressure until the coolant expands to a certain level, and closes a little weighted relief valve at the very bottom of the cap. The type where the little relief valve hangs open loose and is normally open to the atmosphere. The other type has the relief valve always closed. It's obvious when they are compared side to side, top up , but everywhere else they look the same. 

So I install the one off my 67, off the Champion rad, fire up, and within about 2 minutes the hoses are hard, temperature comes up and all is well in the world.

I guess Chrysler didn't have much faith in their engine sealing. But if you are using a overflow tank, you should know that at a minimum, you'll always need one with the little valve at the base, Chrysler or not. And look to the position of the little relief valve to identify the type. Also, the lack of temperature at the sender doesn't mean the engine isn't hot. Without pressure, you run the risk of overheating as you just might be boiling on the surface of the water jackets. No heat transfer to the water through gas! Ouch. Pressure is good.

I learn something every day....

Cheers,