I doubt it's pressure, just a surge from trapped air like Pete said.

Ok, thanks Ross. I know your expertise is far superior to mine, on the flatty's
And the others as well. I just wanted to get a second opinion on my thoughts.

LOL, it's all conjecture from 1,000 miles away

I let the truck run for ten minutes before I put the cap on so it wasn't actually "cold". Wasn't on fire but warm. On the pressure tester....I tried but the tester kit came with several fittings. None would attach to my radiator. Also one of the folks I was talking to today at Dennis Carpenter said if the head gasket was blown out would be bubbling the whole time I was sitting there with the cap off and it wasn't so hopefully no blown hg.

Ok guys! I am glad to hear it. I hope nothing but the best for you Wayne!

I will wait for the good news! I hate annoying little thing like this! Get'er fixed!:-X22

Got a message back from Stant Engineering dept. The neck on my radiator is to deep. No cap is available. :( They confirmed that the cap is not making contact on the bottom lip.

Ooooooh !!! Lots of fun stuff at that site Ross. :) :) :) Good ideas if you plan to move your fuel tank. Thanks! :D

Time to get innovative. Figure out how much addition you need to get the seal part of the cap down into the sealing part of the radiator neck and put your fab skills to work. That or take it to a shop that will install a new neck. My experience is that all of these old vehicles don't burp or bleed quickly.

I cut a disc out of a sheet of rubber as a temporary fix and it's still there 10 years later

Steel washers would do the trick, too, maybe with a rubber one at the bottom.

Note that LMC Truck has a radiator cap #43-0010 for $4.95 that is specifically for stock radiators only. That seems to indicate that it is different than regular caps.

Cool links. I feel better knowing that there are some things available. I'm going to try cutting me a rubber gasket to take up the difference. I appreciate all the time and input guys. Looks like we got enough pages to qualify for a nifty video ad in the thread...lol.
As I've mentioned before this area is horrible for business. Nowhere to get a driveshaft made or balanced. And the list goes on and on. The radiator shop that all us old car guys used just closed last month. Now all thats left is the one that everyone is scared of. When I did my radiator the other day I had to show up in person (after leaving the radiator for a week) to get him to do anything. And when I dropped it off he told me two days. A week was fine but I know him and know he's not trustworthy. Anyway, I stood and watched him check the radiator and he doesn't seem very capable. Hopefully I can fix this without having to go back.

FWIW, I just measured the original Ford script radiator in my '49 F4. It originally had the 226 CId flathead six. Measurement down to the seal ledge surface in the radiator neck is 3/4". Distance from the brass top seal and the lower rubber seal of the 4 psi cap is 13/16", non compressed. So, in my case, there is 1/16" compression.

Thanks Ray. I went by NAPA and dug around in the rubber washers/orings. Found a suitable piece and placed it on the ledge. Now when I put the cap on I feel the spring compress some when I mash down to tighten. I'll keep a check on it and see if it leaks.