The reason the 350U radiator blew..

Running a segmented fan belt without any understanding of how the tension adjustment is accomplished is a big factor. The belt seemed fine at first but after a few minutes it must have seated down in the pulley?and loosened up, which means the fan AND PUMP quit turning. What should have happened next is that the overflow under the radiator cap at the filler neck should have pushed the cap seal spring up and released the pressure at around seven pounds. The counter clerk at NAPA who sold me the cap said it was the correct one for my tractor according to the chart in his computer. He was wrong. This afternoon I checked the cap in the installed position and found that the spring in the cap COMPRESSES COMPLETELY and presses the seal down so tight to the seat in the radiator filler neck that it is locked solid on the seat;there is NO spring relief pressure left. I was able to determine this because the blown open seam at the tank means I can reach my whole arm in the tank top and feel that there is no spring left in the cap seal, it is completely sealed,locked solid at the bottom of the filler neck seat. As another poster said, it must have gotten up to 50 PSI or more before blowing the seam out.

There are several caps that "fit" common old radiators. The correct ones do have some tension on the seat, but only compress about 3/16" maybe 1/4" from tossing it in the hole. The PSI is 7.5 max. Jim

The compression range on this one is a half inch or more.It also has a 'tang' on the top which appears to be spot welded on and protrudes an inch or so out. Probably makes it easier to grasp with gloves on.
Honestly, I've never seen a failure like this before.

THe tang on the top is standard OEM for farmalls and Utilities from the supers on up IIRC. that does not make it the correct cap, but it would confuse many. What PSI is listed on it (if any)? Jim

That there was NO PSI indicated on the cap threw me but now, with it securely turned and latched in place on the filler neck I can see....feel that there is NO PSI release point. Can't even push it up with a bar from inside the tank. I just had a new one ordered with a radiator and I know it should be 7 PSI so we'll see what it says.

Heck, on my '40 H is a gas cap off a Volkswagen. It was that way when I inherited the tractor and my theory is if it isn't broke don't fix it. This is my go tractor and take long rides with the wife and never had any issues with......Kenny

(quoted from post at 06:31:25 06/07/18) Heck, on my '40 H is a gas cap off a Volkswagen. It was that way when I inherited the tractor and my theory is if it isn't broke don't fix it. This is my go tractor and take long rides with the wife and never had any issues with......Kenny

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A 1940 H would have the non-pressurized radiator. The radiator cap and the fuel cap WILL interchange.