I've encountered the same thing with a Champion CC369 radiator. I ordered the "black" adapters to go to 3/8" JIC, and they felt loose. I thought it was the fitting because I mic'd the male threads at 0.492". Then I decided to flare and bend tubing, so I ordered 1/2"-20 stainless steel tube nuts. They fit loosely also. Then I looked inside the bung, to see a cone and an O-ring. WTH???
This is my go to for hydraulic fittings:
https://www.discounthydraulichose.co...read_guide.pdf
If you'll notice the male threads of the 1/4" BSPP -04 fittings are listed at 0.52". I've ordered these, and I'll be seeing how they fit. My issue will be that I have to go back to using JIC fittings instead of inverted flare.
My point would be that Champion has no business selling radiators specifically for our trucks "C/K series, 67-72, L6 or V8, 2 core, 3 core, and 4 core" without radiator supports and without the proper inverted flare bungs. I made the decision for myself to save the shipping fees to return the radiator just to get my money back and then pay Be Cool $600 for a "direct fitting" radiator. I had a TIG welder weld in aluminum to fit the factory support saddles, and it fits perfect now. So I have to live with the situation of these improper bungs because I can't return it.
As you can see, I'm posting about this same issue 6 years later, so Champion must have known about it, but chose not to do anything. Just keep in mind that the issue isn't China, it's American Corporations choosing the cheap labor. Shame on me for choosing the cheap product. <<<I can delete this last part if it's improper in this forum.