Pin lock CO2 port issue

Hi. I’ve got a pin lock corny keg, one of a batch of four I got from an online special. The other three are perfectly fine. The fourth seems to have an issue with the CO2 post. I took both the CO2 and liquid posts apart to clean them, but the CO2 post won’t screw back on. I don’t want to cross the threads, so I’m not forcing it. Here’s the strange thing: I did two things. First, I bought a new CO2 post, thinking maybe there’s something with the old one, but the new one did the same thing. But stranger is that the liquid post screws down just fine onto the CO2 side. I’d just switch them, but you can’t with at least this pin lock corny - the liquid post stick is very specific to that spot. Any idea why the CO2 post won’t let me reattach the top post? Thanks.

Are you saying it wont even begin to catch the threads, or that it will go, but wont seal?

If it came off, it should go back on. Sounds to me like the threads are buggered or maybe the poppet dropped and is binding.

Are all 4 kegs the same style? mismatched? reverse threaded?

Just thowing stuff out there. Very odd though.

FYI…First time I cleaned my new kegs, reassembled and coule not figure out why my co2 side would not seal.

Later found the gas dip tube still in my little pail of starsan. Once I put that back where it belonged, All was good!