At this point I have fixed the problem by switching to the civilian filler tube & cap. Which works much better BTW, then the big CUCV tube. It holds the pump nozzle better so it doesn't keep shutting off, and doesn't splash back when it's full. But I am still curious about the original CUCV cap & filler? I let my A.D.D. kick in and I fell down the rabbit hole of fuel caps today on the interwebs, but still didn't find a simple solution. The inside of the tube measures just over 2-1/2" I.D. which is where the problem lies. The cap is basicly a (bolt) or a (male), and the tube is the (nut) or (female). Everything I found that size is reversed. The only avenue that is harder to find info on the internet is the aviation & European markets. So you'd almost have to ask an old airplane mechanic or Euro truck expert?