This is a cap for a ventless gas can with a push on connector. works excellent. I lost the cap for this tank a few years ago and finally got around to designing a replacement. The cap ID is 22mm which fits my 5-gallon tank perfectly. It's I made it from PLA which is potentially not very gas resistant, but if I have issues, I'm only 20 minutes away from having another. If you try it out and it does not fit the first time, just scale in the X-Y-Z direction as necessary to make one that fits.