Originally Posted by Bobby_M
I think the tolerance on them is really piss poor. I have a few that can easily be spun on the 6g better bottle. I've since been collecting water cooler bottle caps from the office. They fit REALLY tight and you can drill a hole for the airlock or blowoff tube.
It must be poor tolerances, as I've got five and they all fit very snugly on both better bottles and 5g carboys, and yet this issue keeps coming up. If I ever end up with a loose one, I'll just use a zip-tie.
I've save a few of those water bottle caps too, they're bound to come in handy at some point.