The issue we have is that when we take them on and off to refill our cartomizers, after a few days the ring starts to get pinched on a side between the cart and the tip, so after awhile of that, it wont really hold on anymore as the o-ring is breaking down. The real problem with finding a replacement o-ring is the thickness or cross-section, not the diameter, that is the real measurement I need to know.
The guy at Lowes gave us #47, but it was too thick