If you leave the white silicone cap on the carto, the small lip that extends outside the carto will act as a seal. I've never had any problems with juice seeping past, and I'm vaping some 100% PG at the moment. There is only .002" clearance per side on the top cap to the carto by 1/4" deep. I wanted to run an even tighter fit on the top cap, but there is too much variation from carto to carto. I had to use the largest diameter carto to get my dimensions. The tanks are slightly loose on the Boges, but place a IKV carto in there, and they get really snug. IKV cartos are .003/.004 larger than Boges. If I added an o-ring to the top cap, I believe I'd run into vacuum problems in the tanks, and they wouldn't wick very well. You need just a little outside air to get in to help equalize the vacuum pressure in the tank. I ran into this problem while prototyping my 510 atty tank.