These plastic caps do not seal at all. I had one break the threads, snugged down they do not seal. I had three growlers pour flat because of these caps. It was not my beer, regular bottles with traditional caps all had great carbonation. Last night I thought my last growler would be fine, the cap was tight enough it was a little hard to get off, but again, the beer was flat. Stick with the metal caps and replace when they rust.