Works on the 3, 5, 6 and 6.5 gallon glass carboys. These bungs have a graduated taper which provides a snug fit and seal on any size carboy. This bung is solid so you won't be able to install an airlock. No longer worry if you have the right size bung for your carboy. The lip prevents the bung from falling into the carboy. The bung will seal a carboy even if it is wet.