Food-grade silicone is always beet than rubber as it does not leave any taint on the wine or spirit. These bungs are designed to fit all Demijohn sizes between 10 and 34 Litres. It is a tapered bung and the size is a diameter of 37mm at the base and 48mm at the top. This silicone bung comes complete with a hole to fit a standard sized airlock. Fits Demijohn sizes 10lt, 15lt, 20lt, 25lt & 34lt