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 the 54 litre Demijohn and wine barrels. It is a tapered bung and the size is a diameter of 47mm at the base and 58mm at the top . This silicone bung comes complete with a hole to fit a standard sized airlock. Fits 54lt Demijohns