#6 Bored Rubber Stopper Bung - Food grade. Center has 3/8" drilled hole for an airlock or blowoff tubing. You may optionally plug the hole with a #000 solid stopper. Top diameter: 1-1/2" Bottom diameter: 1-1/32" . Fits OUR 1 gallon, 1/2 gallon and 1/4 gallon glass jugs WITH AN AIRLOCK.