#10 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: 2" Bottom diameter: 1-11/16" . Fits OUR 3 gallon, 5 gallon, 6 gallon PLASTIC carboys, and fermonster lids WITH AN AIRLOCK.