Plastic undrilled stopper. This universal style has a longer taper than rubber stoppers, for a snug fit and better seal. Fits plastic carboys, Better Bottles and Bubbler PET fermentors. Universal bungs have a longer taper than standard rubber stoppers, for snugger fit and better seal. Plus a lip prevents bung from falling into carboy. Top Diameter at the lip: 2.09"Top Diameter below the lip: 1.98"Bottom Diameter: 1.69" Not sure if this is the correct stopper for your purposes? Consult our stopper information table below: Stopper Type Compatible With: No. 2 Drilled Stopper Standard Beer bottles No. 5 Drilled Stopper Big Mouth Bubbler® EVO 2 w/screw-on lid (older EVO 2 models) & Little BMB No. 6 Stopper Drilled or Solid 1 Gallon Jugs No. 6.5 Stopper Drilled or Solid Glass carboys, Big Mouth Bubbler® plastic (5 or 6.5 gallon) or EVO 2 w/ Universal Lid. Ideal fit is 6 gallon carboy. No. 7 Drilled Stopper 5 Gallon glass carboys No. 10 Drilled Stopper PET Plastic Carboy 5 or 6 gallon Small Universal Stopper - Drilled or Solid Glass Carboys (3, 5, 6, 6.5), Big Mouth Bubbler® Plastic 5 or 6.5 gallon or EVO 2 w/ Universal Lid. Medium Universal Stopper - Drilled or Solid PET Plastic Carboy 5 or 6 gallon California Proposition 65 ⚠ Proposition 65 Warning: This product can expose you to chemicals that are known to the State of California to cause cancer and birth defects or other reproductive harm.