If you bought a 3" TC dome lid for your standard Chronical this covers up that large 3" tri clamp opening. 1/2" tubing or an airlock and #2 stopper fit nicely in the 17mm hole. This cap comes standard with Brewmaster Chronicals and only fits on a 3" tri clamp flange. Comes with a 1/2" hose barb and pressure relief valve. 3" tri clamp gasket and clamp not included.