The adapter has 1/2 inch threads on the male end and is a smooth 3/4 inch hole on the opposite end. This fitting screws into the threads in the lid of all our fermenters (f8b, f15b, f25, f40 f98 and f101)except the f6.5. The airlock cork presses into the smooth 3/4 inch hole sealing the airlock to the lid. The f6.5 Affordable Conical has a 3/4 inch hole on the shoulder next to the yellow cap and come with an airlock. This hole supports the airlock so this fitting is not needed with the Affordable Conical fermenter.