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 later model 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 new model Conical Fermenters have a 3/4 inch hole on the shoulder, next to the lid and come with an airlock. This hole supports the cork & airlock. Please see the fermenter kits before you buy this fitting and save on shipping.