The 1/2 inch threaded plug is used on all of our fermenters except the f6.5. It is also used on our MiniMash Lauter Tun and the MiniHot Liquor Tank. There are two threaded holes in the lid of the fermenter and one just below the label. The fermenters are delivered with three plugs. The threaded ports are used for a thermometer below the label and the in the lid for the MiniInjector Kit and to hold the Airlock fitting should you decide to use the new Blue Gasket. The three plugs screws into those threaded ports.