British standard pipe thread taper pressure plugs.
Taper pressure plugs are made to close tolerances and will seal at pressure up to 500psi without the use of sealing compounds.
They are often used for hydraulic and pneumatic applications.
Sealing is achieved by crushing the crest of one thread against the root of the mating thread. Dryseal plugs are used more frequently especially where re-use is required. More threads are engaged therfore creating a better seal. They are also prefered for use in softer materials to reduce the risk of over-torquing.