Standard plastic drums Steel drum topusually have two different openings: a fine thread which is 2″ NPS (National Pipe Straight) and a coarser thread which is 2″ buttress. Both threads use an o-ring to provide a liquid-tight seal. (NPT threads which rely on the taper of the thread to form a tight fit, don’t form a reliable seal without tape)

 

 

 NPS threads have 11.5 threads per inch and fit into a drum opening size of 57mm.NPS drum plug

Buttress threads come in various different sizes but the one on standard drums is 63 mm, also known as 2″.Buttress drum plug

Steel drums usually have a large and small opening. The large opening is 2″ NPS thread, and a smaller thread which is 3/4″ NPT thread. (National Pipe Taper )

2″ NPT thread co-incidentally has the same thread pitch as NPS (11.5 t.p.i.) and the two are somewhat interchangeable; you can turn an NPT plug into an NPS opening about 3 turns before it locks but it is not liquid tight.

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