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