Steel drums usually have a 2 inch NPS opening, and a 3/4″ NPS bung. The large bung is a 2″ NPS thread (National Pipe Straight) and the smaller bung is 3/4 inch NPSM. (National Pipe Straight Mechanical)
IBC tanks use a 2 inch NPS thread in the lid.
Standard drums can have different drum plug types. Usually drums will have two different openings. (or bungs) One of them is always going to be a fine thread which in North America is 2″ NPS. (National Pipe Straight) In addition, the drum will always have a second bung which will either be large or small. If this secondary bung is large it is most likely a 63 mm buttress. technical details here However it is often mistakenly called 2 inch Buttress, but this is an inaccuracy. In any case both of these threads are straight (not tapered like the NPT) and therefore they use an o-ring to provide a liquid-tight seal. If the second bung is small in diameter, is is almost certainly a 3/4 inch NPS thread in North America. Unfortunately the NPS part of the the description is usually omitted by suppliers of these bung plugs, or worse, incorrectly described as NPT. Fortunately they can be easily distinguished: if it has an o-ring it is must be NPS, because NPT threads are not designed to seal with an o-ring. Also, a discerning eye can tell if the threads are tapered or not.
NPS threads have 11.5 threads per inch. Interestingly, 2 inch NPT threads (commonly found on tanks) have the same thread pitch which allows the two threads to be somewhat interchangeable. You can for example thread a 2″NPT plug into a 2″NPS bung, however the plug will jam after about 3 turns, and the seal will not be liquid-tight. Similarly, a ¾”NPT plug will thread into a ¾” NPS bung.
THE CONFUSING WORLD OF BUTTRESS THREADS
Buttress threads come in many different sizes around the world, and each container manufacturer seems to favor their own particular thread pitch and depth. Narrowing this to the chemical drum industry in North America, the 63 mm size is the most common. Occasionally this is also called a 2″ Buttress but this is an imprecise description. A nice feature of the Buttress plug is that it threads in place after only 3 turns or so. Also, the Buttress threads are much coarser than NPS and will never get stripped. Finally, because the threads are so course, it is easier to start the thread engagement than with a fine thread like the 2 inch NPS.
To add more confusion, there are Buttress threads known as S type of which the most common in North America is the S64-5. This means that the major diameter is 64 mm and the thread hs a pitch of 5 mm. (see image on the left pane) Although close in size, these are not the same as the mm sizes. So for example the 63 mm Buttress is different than the S64-5.
A newer drum plug on the market is a “tall form” plug that allows mounting of apparatus such as drum suction pipes (or draw tubes, drum wands, drum pickup tubes, etc.) or level alarm pipes. This type of drum plug is made exclusively by Gizmo Engineering and is used in many of their products such as their Drum Alarms and IBC alarms. They come in either NPS or Buttress thread.
The plugs discussed above are for the North American market. International plugs and thread converters are beyond the scope of this article. Good sources of information is at drummates.com or Action Pump.