What Are NPSM Fittings?
National Pipe Straight Mechanical fittings are adapters, couplings, valves, connectors and related objects that are compatible with the NPSM non-tapering pipe thread standard. These items can be used to join NPSM pipes or adapt them to other sizes, shapes and uses.
The National Pipe Thread is the standard used in the United States for plumbing, while the British Standard Pipe standard is the international plumbing standard. Since NPSM pipe threads do not taper, they do not form as effective a seal as NPT or BSP threads. They are frequently used as structural pipes and require a sealer, such as an O-ring, to transport liquids and gases.