Thread standards exist so that components from different manufacturers fit together predictably — but only within the same standard. The moment two standards meet at a joint, you have a leak waiting to happen.
BSP vs NPT
BSP (British Standard Pipe) and NPT (National Pipe Taper) are the two you will meet most often. They differ in thread angle and pitch, so they are not interchangeable even when the nominal diameters look identical. Forcing them together feels fine for the first few turns and then seals poorly under pressure.
Tapered vs parallel
BSP comes in both tapered (BSPT) and parallel (BSPP) forms, and they seal differently — tapered threads seal on the threads themselves, parallel threads need a washer or O-ring. Knowing which you have changes the sealing method entirely.
- BSPT / NPT — tapered; seal on the thread with PTFE tape or pipe paste.
- BSPP — parallel; seal with a bonded washer or O-ring at the face.
- Never combine BSP and NPT at the same joint — use a proper adaptor instead.
A mismatch found on the drawing costs nothing; the same mismatch found on site costs a return visit.
Avoiding mismatches on site
On any project that mixes imported and local components, label thread standards on the drawings and verify them at goods-in. A thread gauge in the toolbox settles arguments in seconds and saves hours of rework.
Thread verification checklist
Threads are unforgiving but entirely predictable. Get the standard right on paper and the joints behave themselves on site.
