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Threads and connections · Pneumatic glossary

NPT, BSPP, and BSPT Threads

National Pipe Taper thread

Last updatedMaintained by

NPT
Tapered, 60° thread form
BSPP / G
Parallel, 55° thread form; separate seal
BSPT / R
Tapered BSP family, 55° thread form

01

Why nominal size is misleading

Pipe-thread size is a designation, not a direct measurement of the outside diameter. Some NPT and BSP sizes look close enough to start threading, yet their pitch, flank angle, taper, diameter, and sealing design differ. Partial engagement is not proof of compatibility.

02

The sealing location changes the assembly

NPT and suitable ISO 7-1 tapered combinations are designed around thread engagement with the specified sealant or sealing practice. A G/BSPP connection normally uses parallel threads for retention and a separate sealing surface. The mating port geometry must provide that face or seal seat.

03

How to identify an unknown pneumatic port

Use the product drawing or model code first. If that is unavailable, check taper, major diameter, threads per inch or pitch, flank form with a suitable gauge, and the presence of an O-ring seat or sealing face. Do not force a sample fitting into an expensive valve or cylinder port as an identification method.

Selection checklist

What to verify before selection

  1. 01

    Manufacturer drawing or model code

  2. 02

    Parallel or tapered thread

  3. 03

    Measured diameter and pitch

  4. 04

    Thread-form gauge where available

  5. 05

    Required sealing face or sealant

  6. 06

    Pressure, material, and tightening limits

Application caution

Common mistakes

  • Assuming equal nominal sizes mean equal threads
  • Applying thread tape to a BSPP face-seal connection instead of using the specified seal
  • Forcing a partly engaged fitting and damaging the port

FAQ

Questions engineers often ask

01Terminology checkCan an NPT fitting be installed in a BSPT port?

Do not treat them as interchangeable. Their thread angles, pitch combinations, diameters, and standards differ even when a sample appears to engage.

02Terminology checkDoes BSPP seal on the thread?

Normally no. G/BSPP threads usually provide mechanical retention while an O-ring, washer, bonded seal, or sealing face provides pressure sealing.

Source notes

Technical references

This lookup table explains practical usage. Definitions, reference conditions, symbols, and abbreviations may differ by standard or manufacturer. Treat the applicable datasheet and standard as authoritative.

  1. ASME B1.20.1 — Pipe Threads, General Purpose, Inch

    Dimensions and gauging requirements for NPT and related inch pipe threads.

  2. ISO 228-1:2000 — Parallel pipe threads

    Requirements for parallel fastening pipe threads where the pressure-tight joint is not made on the threads.

  3. ISO 7-1:1994 — Pressure-tight pipe threads

    Dimensions, tolerances, and designations for pipe threads where pressure-tight joints are made on the threads.