In brief
NPT and BSPT belong to different thread systems even though both use tapered external threads. BSPP uses parallel threads and requires a defined sealing arrangement outside the threads. A similar diameter or a few turns of engagement does not establish compatibility.
Compare the system, not just the diameter
NPT has a 60-degree thread profile; the ISO pipe threads commonly called BSPT and BSPP have a 55-degree profile. Size, pitch, taper and the sealing arrangement must all agree. Use an identification guide to narrow the type, then check the specified standard and mating drawing.
| Common name | Reference | Form and sealing distinction |
|---|---|---|
| NPT | ASME B1.20.1 | Tapered; specify the approved sealing and assembly procedure |
| BSPT | ISO 7-1 | R external taper; Rc internal taper or Rp internal parallel within the specified joint system |
| BSPP | ISO 228-1 | G parallel; pressure sealing is provided outside the threads |
References: Swagelok: Thread and End Connection Identification Guide; ASME B1.20.1: Pipe Threads, General Purpose, Inch; ISO 7-1:1994: Pressure-tight joints made on the threads; ISO 228-1:2000: Joints not made pressure-tight on the threads
Show where the joint is intended to seal
For an ISO 228-1 connection, show the mating sealing surfaces and the seal used between them. Do not treat the thread designation as a complete definition of a washer, O-ring, cone or seat. The selected interface design determines which details belong on the drawing.
For a tapered thread connection, specify the required thread system and the approved assembly instructions. This guide does not prescribe sealant, tightening torque or pressure ratings. Those choices require the actual mating components, materials and service conditions.
References: ISO 228-1:2000: Joints not made pressure-tight on the threads
When identifying an existing component
Keep the mating part available where possible. Record measurements and photographs as observations, not as a final thread callout. A worn or plated sample may not provide enough information for a confident identification.
- Record whether the thread is internal or external
- Check parallel versus tapered form and measure pitch with suitable tools
- Locate any separate sealing seat, shoulder or washer face
- Ask for the mating drawing or original part specification
- Resolve uncertainty before manufacturing or assembling a substitute
References: Swagelok: Thread and End Connection Identification Guide
Turn identification into a quotation requirement
Name both sides of an adapter separately. A part may connect two different systems, so one thread label for the whole part is insufficient. Include the thread reference, nominal size, applicable tolerance designation and any orientation or access requirement.
The next step is a drawing callout and inspection agreement. Keep this comparison for choosing the interface; use the drawing checklist to communicate the selected interface without ambiguity.
- Instrument-side connection identified
- Process-side connection identified
- Sealing components and supplied items separated
- Inspection and any assembly test requested explicitly
Sources & scope
Source pages checked on 2026-09-24. Publisher summaries and manufacturer references help identify the subject; they do not replace the specified standard or a reviewed part drawing. The checklists are editorial guidance for quotation preparation. External suppliers’ capabilities and certifications are not BOREVIA claims.
- ASME B1.20.1: Pipe Threads, General Purpose, Inch
Publisher overview of dimensional and gauging scope. Consult the specified edition for acceptance requirements.
- ISO 7-1:1994: Pressure-tight joints made on the threads
Publisher description of thread dimensions, tolerances and designation.
- ISO 228-1:2000: Joints not made pressure-tight on the threads
Publisher abstract describing parallel threads and sealing outside the threads.
- Swagelok: Thread and End Connection Identification Guide
Thread identification reference. Its product specifications are not BOREVIA capability claims.
From preparation to a defined RFQ
Bring the drawing and the open questions
State the part revision, material, quantity and inspection requirements. Pricing and feasibility need a review of the actual request.
Request an engineering quote