# NPT, BSPP, and BSPT Threads

> Source: https://rodlesspneumatic.com/resource-center/pneumatic-glossary/npt-vs-bspp-vs-bspt/
> Language: en-US
> Category: Threads & connections

## Definition

NPT is a tapered inch pipe-thread system defined by ASME B1.20.1. BSPP, commonly marked G, is a parallel thread based on ISO 228-1 and normally seals on a separate washer, O-ring, or sealing face. BSPT/R is a tapered ISO 7-1 thread family. Similar nominal sizes do not make these thread forms safely interchangeable.

## Key facts

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

## 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.

## 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.

## 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.

## What to verify before selection

- Manufacturer drawing or model code
- Parallel or tapered thread
- Measured diameter and pitch
- Thread-form gauge where available
- Required sealing face or sealant
- Pressure, material, and tightening limits

## 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

## Frequently asked questions

### Can 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.

### Does 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.

## Technical references

- [ASME B1.20.1 — Pipe Threads, General Purpose, Inch](https://www.asme.org/codes-standards/find-codes-standards/b1201-pipe-threads-general-purpose-inch): Dimensions and gauging requirements for NPT and related inch pipe threads.
- [ISO 228-1:2000 — Parallel pipe threads](https://www.iso.org/standard/33777.html): Requirements for parallel fastening pipe threads where the pressure-tight joint is not made on the threads.
- [ISO 7-1:1994 — Pressure-tight pipe threads](https://www.iso.org/standard/20819.html): Dimensions, tolerances, and designations for pipe threads where pressure-tight joints are made on the threads.