NPT

Illustrated explanation

Parallel and tapered entry threads

3NPT
Entry thread envelopesFour labelled thread envelopes. Callout 1 is a parallel metric thread, 2 a parallel PG thread, 3 a tapered NPT thread and 4 a parallel G pipe thread. Dashed lines show the parallel or tapered outside envelope. Tooth profiles are simplified and not gauges; the four thread families are not interchangeable. Current term: NPT, callout 3. 1 2 3 4 MetricPGNPTG ParallelParallelTaperedParallel / separate sealSimplified envelopes / not to scale
  1. Metric thread

    Metric entry threads are parallel. Specify nominal diameter, pitch, fit and the complete entry sealing arrangement.

  2. PG thread

    PG is a separate parallel electrical-entry thread family. Its designation and profile differ from metric threads.

  3. NPTThis term

    NPT has a tapered envelope. Confirm its exact designation, engagement and permitted thread sealing procedure.

  4. BSPP / G

    G threads are parallel pipe threads. Pressure-tight sealing is provided outside the thread flanks by the specified arrangement.

The dashed envelope highlights constant diameter on parallel threads and changing diameter on a tapered thread. Each family still has its own profile, size designation and mating requirements.

Simplified thread envelopes, not gauge profiles or dimensioned drawings. Similar diameter or a parallel shape does not establish interchangeability.

ReferencesIEC 60423:2007 — Conduit diameters and threads for conduits and fittingsISO 965-1:2026 — General-purpose metric screw-thread tolerancesASME B1.20.1 — Pipe Threads, General Purpose, InchISO 228-1:2000 — Pipe threads where pressure-tight joints are not made on the threads