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Technical Specifications

NPTF Taper Plug and Ring Gauges

NPTF Thread Gauges for testing threads to American National Standard ASME B1.20.5 - 1991.

This standard provides information on gauges used for testing dryseal threads. The gauges are intended to be used to test unused pipe threads.

The types of thread designated are NPTF, PTF-SAE SHORT, NPSF and NPSI.

The thread form and taper are the same as for NPT threads.

The pipe fittings range in size from 1/16 inch to 3.0 inch.

NPTF Class 1 and Class 2, and PTF-SAE SHORT threads are tested by the coordinated use of L1 and L2 ring gauges for external threads, and L1 and L3 plug gauges for internal threads.

Functions of L1 and L3 Plug gauges

  • L1 plugs check the threads when assembled hand tight.
  • L3 plugs check the threads when assembled with a wrench.

Functions of L1 and L2 Ring gauges

  • L1 rings check the threads when assembled hand tight.
  • L2 rings check the threads when assembled with a wrench.

NPSF and NPSI are internal threads and are tested using the appropriate L1 taper thread plug gauge. These parallel product threads are intended to be assembled with taper dryseal external threads.

Provision is made for two types of gauge.

Master gauges: used to check new working gauges. Each set consists of an L1/L3 master ring gauge and an L1/L2 master plug gauge.

Working gauges: used to check product threads during and after manufacture. Information on the following types of working gauge is provided in this standard.

For class 2 threads, crest and root check gauges should also be included in the set of working gauges. Root check ring and plug gauges have a narrower thread angle which enables the gauges to contact only at the root of the product thread, so that the root truncation can be checked. Crest check rings and plugs are plain taper gauges. Root and crest gauges are used in conjunction with the 4 step limit gauges.

Gauging external taper threads using a taper ring gauge
  • Gauging external taper threads using a taper ring gauge
Gauging internal taper threads using a taper plug gauge
  • Gauging external taper threads using a taper ring gauge

These diagrams show the principle of min/max/basic gauging of tapered pipe threads
The individual specifications should be consulted for full information