Here's a Quick Review of the ANPT L-1 Plug Gage


ANPT L-1 Plug Gage is used to check the internal thread rings of the aeronautical equipment such as pipes, fittings, plugs, etc. ANPT or Aeronautical National Form Pipe Taper Thread Plug Gages are used in applications where gasket, straight threads, or O-rings are not applicable; particularly in aeronautical applications.

More About ANPT L-1 Plug Gage:

ANPT L-1 Plug Gage is used to first gauge the internal threads. The gaging notch that most accurately represents the size of the thread is hence taken a note of. It is used to check the major diameter, pitch diameter, lead, and taper. The major diameter is only measured to the upper threads which are hand-tied. The later threads, which are measured by ANPT L-3 Plug Gage are measured for wrench tight position.

The parts checked with ANPT L-1 Plug Gage are considered acceptable when they come flush with the notch. This means they will come close to either plus one or minus one turn and check the first four screw threads. Similar to NPTF, ANPT threads also employ an L1 and an L3 gage to measure the first four and the last three thread rings. Likewise, they both require plain taper 6-Step Crest-Check pipe plug gauges. The L-1 gage, more particularly, the ANPT L-1 Plug Gage is first used to gauge the size of the hole in contrast to the screw thread tolerance which could be either basic, minimum, or maximum.

Albeit the ANPT internal pipe threads are practically the same as the pipe threads of NPT, they have a distinct application for their use in aeronautical applications which is why their diameter, taper, and thread form are very carefully controlled to form a seal that doesn’t need application of external sealants. These sealed threaded joints are also needed in applications where O-rings, gaskets, or straight threads won’t get the job done.

Similar to ANPT L1 plug gage which checks the external diameter, the ANPT ring gages are used for checking the external diameter of pipes and ends with rings. The plug has a distinct difference from the basic NPT plug where in addition to its basic steps, we have the maximum and minimum steps that are used for measurement as follows:

  • The product is in tolerance if its face is between the maximum and minimum steps of the plug.
  • To check the depth beyond L1 depth which is measured with ANPT L1 plug gage, the ANPT L3 plug gage is used.
  • Furthermore, the truncation is checked with a six-step plain taper plug gage which includes maximum and minimum truncation steps for each of the three-pitch diameter conditions which are maximum, minimum, and basic.

It is also noteworthy that there should be no shake or wobble during the measurement. This is an indicator of taper error and must be tended to on an immediate basis.


At WESTport Corporation, we have been providing the industry worldwide with precision measuring instruments and calibration services since 1989. WESTport employs state-of-the-art measures to ensure all our products comply with the ISO 9001, and ISO/IEC 17025 standards. Moreover, our expanded product line includes thread and plain gages, API Plug and Ring Gages, taps and dies, medical gages, spline gages, and full-service metrology laboratories for tool calibration, repair, and onsite services.

Written by WESTport Corporation. All rights reserved. Any reproduction is strictly prohibited.