Taper pipe NPTF L1 Plug Gage Overview


The Taper pipe NPTF L1 plug Gage, also called relationship gage are considered dry-seal pipe threads. Essentially, Taper pipe NPTF L1 plug Gage is used for the inspection of the pitch diameter of the hand tightened L1 length of any engagement. The NPTF part stands for “National Pipe Taper Fuel” where their requirements are spelled out in ANSI B1.20.3.


Comparatively speaking, both, the Taper pipe NPTF L1 plug Gage and the NPT version will have identical taper-per-inch, threads-per-inch, and pitch diameter. What is different is their crest and roots of threads as well as their major and minor diameters.

Though NPTF threads crest is well within the required parameters of NPT, they still have a smaller range in comparison. Where NPT threads are considered general-purpose pipe threads, NPTFs are reserved for more critical applications. In a sense, both are designed to screw together and that much is simple enough, but where NPT requires a “seal” to be truly leak-proof, NPTF doesn’t.

Both NPT and NPTF standards are strikingly different in terms of threads as even the L1 gage requirements both have a difference of ½ in. size-wise. They are not interchangeable which is where the Taper pipe NPTF L1 plug Gage comes into play. The NPTF threads require more assembly for size check which is not possible with the NPF plug gage. Only a Taper pipe NPTF L1 plug Gage can check NPTF threads.

More About Taper Pipe NPTF L1 Plug Gage

Taper pipe NPTF L1 plug Gages are specifically designed for inspecting the functional thread and pitch diameter for the components with internal threads. The L1 indicates that Taper pipe NPTF L1 plug Gage belongs to class 1 thread gages for a functional check of taper deviations and continuation of thread beyond the L1 length of engagement. Being an L1 plug, the Taper pipe NPTF L1 plug Gage inspects the hand tight engagement of an internal thread or couplings as well as the functional size.

At WESTport, all Taper pipe NPTF L1 plug Gages are manufactured as per ANSI/ASME B1.20.5 and come along with a Free NIST certificate of accuracy. They are designed with hardened steel or similar materials which prevent the degradation of the threads. Moreover, these gages are designed with recessed minor thread diameter for safe functionality. The ergonomic handle provides a maximum comfortable grip. Due to the reduced thread start diameter design, it ensures easy insertion of the gage.


The Taper pipe NPTF L1 plug Gages have tolerance of one turn (+/-) from either the small end face of the ring or notch on the plug and it falls between one of the following sets of notches:

  • MN & MNt
  • B & Bt
  • MX & MXt

The notches are in relation to from where L1 gets measured.

At WESTport, you can expect to buy top quality Taper pipe NPTF L1 plug Gages with quality you can trust. With an extended line of products pertaining to Quality Assurance Products and Services, we are at the forefront of serving the industry as leaders in precision tools and related services.

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