Thread plug gauges

A working thread plug checks an internal thread. Select the full thread designation and governing inspection rule before applying GO or NO-GO.

Illustrative decision: an inspector records 2.5 turns of NO-GO entry. Under an applicable two-turn rule that exceeds the allowance; under a three-turn rule that observation alone does not exceed it. Neither conclusion establishes complete part acceptance: GO engagement, separate checks and any exceptions still apply. The example shows why the standard must accompany the observed turn count.

Identify the thread and gauge purpose

Specify nominal diameter, pitch or threads per inch, thread form/series, tolerance class, handedness and governing standard/edition. Add the required engagement depth and any special thread or coating condition. A gauge with a matching diameter but wrong pitch, class or thread family cannot establish acceptance.

Glastonbury Southern Gage distinguishes working plug styles, truncated ring-setting plugs, helical-coil plugs and hole-location plugs. Its setting plugs have deliberately different crest geometry, and its helical-coil plugs inspect the hole before installing the insert. These are not interchangeable merely because they carry the same nominal fastener size. GSG, Thread Plugs, undated manufacturer guidance

“Taperlock” describes a member's mounting style, not a taper-pipe thread. Likewise, reversible and trilock constructions concern mounting and handling; obtain the actual product-thread designation independently.

What GO and NO-GO establish

Vermont Gage's August 1, 2016 technical article describes the GO member as a cumulative functional check of thread elements, with clearance at its minor diameter. It checks whether the threaded geometry permits the required engagement; it does not independently report all element dimensions. The NO-GO member checks the upper pitch-diameter limit with major/minor diameters cleared. A part can therefore produce apparently contradictory GO/NO-GO behaviour when different thread elements interfere. Investigate the geometry rather than treating either member as a cutting or sizing tool. Vermont Gage, Internal Product Thread Inspection

The internal thread's minor diameter needs a separate check under the applicable inspection system. Passing both threaded members is not evidence that every diameter, thread depth, location or surface requirement is satisfied. Vermont Gage, separate diameter inspection guidance

Acceptance rules are standard-specific

OSG's technical guidance says a GO gauge should traverse the required threaded length by hand without excessive force. Its straight-thread example permits a NO-GO member no more than two turns; OSG separately identifies three turns for its ANSI example in the general gauge guide. These limits belong to the identified gauge systems, not to every item sold as a thread gauge. Check the governing edition, short-thread exceptions, entry direction and definition of engagement before issuing a shop instruction. OSG, gauge judgment guidance, OSG, About gauges

OSG also distinguishes older JIS WP working and IP inspection NO-GO gauges from the NP designation in its current-JIS explanation. Do not mix historical and current gauge systems without resolving the specified limits. For taper threads, its instructions judge the relationship between the part end and gauge notches/gauge plane, rather than transferring the straight-thread turn allowance. OSG, gauge types and taper-thread inspection

Bounded catalogue example

Vermont Gage's 2025 Product Catalog, whose contents page states Print Date 03/2025, lists these 1/4-20 UNC taperlock configurations on printed page 40:

The shared GO member does not make the assemblies equivalent: the NO-GO member differs. The same catalogue separately lists STI plugs for the nominal 1/4-20 fastener. Verify member markings when replacing a worn end, and confirm the actual supplied gauge/certificate against the order. These catalogue identifiers are selection examples, not universal gauge limits or current availability promises. Vermont Gage, 2025 catalogue

Practical use and depth limits

Clean and inspect the part and gauge before engagement. Start squarely and use the prescribed hand technique. Vermont warns never to force a gauge: if it stops, remove it and check for damage or contamination. It is not a deburring tool. Vermont Gage, use precautions

For a blind hole, distinguish contact with the hole bottom from interference with the thread. Ensure the member can reach the required full-thread region without a shoulder or incomplete end thread stopping it prematurely. A conventional limit plug is not automatically a calibrated depth gauge. If engagement depth is an acceptance requirement, define how it is established and how incomplete entry/end threads are treated.

GSG's published range includes reversible plugs and notes the possibility of reversing a member in its handle. Treat reversal as a controlled configuration change: identify the usable end and its condition rather than assuming an unused-looking end has valid calibration. GSG, working-plug constructions

Calibration and reporting

Fixed-limit inspection provides a decision rather than a numeric pitch-diameter reading. Gauge manufacturing tolerance, wear, damage, thermal conditions and inspection technique still affect that decision. NPL distinguishes calibration, uncertainty and conformance decisions; calibration status does not make the gauge exact, and calibration intervals should reflect use and evidence. NPL, Fundamental Good Practice in Dimensional Metrology, GPG80, July 2005; corrected reprint October 2012

Retain part/thread designation, standard and rule, gauge/member identity, condition and calibration status, required depth and inspection stage. Record GO engagement and NO-GO entry separately, plus minor-diameter and other required checks. A report should let another inspector reproduce the decision without guessing which “NO-GO rule” was used.

Related reference

Inspection and quality documents