# Thread plug gauges

Selecting and using internal-thread limit gauges with standard-specific acceptance rules, separate dimensional checks and controlled gauge condition.

Reviewed: 2026-11-01

Tags: inspection, thread-inspection, limit-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.

| Gauge or device | Intended task |
| --- | --- |
| Working GO/NO-GO thread plug | Limit inspection of the internal product thread |
| Plain cylindrical plug | Separate size check, including internal-thread minor diameter where required |
| Truncated setting plug | Setting or checking a thread ring gauge |
| Screw-thread-insert/helical-coil plug | Checking the prepared thread before insertion |
| Thread-location plug | Establishing a threaded feature's location using the specified measurement arrangement |

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](https://www.gsgage.com/thread-plugs)

“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*](https://www.vermontgage.com/support/detail/internal-part-thread-inspection-quality-magazine)

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](https://www.vermontgage.com/support/detail/internal-part-thread-inspection-quality-magazine)

## 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](https://www.osg.co.jp/en/media_dl/technical/file/gauge_handle_shape_dimensions.pdf), [OSG, About gauges](https://www.osg.co.jp/en/media_dl/flier/file/p_9.pdf)

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](https://www.osg.co.jp/en/media_dl/technical/file/gauge_handle_shape_dimensions.pdf)

## 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:

| Product class | GO member | NO-GO member | Assembly with handle |
| --- | --- | --- | --- |
| 2B | 301126010 | 301126020 | 301126040 |
| 3B | 301126010 | 301126030 | 301126050 |

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](https://vermontgage.com/assets/catalogs/2025-VG-Catalog/f37ced27da/961032025-R7-9-12-25.pdf)

## 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](https://www.vermontgage.com/support/detail/internal-part-thread-inspection-quality-magazine)

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](https://www.gsgage.com/thread-plugs)

## 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](https://eprintspublications.npl.co.uk/3360/1/mgpg80.pdf)

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](../../sourcing/documents/index.md)