Reference · ISO 1502 · ASME B1.2 · DIN 103 · Metrology

Thread Gauge Glossary: 60 Key Terms Defined

This glossary defines 60 essential terms used in thread gauging, thread metrology, and quality inspection. Each definition is written to be self-contained and accurate to the applicable international standard. Use this page as a reference when reading calibration certificates, engineering drawings, or gauge specifications.

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GO gauge: A full-form gauge member that must pass through or over the complete thread length under hand pressure. Verifies the thread is not too tight to assemble.

NO-GO gauge: A single-element gauge member that must not engage more than two turns. Verifies the thread pitch diameter has not exceeded the maximum material limit.

6H: Standard ISO tolerance class for internal metric threads (nuts, tapped holes). Uppercase = internal thread.

6g: Standard ISO tolerance class for external metric threads (bolts, shafts). Lowercase = external thread.

Pitch diameter: The diameter of the imaginary cylinder where the thread width equals the space width. The most critical dimension in thread gauging.

A B C D E F G H I L M N P R S T U W

ACME Thread ASME B1.5

A power transmission thread form with a 29° included angle, defined by ASME B1.5. Used for lead screws, jacks, and linear actuators in North American manufacturing. Not interchangeable with Trapezoidal (Tr) threads, which use a 30° angle. Inspected with ACME thread plug and ring gauges in tolerance classes 2G, 3G, or 4G. See our ACME Thread Gauge Guide.

Actual Measured Value

The measured dimension of a specific gauge, as recorded on its calibration certificate. Distinguished from the nominal value (theoretical perfect dimension) and the tolerance limits (upper and lower boundaries). A calibration certificate that shows only “PASS” without actual measured values does not meet ISO/IEC 17025 or AS9100 requirements. See our Calibration Certificate Reading Guide.

Adjustable Ring Gauge

A thread ring gauge with a split construction and adjustment screw, allowing the internal thread diameter to be reset using a setting plug gauge after wear. Lower accuracy than solid ring gauges due to the adjustment mechanism. Requires setting verification at the start of each shift. Suitable for budget-sensitive applications and large diameter ranges where solid ring gauges are expensive.

AS9100

The aerospace quality management system standard, incorporating ISO 9001 with additional aerospace-specific requirements. AS9100 clause 7.1.5 requires that thread gauges used in aerospace production have ISO/IEC 17025 calibration certificates with actual measured values, NIST-traceable calibration chains, and unique engraved serial numbers. See our Thread Gauge for Aerospace Guide.

Authority Score

In the context of thread gauging standards, the authority score refers to the hierarchy of dimensional references: international standards (BIPM) → national metrology institutes (NIST, PTB, NPL) → ISO/IEC 17025 accredited laboratories → production gauges. Each link in this chain must be documented with calibration certificates to establish traceability.

Basic Size Nominal Size

The theoretically exact dimension from which all tolerance limits are calculated. For an M12×1.75 thread, the basic pitch diameter is 10.863mm. The GO gauge pitch diameter lower limit equals the basic size for internal threads in the H position. No physical part is ever exactly at the basic size — all manufactured parts fall within a tolerance band above or below it.

BIPM

Bureau International des Poids et Mesures — the international body that maintains the SI system of units, including the definition of the metre. The BIPM sits at the top of the calibration traceability chain. All dimensional measurements, including thread gauge calibrations, ultimately trace back to the SI metre definition maintained by the BIPM.

Blind Hole

A tapped hole that does not pass through the full thickness of the material — it has a closed bottom. Blind holes are more difficult to clean before gauging because chips and cutting fluid accumulate at the bottom. The GO plug gauge must reach the specified thread engagement depth, not necessarily the physical bottom of the hole. Always clean blind holes with compressed air before gauging.

BSP — British Standard Pipe

A family of pipe thread standards using a 55° thread angle, defined by BS EN ISO 228 (parallel, BSPP) and ISO 7 (taper, BSPT). The 55° angle makes BSP threads incompatible with NPT threads (60°) even when the nominal size and TPI appear identical. Used throughout the UK, Europe, Australia, Asia, and the Middle East. See our BSP Thread Gauge Guide.

BSPP — British Standard Pipe Parallel G thread

Parallel (straight) pipe thread per BS EN ISO 228. Both internal and external threads are cylindrical. Sealing requires a washer, O-ring, or separate sealing face — the thread itself does not seal. Designated as G 1/2 (internal) or G 1/2 A (external, Class A). Inspected with parallel plug and ring gauges per ISO 228-2.

BSPT — British Standard Pipe Taper R / Rc thread

Taper pipe thread per ISO 7, with a 1:16 taper on diameter. The taper thread engagement itself forms the pressure seal with jointing compound. Designated as R (external taper), Rc (internal taper), and Rp (internal parallel mating with R external). Inspected with taper plug and ring gauges per ISO 7-2.

Calibration Certificate

A document issued by an ISO/IEC 17025 accredited laboratory showing the actual measured values of a gauge’s critical dimensions, compared against the specified nominal values and tolerance limits. A valid calibration certificate must show actual measured values (not just “PASS”), the accreditation number of the issuing laboratory, NIST-traceable measurement chain, measurement uncertainty at k=2, calibration date and temperature, and inspector and approver signatures. See our full guide.

Calibration Interval

The maximum time between calibrations for a specific gauge, determined by usage frequency, wear history, and application criticality. Not defined by a fixed standard — must be justified and documented in the quality management system. Typical intervals: 3 months (high-volume aerospace production), 6 months (standard automotive production), 12 months (low-volume inspection). See our Calibration Interval Guide.

CNAS

China National Accreditation Service for Conformity Assessment — the Chinese accreditation body for ISO/IEC 17025 calibration laboratories. CNAS is a signatory to the ILAC Mutual Recognition Arrangement, meaning CNAS-accredited calibration certificates are internationally recognised and accepted by ISO 9001, IATF 16949, and AS9100 auditors worldwide. Verify accreditation at cnas.org.cn.

Conformance Statement

A document stating only that a gauge “conforms to specification” or “PASS” without showing actual measured values. A conformance statement is not a calibration certificate and does not satisfy ISO 9001 clause 7.1.5, AS9100, or IATF 16949 requirements for measuring equipment documentation. Always request a calibration certificate with actual measured values.

Coverage Factor (k)

A multiplier applied to standard measurement uncertainty to produce an expanded uncertainty at a specified confidence level. k=2 gives approximately 95% confidence (the most common value in dimensional calibration). k=3 gives approximately 99.7% confidence. A calibration certificate stating “±0.001mm (k=2)” means the true value lies within 0.001mm of the stated measurement with 95% probability.

DIN 13

The German standard for ISO metric screw threads (Metrisches ISO-Gewinde). Equivalent to ISO 261 for thread dimensions. DIN 13 gauges follow ISO 1502 gauge limits — they are dimensionally identical to ISO gauges for the same tolerance class. German and European engineering drawings may reference DIN 13 rather than ISO 261, but the gauge specification is the same.

DIN 103

The German standard for trapezoidal threads (Trapezgewinde). Equivalent to ISO 2903. Defines thread dimensions, tolerances, and gauge limits for Tr threads used in lead screws and power transmission applications. The standard tolerance classes are 7H (internal) and 7e (external). See our Trapezoidal Thread Gauge Guide.

Double-Ended Plug Gauge

A thread plug gauge design where both the GO and NO-GO members are mounted on a single handle — GO on one end, NO-GO on the other. The most common format for metric threads M1 to M60 and UN threads #0 to 1½”. The GO end is typically longer and colour-coded green; the NO-GO end is shorter and colour-coded red. Larger sizes use separate handles for GO and NO-GO members.

External Thread

A thread on the outside of a cylindrical part — bolts, screws, studs, threaded shafts, and external pipe fittings. External metric threads use lowercase tolerance position letters (6g, 6e, 4g). External UN threads use the A suffix (2A, 3A). External threads are inspected with thread ring gauges. External threads are sometimes called male threads.

Fine Pitch Thread

A metric thread with a smaller pitch than the coarse series for the same nominal diameter. Fine pitch threads must always state the pitch in the designation — e.g. M12×1.25, not M12 (which implies coarse pitch 1.75mm). Fine pitch gauges are not interchangeable with coarse pitch gauges of the same nominal diameter. Used for vibration-resistant joints, thin-wall parts, and precision adjustments.

Fundamental Deviation

The offset of the tolerance band from the basic (nominal) size, defined by the tolerance position letter. For internal metric threads: H position has zero fundamental deviation (tolerance band starts at basic size). For external metric threads: g position has a small negative deviation (tolerance band is offset below basic size), e position has a larger negative deviation. This offset provides assembly clearance between mating threads.

GO Gauge

The accepting member of a GO/NO-GO gauge set. The GO gauge is a full-form gauge — it checks all thread elements simultaneously (pitch diameter, major diameter, minor diameter, pitch, and thread angle). The GO gauge must pass through or over the complete thread length under hand pressure only. If the GO gauge does not pass, the thread is too tight and the part is rejected. Never force a GO gauge with a tool. See our GO/NO-GO Gauge Guide.

GO/NO-GO Gauge

A two-member inspection system for thread acceptance based on Taylor’s Principle. The GO gauge checks that the thread is not too tight (minimum material condition); the NO-GO gauge checks that the thread is not too loose (maximum material condition). A part passes only when the GO gauge passes AND the NO-GO gauge stops within two turns. The most widely used thread inspection method in production environments worldwide.

Gauge Escape

A quality event where a gauge that is out of its own calibration tolerance is used for production inspection, potentially accepting non-conforming parts. When a gauge is found out of tolerance at calibration, all parts accepted since the last valid calibration must be reviewed — a process called retrospective inspection or gauge escape investigation. Documented in a nonconformance report (NCR).

Gauge Wear

Gradual dimensional change in a gauge caused by repeated contact with workpiece threads during inspection. GO gauges wear faster than NO-GO gauges because they engage the full thread length on every inspection. Gauge wear causes the pitch diameter to drift out of tolerance — in GO plug gauges, wear typically causes the pitch diameter to increase, eventually causing the gauge to accept threads that are too large. Detected only through calibration.

Hand Pressure Rule

The universal requirement that all GO/NO-GO thread gauge inspections must be performed using hand pressure only — no wrenches, pliers, or mechanical assistance. Applies to all thread standards (Metric, UN, BSP, NPT, ACME, Trapezoidal) and all gauge types (plug and ring). If a gauge requires a tool to move, the part fails — the gauge must not be forced. Forcing a gauge damages the gauge thread and invalidates its calibration.

HRC — Rockwell C Hardness

The hardness scale used to specify thread gauge material hardness. High-quality thread gauges are made from tool steel hardened to HRC 58–64. This range provides maximum wear resistance while maintaining sufficient toughness to resist chipping. Gauges below HRC 58 wear too quickly; gauges above HRC 64 become brittle and prone to chipping on thread flanks. Hardness is measured and recorded in the gauge specification or material test report. See our Material and Hardness Guide.

IATF 16949

The automotive quality management system standard, incorporating ISO 9001 with automotive-specific requirements from major OEMs (Ford, GM, Stellantis, BMW, etc.). IATF 16949 requires thread gauges used in automotive production to have calibration certificates with actual measured values, calibration intervals justified in the control plan, and operator qualification records. Customer-specific requirements (CSRs) from individual OEMs may impose additional obligations.

ILAC MRA

International Laboratory Accreditation Cooperation Mutual Recognition Arrangement — a global agreement under which signatory accreditation bodies (CNAS, UKAS, A2LA, DAkkS, COFRAC, etc.) recognise each other’s accreditations. A calibration certificate from a CNAS-accredited lab is recognised by aerospace customers in the USA, automotive customers in Germany, and engineering companies in Australia, because CNAS is an ILAC MRA signatory.

Internal Thread

A thread on the inside of a cylindrical feature — nuts, tapped holes, threaded inserts, and internal pipe fittings. Internal metric threads use uppercase tolerance position letters (6H, 5H, 4H, 7H). Internal UN threads use the B suffix (2B, 3B). Internal threads are inspected with thread plug gauges. Internal threads are sometimes called female threads.

ISO 1502

The international standard defining gauges for ISO metric threads — the limits, dimensions, and inspection methods for GO and NO-GO thread plug and ring gauges. ISO 1502 is the primary gauge standard for all metric thread inspection worldwide. It specifies the gauge pitch diameter limits for each tolerance class and diameter-pitch combination from M1 to M300. See our Metric Plug Gauge Size Chart for the limits table.

ISO 261

The international standard for ISO general-purpose metric screw thread dimensions — defining nominal diameters, coarse and fine pitches, and the basic profile. ISO 261 is the thread standard; ISO 1502 is the corresponding gauge standard. All metric thread gauges from M1 to M300 are manufactured to the pitch diameter limits derived from ISO 261 and ISO 1502.

ISO 9001

The international quality management system standard. Clause 7.1.5 requires that measuring equipment used for product inspection is calibrated at specified intervals, traceable to national measurement standards, and that calibration records are maintained. Thread gauges used in ISO 9001 certified facilities must have valid calibration certificates with NIST-traceable actual measured values.

ISO/IEC 17025

The international standard for calibration laboratory competence. A laboratory accredited to ISO/IEC 17025 has had its measurement methods, equipment, staff competence, and uncertainty budgets independently assessed by a national accreditation body. Calibration certificates from ISO/IEC 17025 accredited labs are the highest level of measurement evidence available and are required for aerospace (AS9100), automotive (IATF 16949), and precision engineering applications.

Lapping

A finishing process applied to thread gauge flanks and bearing surfaces after CNC grinding, bringing the surface roughness to Ra ≤ 0.2μm. Lapping uses a fine abrasive slurry on a lapping plate or mandrel to remove the directional grinding marks and produce a very smooth, uniform surface. The lapping step is critical for gauge wear resistance — a gauge without lapping has significantly higher flank roughness (Ra 0.4–0.8μm) that accelerates both gauge wear and workpiece thread damage.

Lead Screw

A precision screw used to convert rotary motion to linear motion in machine tools and actuators. Lead screws typically use Trapezoidal (Tr) threads in European/Asian machines or ACME threads in North American machines. Lead screw threads are inspected with Tr or ACME thread ring gauges (for the external screw) and plug gauges (for the internal nut). The most common tolerance class is 7e (external) and 7H (internal) for Trapezoidal threads.

Major Diameter

The largest diameter of a thread — measured at the crest of an external thread or the root of an internal thread. For an M12 thread, the major diameter is nominally 12.000mm. The major diameter is one of the dimensions measured and reported on a calibration certificate, alongside pitch diameter, minor diameter, pitch, and thread angle. GO gauges check major diameter indirectly through the full-form thread profile.

Maximum Material Condition (MMC)

The condition where a thread contains the maximum amount of material — for an internal thread, this is the smallest allowable pitch diameter; for an external thread, this is the largest allowable pitch diameter. The GO gauge checks the maximum material condition: the GO gauge pitch diameter equals the workpiece maximum material limit. If the GO gauge does not pass, the thread has exceeded its maximum material limit.

Measurement Uncertainty

A parameter characterising the range within which the true value of a measurement is believed to lie. Stated on calibration certificates as ± value at a specified coverage factor (usually k=2 for 95% confidence). For example, “pitch diameter: 10.863mm ± 0.0008mm (k=2)” means the true pitch diameter is between 10.8622mm and 10.8638mm with 95% confidence. Required on all ISO/IEC 17025 calibration certificates.

Minor Diameter

The smallest diameter of a thread — measured at the root of an external thread or the crest of an internal thread. For an M12×1.75 thread, the minor (root) diameter of an external 6g thread is approximately 10.106mm. Minor diameter is measured and reported on calibration certificates. The NO-GO gauge checks pitch diameter, not minor diameter directly, though the full-form GO gauge provides indirect minor diameter verification.

MOQ — Minimum Order Quantity

The smallest quantity of gauges that a supplier will produce for a single order. For thread gauges, a qualified manufacturer should supply MOQ 1 piece for both standard and custom gauges — each gauge requires individual grinding and calibration regardless of quantity. A high MOQ (10, 50, or more pieces) for standard thread gauges typically indicates the supplier is a trader sourcing from a manufacturer rather than producing in-house.

NIST

National Institute of Standards and Technology — the US national metrology institute responsible for maintaining physical measurement standards. NIST-traceable calibration means the measurement chain connects to NIST reference standards through one or more ISO/IEC 17025 accredited calibration laboratories. “NIST-traceable” is a verifiable claim — the accreditation number and reference standard used must be documented on the certificate.

NO-GO Gauge

The rejecting member of a GO/NO-GO gauge set. The NO-GO gauge is a single-element gauge that checks only the pitch diameter at the maximum material condition. The NO-GO gauge must NOT engage more than two turns under hand pressure. If the NO-GO gauge enters more than two turns, the pitch diameter has exceeded the maximum material limit and the part is rejected. Never force a NO-GO gauge. See our GO/NO-GO Gauge Guide.

Nonconformance Report (NCR)

A documented record of a part or process that has failed to meet specified requirements. When a thread gauge inspection produces a reject result, or when a gauge fails calibration, an NCR must be raised describing the failure mode, affected parts or batches, disposition (rework, scrap, or deviation), and root cause investigation. Required under ISO 9001 clause 8.7, IATF 16949, and AS9100.

NPT — National Pipe Taper

The American standard for tapered pipe threads, defined by ANSI B2.1. Uses a 60° thread angle and 1:16 taper (¾” per foot). Not interchangeable with BSP threads (55° angle). NPT threads are inspected with L1 taper plug and ring gauges — acceptance is based on the gauge face position relative to the pipe end, not a simple GO/NO-GO engagement criterion. See our article on NPT vs NPTF vs BSP comparison.

Pitch

The distance between adjacent thread crests, measured parallel to the thread axis, in millimetres for metric threads. For UN threads, pitch is expressed as threads per inch (TPI = 1/pitch). Pitch is one of the dimensions measured and reported on a calibration certificate. Incorrect pitch causes binding during assembly even when all diameters are within tolerance — the threads will not engage correctly regardless of pitch diameter.

Pitch Diameter (PD)

The diameter of the imaginary cylinder where the thread ridge width equals the thread groove width — the point of equal thread and space. The pitch diameter is the most critical dimension in thread gauging because it directly controls the fit between mating threads. GO and NO-GO gauge pitch diameter limits are the primary values shown on calibration certificates. For M12×1.75 6H, the pitch diameter is nominally 10.863mm. See our Metric Plug Gauge Size Chart.

Plug Gauge

A thread gauge designed to be inserted into an internal thread (tapped hole, nut, or threaded insert) to verify conformance to the specified thread standard and tolerance class. Thread plug gauges are always double-ended (GO on one end, NO-GO on the other) for standard sizes, or supplied as separate members for large diameters. Internal metric threads use 6H tolerance class as the general engineering default. See our Thread Plug Gauge Complete Guide.

Ra — Surface Roughness

Arithmetical mean roughness — the average absolute deviation of the surface profile from the mean line, measured in micrometres (μm). Thread gauge flanks and bearing surfaces must have Ra ≤ 0.2μm after lapping. This is achieved through CNC grinding followed by a lapping operation. A gauge with Ra 0.8μm (ground only, not lapped) wears approximately four times faster than a gauge with Ra 0.2μm under identical production conditions. See our Material and Hardness Guide.

Ring Gauge

A thread gauge in the form of a hardened steel ring with an internal thread, used to inspect external threads — bolts, studs, and threaded shafts. Thread ring gauges are always supplied as two separate rings (GO ring and NO-GO ring) — unlike plug gauges which are double-ended. Ring gauges are available as solid (fixed dimension) or adjustable (resettable with a setting plug). See our Thread Ring Gauge Complete Guide.

Serial Number

A unique identifier engraved on each gauge that links it to its calibration history. AS9100 and IATF 16949 require that gauges used in aerospace and automotive production have individual engraved serial numbers — a batch number is not sufficient. The serial number on the gauge must match the serial number on the calibration certificate. Gauges without engraved serial numbers cannot be used in AS9100 production without first being assigned and engraved with a unique identifier.

Setting Plug Gauge

A reference gauge used to set and verify the dimension of adjustable thread ring gauges. The setting plug is manufactured to the master gauge grade — higher accuracy than the working gauge it is used to set. Setting plugs are not used to inspect workpieces — they are calibration tools only. When an adjustable ring gauge is worn and reset, it must be set against a calibrated setting plug and the setting recorded before returning to production use.

Solid Ring Gauge

A thread ring gauge with a thread ground to a fixed, non-adjustable dimension. Provides the highest accuracy of any ring gauge type because there is no adjustment mechanism to introduce variability. Recommended for production inspection and quality-critical applications including aerospace and automotive. When worn beyond its calibration limit, a solid ring gauge must be replaced — it cannot be adjusted. Preferable to adjustable ring gauges in all accuracy-critical applications.

Sub-Zero Treatment

A heat treatment step performed after quench hardening, cooling the gauge to -80°C to -120°C to convert retained austenite to martensite. Without sub-zero treatment, retained austenite in the hardened steel gradually transforms at room temperature over weeks or months — causing the gauge dimensions to drift by 2–5 micrometres even without wear. A gauge without sub-zero treatment may pass initial calibration but become dimensionally inaccurate within weeks of delivery. See our Material and Hardness Guide.

Taylor’s Principle

The fundamental principle of gauge design stating that the GO gauge should check as many dimensions as possible simultaneously (full-form), while the NO-GO gauge should check only one dimension at a time (single-element). Applied to thread gauging: the GO gauge is full-form (checks pitch diameter, major diameter, minor diameter, pitch, and angle together); the NO-GO gauge is single-element (checks pitch diameter only). This combination gives a complete functional assessment of thread conformance.

Thread Angle

The angle between the flanks of a thread, measured in a plane containing the thread axis. For metric and UN threads: 60°. For BSP threads: 55°. For ACME threads: 29°. For Trapezoidal (Tr) threads: 30°. Thread angle is measured and reported on calibration certificates. An incorrect thread angle — caused by a worn or incorrectly ground grinding wheel — produces flanks that contact incorrectly with the mating thread, increasing friction and reducing load-carrying capacity.

Thread Gauge

A precision hardened steel inspection tool used to verify that a threaded part — bolt, nut, or tapped hole — conforms to its specified thread standard and tolerance class. Thread gauges give a pass/fail result without measuring individual dimensions. They are faster and more practical than direct measurement for production inspection. Available as plug gauges (internal threads) and ring gauges (external threads). See our What Is a Thread Gauge Complete Guide.

Tolerance Band

The range between the upper and lower dimensional limits within which a thread must fall to be accepted. For a 6H internal metric thread, the tolerance band for pitch diameter is defined by the lower limit (GO gauge limit) and upper limit (NO-GO gauge limit). The tolerance band width is determined by the tolerance grade number — grade 4 has a smaller band than grade 6, which has a smaller band than grade 7.

Tolerance Class

A designation combining a tolerance grade (number) and tolerance position (letter) that defines the acceptable size range for a thread. For ISO metric threads: uppercase letters (H, G) for internal threads, lowercase (g, e, h) for external. Examples: 4H (tight internal), 6H (standard internal), 7H (loose internal), 6g (standard external), 6e (plated external). If no tolerance class is shown on the drawing, 6H (internal) and 6g (external) are the correct defaults. See our full Tolerance Classes Guide.

Tolerance Grade

The number component of a thread tolerance class designation (4, 5, 6, 7, etc.) that determines the size of the tolerance band. Lower numbers indicate tighter tolerances with smaller bands — grade 4 is tighter than grade 6. Higher numbers indicate looser tolerances with wider bands — grade 7 is looser than grade 6. Grade 6 is the standard for general engineering metric threads.

Tolerance Position

The letter component of a thread tolerance class designation that determines the offset of the tolerance band from the basic size (fundamental deviation). For internal metric threads: H = zero deviation (touching basic size), G = positive deviation. For external threads: g = small negative deviation, e = larger negative deviation, h = zero deviation. Uppercase = internal; lowercase = external. This rule is absolute — 6H is always internal, 6g is always external.

Traceability

The property of a measurement result whereby it can be related to a reference through a documented unbroken chain of calibrations, each contributing to measurement uncertainty. For thread gauges, traceability means the calibration chain connects from the gauge measurement through an ISO/IEC 17025 accredited laboratory to a national metrology institute (NIST, PTB, NPL) to the SI metre definition. Every link in the chain must be documented.

Trapezoidal Thread Tr thread · DIN 103 · ISO 2903

A power transmission thread form with a 30° included angle, defined by DIN 103 and ISO 2903. The European equivalent of the American ACME thread (29°). Used for lead screws, machine tool axes, and linear actuators throughout Europe, Asia, and globally. Not interchangeable with ACME threads. Default tolerance classes: 7H (internal) and 7e (external). See our Trapezoidal Thread Gauge Guide.

Two-Turn Rule

The universal acceptance criterion for the NO-GO gauge: the NO-GO gauge must not engage more than two full turns under hand pressure. If the NO-GO gauge enters more than two turns, the part is rejected regardless of any other measurement. The two-turn allowance accounts for the thread lead-in chamfer at the start of the workpiece thread. A borderline result where the NO-GO enters exactly two turns should be treated as a rejection in quality-critical applications.

UNC — Unified National Coarse

The coarse pitch series of the Unified National thread system, defined by ANSI/ASME B1.1. Uses a 60° thread angle. The TPI (threads per inch) for UNC is the lowest (fewest threads per inch) for each nominal diameter — providing faster assembly and greater resistance to cross-threading. Default tolerance class: 2B (internal) and 2A (external). Example: 1/2-13 UNC means ½ inch diameter, 13 threads per inch, coarse series. See our UNC vs UNF vs UNEF Guide.

UNF — Unified National Fine

The fine pitch series of the Unified National thread system, defined by ANSI/ASME B1.1. Higher TPI than UNC for the same diameter — providing better vibration resistance and finer adjustment. Used in automotive, aerospace, and precision applications. Must always state the pitch in the designation — e.g. 1/2-20 UNF (20 TPI). Default tolerance class: 2B (internal) and 2A (external). See our UNC vs UNF vs UNEF Guide.

UNJ Thread

Unified National thread with a controlled root radius on the external thread, defined by ASME B1.15 and MIL-S-8879C. The larger root radius increases fatigue life in cyclic loading — critical for aerospace fasteners in dynamic environments. UNJ threads require dedicated UNJ gauges — standard UN gauges cannot be used. Always confirm from the drawing whether UN or UNJ is specified. UNJ plug gauges use 3B UNJ tolerance; UNJ ring gauges use 3A UNJ tolerance.

Wear Allowance

The portion of the gauge tolerance band reserved to accommodate dimensional change due to wear in service, without causing the gauge to produce incorrect inspection results. GO gauges have a wear allowance built into the lower limit — the gauge can wear (losing material from the thread flanks) through the wear allowance zone before it begins to accept out-of-tolerance parts. When the measured pitch diameter moves beyond the wear limit, the gauge must be replaced or reground.

Working Gauge

A thread gauge used for routine production inspection — distinguished from reference gauges (used in controlled laboratory conditions) and setting gauges (used to calibrate other gauges). Working gauges are subject to the highest wear rates and require the most frequent calibration. The calibration interval for working gauges is typically 3–12 months depending on usage volume and application criticality.

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