// Direct Answer
Thread gauge inspection follows six steps: (1) verify gauge calibration status; (2) clean workpiece thread and gauge; (3) apply light oil for fine pitch threads; (4) apply GO gauge under hand pressure only — must pass fully; (5) apply NO-GO gauge under hand pressure only — must not engage more than two turns; (6) record the result and decision. A part passes only when GO passes AND NO-GO stops within two turns. Never force a gauge. Never use tools. Always record the result with the gauge serial number and calibration date.
Before Inspection — Pre-Use Checks
Before any thread gauge is used for production inspection, three pre-use checks must be completed. Skipping these steps is the most common cause of false inspection results and undetected gauge wear.
A
Check Calibration Status
Required before every production session — not just once per day
Check the calibration label on the gauge or its case. The label must show a calibration date within the valid interval for your quality management system — typically 6 to 12 months for production gauges. If the gauge is overdue for calibration, remove it from service immediately and quarantine it.
✅ Gauge is ready to use when:
- Calibration label is present and legible
- Calibration date is within valid interval
- Gauge serial number matches the label
- No quarantine tag is attached
❌ Remove from service when:
- No calibration label present
- Calibration date has expired
- Gauge has been dropped since last calibration
- Quarantine tag is attached
B
Visual Inspection of the Gauge
Check for visible wear, damage, or contamination
Examine the gauge thread under good lighting. Look for polished wear marks on thread flanks (indicates dimensional loss), flat spots on thread crests, surface rust or corrosion, burrs or nicks, and any contamination in thread roots. A gauge with visible damage must be quarantined and sent for calibration — not used.
For adjustable ring gauges, also verify the gauge has been set against its calibrated setting plug at the start of the current shift. Record the setting verification in your inspection log. See our Thread Gauge Calibration Guide for more on wear indicators.
C
Confirm Gauge Matches the Drawing
Thread designation, pitch, and tolerance class must match
Read the engraving on the gauge handle or body. Confirm that the thread designation (e.g. M12×1.75), tolerance class (e.g. 6H), and gauge member (GO or NO-GO) match the engineering drawing callout for the part being inspected. Using the wrong gauge — correct size but wrong tolerance class — is a common error that produces incorrect acceptance decisions.
| Drawing shows | You need | Wrong gauge (common mistake) |
|---|
| M12×1.75-6H (internal) | M12×1.75 6H Plug Gauge | M12×1.75 6g Ring Gauge |
| M12×1.75-6g (external) | M12×1.75 6g Ring Gauge | M12×1.75 6H Plug Gauge |
| M12×1.75-4H (tight) | M12×1.75 4H Plug Gauge | M12×1.75 6H Plug Gauge |
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Thread Plug Gauge Inspection Procedure — Internal Threads
Use this procedure for inspecting internal threads — tapped holes, nuts, threaded inserts, and internal pipe threads. For ring gauge procedure (external threads), see the next section.
1
Clean the Tapped Hole and the Gauge
Remove all chips, cutting fluid, and contamination
a
Blow out the tapped hole with compressed air to remove chips and swarf from thread roots. If compressed air is not available, use a small brush.
b
Wipe the gauge thread with a clean lint-free cloth to remove storage oil, dust, and any contamination from previous use.
c
For fine pitch threads (pitch below 1.0mm), use a soft brass brush to clean debris from the thread roots of both the hole and the gauge.
d
Apply a thin film of light instrument oil to the gauge thread if the thread surface appears dry. Do not use grease, heavy oil, or cutting fluid.
2
Apply the GO Plug Gauge
Hand pressure only · Must thread through the full hole length
a
Identify the GO end of the double-ended plug gauge — it is the longer end, typically marked with a green band or groove, or labelled GO on the handle.
b
Align the GO gauge thread with the tapped hole thread. Insert and rotate by hand — two to three fingers on the handle only. Apply no mechanical assistance.
c
The GO gauge must thread through the complete length of the tapped hole under hand pressure only. For blind holes, the GO gauge must reach the full thread engagement depth.
d
GO test PASSED — gauge threads through fully under hand pressure.
GO test FAILED — gauge does not pass freely at any point. Do not force. Record as rejected.
3
Apply the NO-GO Plug Gauge
Hand pressure only · Must not engage more than 2 turns
a
Identify the NO-GO end — it is the shorter end, typically marked with a red band or groove, or labelled NO-GO on the handle.
b
Align the NO-GO gauge thread with the tapped hole thread. Insert and rotate by hand under light finger pressure only.
c
Count the turns as the NO-GO gauge enters the hole. Stop immediately if it enters freely past two turns — do not continue rotating.
d
NO-GO test PASSED — gauge stops within two turns under hand pressure.
NO-GO test FAILED — gauge enters more than two turns. Record as rejected.
Thread Ring Gauge Inspection Procedure — External Threads
Use this procedure for inspecting external threads — bolts, studs, threaded shafts, and external pipe fittings. For thread ring gauge specifications, see our full ring gauge guide.
1
Clean the External Thread and the Ring Gauge
Remove all swarf, burrs, and cutting fluid from the thread
a
Wipe the external thread with a lint-free cloth to remove cutting fluid, chips, and surface contamination. Pay attention to thread roots and the lead-in chamfer.
b
Check the thread start for burrs from the facing operation. A burr at the thread start will prevent the GO ring from engaging even if the thread is dimensionally correct — causing a false rejection.
c
Wipe the ring gauge bore with a clean lint-free cloth. Apply a thin film of instrument oil to the ring gauge internal thread if it appears dry.
d
For adjustable ring gauges: verify the gauge setting against the calibrated setting plug before starting the inspection session. Record the setting check.
2
Apply the GO Ring Gauge
Hand pressure only · Must thread over the full bolt length
a
Identify the GO ring — it is usually marked GO with a green colour code or engraving on the outer face. GO and NO-GO ring gauges are always supplied as separate rings.
b
Place the GO ring over the start of the external thread. Align carefully and begin rotating by hand, applying only finger pressure.
c
The GO ring must thread over the full length of the external thread under hand pressure. For studs and long bolts, the ring must reach the full threaded length.
d
GO test PASSED — ring threads over the full length under hand pressure.
GO test FAILED — ring does not pass at any point. Do not force. Record as rejected.
3
Apply the NO-GO Ring Gauge
Hand pressure only · Must not engage more than 2 turns
a
Identify the NO-GO ring — marked NO-GO with a red colour code or engraving. Never confuse the GO and NO-GO rings — they have different internal thread dimensions.
b
Place the NO-GO ring over the start of the external thread. Rotate by hand under light finger pressure only.
c
Count the turns as the NO-GO ring engages. Stop immediately if it passes two turns freely.
d
NO-GO test PASSED — ring stops within two turns under hand pressure.
NO-GO test FAILED — ring engages more than two turns. Record as rejected.
Hand pressure rule — applies to all gauges, all thread standards: Every step of the GO/NO-GO inspection must be performed under hand pressure only — no wrenches, pliers, spanners, or any mechanical assistance. A gauge that requires tool assistance to move is either indicating a failing part or is itself damaged. Forcing a gauge damages the gauge thread, invalidates the calibration certificate, and may cause injury.
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Reading and Interpreting Inspection Results
| GO Result | NO-GO Result | Final Decision | Root Cause Investigation |
|---|
| ✓ Passes fully | ✓ Stops ≤ 2 turns | ACCEPT | Thread is within tolerance — proceed to next part |
| ✗ Does not pass | Any | REJECT | Thread oversize, wrong pitch, tap broken, or work-hardened material — inspect visually and check tap condition |
| ✓ Passes fully | ✗ Passes > 2 turns | REJECT | Thread undersize — pitch diameter below minimum material limit — check tap size, tool wear, or machining parameters |
| ✗ Does not pass | ✗ Passes > 2 turns | REJECT | Severe damage — inspect visually before scrapping. Consider gauging with a different gauge to confirm |
Borderline NO-GO result (exactly 2 turns): If the NO-GO gauge enters exactly two turns and stops, the thread is at the boundary of acceptance. Industry practice is to reject the part when there is any doubt — a borderline result close to the NO-GO limit may indicate the thread is at the extreme of tolerance, and gauge wear or measurement uncertainty could make the true condition unclear. When in doubt, reject and escalate to your quality supervisor.
Recording Inspection Results — ISO 9001 Requirements
ISO 9001 clause 7.1.5 requires that inspection results are recorded with sufficient information to demonstrate that the measurement was performed with calibrated equipment. The following minimum information must be recorded for each inspection event.
// MINIMUM INSPECTION RECORD — PER PART OR BATCH
Date and time
2025-09-15 · 08:42
Part number / revision
PN-4521-A / Rev C
Batch / lot number
LOT-2025-0915-001
Thread callout inspected
M12×1.75-6H
Gauge type and serial number
Plug Gauge · SN: GT-2024-0892
Calibration date / next due
2025-03-10 · Next due: 2026-03-10
GO result
PASS — threaded through fully
NO-GO result
PASS — stopped within 2 turns
Inspector name / ID
J. Chen · EMP-042
Quantity inspected / rejected
50 inspected · 0 rejected
For IATF 16949 and AS9100 suppliers, additional records may be required — including the control plan reference, operation number, and sampling plan used. Check your customer-specific requirements (CSRs) for any additional documentation obligations.
Handling Rejected Parts
When a part fails the GO/NO-GO inspection, a defined nonconformance procedure must be followed. Ad-hoc handling of rejected parts — leaving them on the inspection bench, returning them to the production flow, or disposing of them without documentation — is a major audit finding under ISO 9001 clause 8.7.
🏷️
Step 1: Tag and Quarantine
Attach a red nonconformance tag immediately. Move to the designated quarantine area. Prevent any possibility of the part re-entering the production flow.
📝
Step 2: Record and Report
Complete a nonconformance report (NCR) with part number, batch, failure mode (GO fail or NO-GO fail), and quantity. Report to the quality supervisor and production team.
🔍
Step 3: Investigate Root Cause
Identify whether the cause is a worn tap, wrong tool, incorrect machining parameters, or gauge issue. Check the gauge calibration status before assuming the gauge is correct.
If the gauge itself may be at fault: When a high or unexpected rejection rate occurs — particularly if a known-good reference part fails — quarantine the gauge and send it for calibration before continuing production. Do not assume the gauge is correct. An out-of-tolerance GO gauge will reject good parts; an out-of-tolerance NO-GO gauge will accept bad parts. See our Calibration Guide for when to recalibrate immediately.
After Inspection — Gauge Care and Storage
Proper post-inspection care is as important as the inspection procedure itself. Gauges that are not cleaned and stored correctly after use wear faster and produce unreliable results. For a complete care guide, see Thread Gauge Storage and Handling.
Immediately After Use
- Wipe gauge thread with clean lint-free cloth
- Remove all chips, cutting fluid, and workpiece debris
- Apply thin film of instrument oil to thread surface
- Return gauge to individual case or labelled slot
- Never leave gauge on machine table or bench
End of Shift
- Confirm all gauges are returned to storage
- Check no gauges have been left at workstations
- Update inspection log with gauges used
- Flag any gauges showing unusual wear for supervisor review
- Confirm adjustable ring gauge settings are locked
ISO 9001 Thread Inspection Requirements Summary
| ISO 9001 Clause | Requirement | How Thread Gauging Satisfies It |
|---|
| 7.1.5 | Measuring equipment must be calibrated at specified intervals, traceable to national standards | ISO/IEC 17025 calibration certificate with NIST-traceable values, within valid interval |
| 7.1.5 | Calibration status must be identifiable | Calibration label on gauge/case showing date and next due date |
| 8.6 | Product must be inspected and released only when it meets requirements | GO/NO-GO pass decision documented before release |
| 8.7 | Nonconforming outputs must be identified and controlled | Rejected parts tagged, quarantined, and documented in NCR |
| 9.1 | Inspection results must be retained as documented information | Inspection records with gauge serial number, date, and result |
For the wider picture — inspecting parts with different gage types and verifying the gages themselves — read our gage inspection guide.
Frequently Asked Questions
How many times should I inspect each part — GO and NO-GO once each?
Yes — one GO inspection and one NO-GO inspection per thread per part is standard practice. Both must be performed for a complete inspection. In high-volume production, 100% inspection (every part inspected) is required for safety-critical threads. For non-critical applications, statistical sampling plans (AQL sampling) may be used — the plan must be defined in your control plan and accepted by your customer. Under no circumstances should the NO-GO inspection be omitted to speed up production.
What should I do if the GO gauge passes but leaves black marks on the thread?
Black marks or smearing on the thread surface after the GO gauge passes indicate metal-to-metal contact — either the thread is at the tight end of tolerance or the gauge thread is not properly lubricated. Apply instrument oil to the gauge thread and re-inspect. If the marks persist, the thread may be borderline acceptable — the GO gauge is passing, but there is minimal clearance. Document the observation and consider sending the gauge for calibration to verify its dimensions are still within specification.
Can the same gauge be used by multiple operators on the same shift?
Yes, provided each operator follows the same pre-use check procedure — verifying calibration status and cleaning the gauge before use. The inspection record must identify the individual operator (name or ID) who performed each inspection, not just the shift. For IATF 16949 suppliers, gauge handling training and operator qualification records are typically required and subject to audit.
Does the inspection procedure change for taper threads like NPT or BSP-T?
Yes. Taper thread gauges (NPT, BSPT) use a position-based acceptance criterion rather than the simple GO/NO-GO pass/fail of parallel thread gauges. For NPT, the L1 plug gauge is inserted and the gauge face position relative to the pipe end is checked against specified limits — typically ±1 turn from the hand-tight position. The principle is similar (hand pressure only, no tools) but the acceptance criterion is dimensional position rather than whether the gauge passes through fully.
What records are required for an IATF 16949 audit?
For IATF 16949, inspection records must include: part number and revision, operation number from the control plan, gauge identification and calibration status, inspection result (pass/fail) with date and operator ID, sampling plan reference if applicable, and nonconformance report reference for any rejected parts. Gauge calibration records must be maintained with the calibration certificate, interval justification, and any out-of-tolerance disposition records. Customer-specific requirements (Ford, GM, Stellantis, etc.) may add further documentation obligations.
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