Published July 12, 2026 at 16:43
A bolt-manufacturing line produces 20,000 M6 screws per day. Quality control needs to verify that the threads meet ISO 1502-6g tolerance. MEASURING every screw with a thread micrometer would take 30 seconds per piece — 167 hours for a full day’s production. Impossible.
With thread ring gauges, the same verification takes 2 seconds per screw:
- Take an M6 screw from the production line.
- Screw it into the M6 GO thread ring gauge. If it goes in smoothly all the way — the thread’s PITCH DIAMETER is within the tolerance band.
- Screw it into the M6 NoGo thread ring gauge. If it GOES IN more than 1-2 turns — the thread’s PITCH DIAMETER is too small (reject).
This is GO/NO-GO gauging for threads — the counterpart to the plug gauges I wrote about earlier (for hole diameters). The difference is that here you measure EXTERNAL threads (screws, bolts) instead of INTERNAL threads (nuts) or holes.
Thread tolerance per ISO 1502
ISO 1502 defines tolerance classes for metric threads:
- 3g: Tight tolerance for precision applications. Optical instruments, medical equipment.
- 4g: Fine tolerance. Precision mechanics.
- 6g: STANDARD tolerance for general industry. This is what this set covers.
- 8g: Coarse tolerance. Heavy-duty construction, less critical fit.
”g” stands for the CREST tolerance (external thread tolerance). The corresponding ”h” defines the root tolerance. A 6g screw has its pitch diameter at the lower end of the tolerance range; a 6h nut has its pitch diameter at the upper end of the tolerance range. The combination provides a fit with defined clearance.
For M6-6g:
- Nominal pitch diameter: 6.000 mm.
- Upper limit: 5.974 mm.
- Lower limit: 5.750 mm.
Tolerance band: 224 μm. An acceptable M6-6g screw has a pitch diameter between 5.750 and 5.974 mm.
GO vs NoGo ring gauge
A complete thread gauging system consists of a pair:
GO ring gauge: Diameter matched to the UPPER TOLERANCE LIMIT (5.974 mm for M6-6g). SHOULD fit onto an acceptable screw. If the screw does NOT thread through → the pitch diameter is too LARGE.
NoGo ring gauge: Diameter matched to the LOWER TOLERANCE LIMIT + 1-2 turns of margin (5.750 mm + tolerance). SHOULD NOT fit onto an acceptable screw. If the screw THREADS THROUGH more than 1-2 turns → the pitch diameter is too SMALL.
Acceptable screw: GO fits AND NoGo does not fit. The standard workshop workflow.
This set contains ONLY the NoGo versions (M3-M12). The GO rings are sold separately (typically ~4,000 SEK per set of the same range). A combined set (GO + NoGo for M3-M12) would cost ~10,000 SEK.
Why only NoGo in this set?
Practical workshop experience: the NoGo gauge is the CRITICAL tester. Reason:
- Threads are often manufactured slightly UNDERSIZED relative to nominal diameter (it’s hard to make them ”too large” with common manufacturing methods).
- The common defect is a thread that’s TOO SMALL — undercutting, tool wear, an incorrect thread tap.
- The NoGo gauge catches this defect immediately.
Many workshops keep GO rings as a reference and only routinely check NoGo on every production batch. This set strategy covers the most common defect with the smallest investment.
Composition: 7 sizes
This set includes NoGo thread rings for:
- M3 (Ø 3 mm nominal, thread pitch 0.5 mm).
- M4 (Ø 4 mm, 0.7 mm).
- M5 (Ø 5 mm, 0.8 mm).
- M6 (Ø 6 mm, 1.0 mm).
- M8 (Ø 8 mm, 1.25 mm).
- M10 (Ø 10 mm, 1.5 mm).
- M12 (Ø 12 mm, 1.75 mm).
These 7 sizes cover 80-90% of common lathe and press-thread sizes in medium-sized workshop production. Larger threads (M14+) and smaller ones (M2 and below) require specialized versions.
Material and manufacturing
The ring gauges are made of HARDENED STEEL (HRC 62+) with a precision-ground internal thread. Manufacturing tolerance on the internal diameter is typically ±5 μm. The material’s wear tolerance is sufficient for 20,000-50,000 usage cycles before the measured value drifts outside the tolerance band.
Alternative materials (specialized versions):
- Carbide (for higher wear resistance): 3-5× more expensive, 5-10× longer service life.
- Ceramic: rare, for applications requiring corrosion resistance.
Standard steel is cost-effective for most workshops and can be reconditioned at an accredited lab once worn.
Usage workflow
Standard production QC:
- Take a screw from the production line.
- Clean the screw of oil, chips, dust.
- Hold the screw vertically.
- Place the NoGo ring gauge on the tip of the screw and try to thread it down.
- If the ring jams after 0-1 turns: the screw passes.
- If the ring threads down 2+ turns: the screw is rejected.
Total time per screw: 5-10 seconds.
For batch QC: 10-20 randomly selected screws per batch. If ≥ 1 is rejected — check the entire batch or stop the production line for investigation.
DIN 13 and ISO 1502 compatibility
These Schut rings are compatible with:
- ISO 1502: International standard for thread gauges.
- DIN 13: German standard for metric threads (identical to ISO 262 for thread profiles).
This means the rings can be used INTERCHANGEABLY with gauges from other manufacturers that follow the same standards. No vendor lock-in.
Calibration and maintenance
- Initial verification on delivery against the manufacturer’s included measurement report.
- After 20,000 usage cycles (typically 1-2 years of daily workshop use): recalibration.
- At an accredited calibration lab: 500-1,000 SEK per ring.
- If drift exceeds 10 μm from spec: replace (or regrind if possible).
- Store in the included wooden case. Avoid drop impacts (hardened steel can crack).
- Clean periodically with an oiled microfiber cloth.
What you get for your money
Schut set of thread ring gauges (NoGo) for metric threads M3, M4, M5, M6, M8, M10, M12 (7 pieces), tolerance 6g per ISO 1502 and DIN 13, hardened and ground steel (HRC 62+), the manufacturer’s individual measurement report for each ring, delivered in a wooden case. 5,060 SEK per set.
The standard tool for workshop QC of external thread products. A 5-second NoGo test replaces a 30-second thread micrometer measurement — 6x faster, no operator interpretation error, GO/NoGo answer. For series production of bolt and screw manufacturing, thread ring gauges are indispensable. Naturally complemented by GO versions (sold separately) for full verification.
