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Description
THREADM20x2.5
TOLERANCE6g
STANDARDISO 1502
MATERIALTool steel
M20x2.5 thread ring gauge — GO and NO-GO set
Two ring gauges for a coarse-pitch external thread per PN-ISO 1502:1998
| TECHNICAL DATA | |
|---|---|
| Thread type | ISO metric (M) — coarse pitch |
| Thread size | M20x2.5 |
| Thread pitch | 2.5 mm |
| Nominal diameter | 20.000 mm |
| Tolerance class | 6g |
| Pitch diameter d2 | 18.164 – 18.334 mm |
| Major diameter d | 19.623 – 19.958 mm |
| Gauge type | Set: GO ring + NO-GO ring |
| Material | Tool steel, hardened |
| Hardness | 58-62 HRC |
| Standard | PN-ISO 1502:1998 |
| Manufacturer | Varel |
T
GO ring gauge — screwed on by hand along the full bolt thread.
Z
NO-GO ring gauge with a groove in the knurled surface — stops after no more than 2 turns.
The M20-6g engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
M20x2.5 thread ring gauge checks a class 6g external thread — a screw, stud or threaded shaft with a coarse pitch. The GO ring must screw on by hand over the full thread length; the NO-GO ring may engage by no more than two turns. The set checks both tolerance limits; a single ring answers only half the question.
Class 6g differs from 6H in more than just the letter. The letter g shifts the entire tolerance zone downward by 0.042 mm — this is a deliberate allowance left for a coating. Electroplating increases the pitch diameter by approximately four times the coating thickness, so the class 6g allowance accommodates a layer up to approximately 10 µm thick, typical of electroplated zinc. The gauge dimensions apply to the thread before coating. For an M20 coarse thread, this gives a pitch diameter range of 18.164 – 18.334 mm, calculated from the basic value of 18.376 mm minus the allowance.
Checking a bolt with two ring gauges
The principle of limit gauging per ISO 1502
1
The GO ring gauge is screwed on by hand, without force, along the full thread of the M20 x2.5 bolt.
2
The NO-GO ring gauge stops after no more than 2 turns — turns are counted when unscrewing.
3
The GO ring gauge passed, NO-GO stopped — the bolt meets class 6g.
Thread inspection in 3 steps
1 GO ring (GO gauge)
Screwed on by hand — passes over the full length
2 NO-GO ring (NO-GO gauge)
Screwed on by hand — no more than 2 turns
✓ Acceptable thread
Class 6g confirmed
Screw the ring on without a wrench or hammering. Count turns when unscrewing it. If the thread on the part has three threads or fewer, the NO-GO ring must not pass over its entire length.
GO ring (GO gauge)
Corresponds to maximum material condition — a pitch diameter of 18.334 mm. It checks the thread as a whole: diameter, pitch and thread angle simultaneously. A thread rolled with incorrectly set dies will stop the ring after just a few turns, even if a micrometer shows nothing unusual.
NO-GO ring (NO-GO gauge)
Corresponds to minimum material condition — a dimension of 18.164 mm. The class 6g tolerance zone is 0.170 mm wide, less than one fourteenth of the pitch. The standard notes that a NO-GO ring used on rigid components departs from Taylor's principle, so its indication relates to the pitch diameter rather than the entire profile.
✓ Thread acceptable — GO screws onto the full thread, NO-GO stops.
✗ Thread oversized — GO does not screw onto the bolt.
✗ Thread undersized — NO-GO screws on more than 2 turns.
Tolerance classes for external metric threads
Why 6g rather than 6h, 4g, or 8g
Comparison of classes 4g, 6g, 6h, and 8g
The basic pitch diameter for the M20x2.5 thread is 18.376 mm. The letter defines the position of the tolerance zone: h denotes a zone adjacent to the basic size, while g shifts it downward by 0.042 mm. The digit defines the zone width.
| Class | Upper deviation [mm] | d2 permissible [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 4g | −0.042 | 18.228 – 18.334 mm | 0.106 mm | Close-fitting threads, guiding connections | |
| 6g | −0.042 | 18.164 – 18.334 mm | 0.170 mm | Standard — commercial bolt, mating with a class 6H tapped hole | THIS PRODUCT |
| 6h | 0.000 | 18.206 – 18.376 mm | 0.170 mm | Uncoated thread, made to size | |
| 8g | −0.042 | 18.069 – 18.334 mm | 0.265 mm | Coarsely rolled threads, large production runs |
Class 6g is the default for commercial screws and the natural mating class for a class 6H internal thread. Class 6h has the same tolerance zone width but no allowance — it is used where the thread will not be coated and the joint must maintain its specified fit. A gauge for 6g will not correctly inspect a 6h thread, and vice versa.
What the ring gauge set cannot check
Method limitations and thread major diameter
The rings check the pitch diameter and profile, but do not measure the thread major diameter. For class 6g, it lies within 19.623 – 19.958 mm and is checked with a micrometer or plain gauge. A thread with an undersized crest will pass ring gauging yet carry less load than assumed in the joint calculation.
The set will not assess shank straightness or coating quality. It determines only whether the thread lies within the class 6g tolerance zone.
▲
Thread inspection before coating
Class 6g dimensions apply to the thread before coating. A GO ring screwed onto a freshly zinc-plated screw can jam and scrape off the zinc layer, and the result of this measurement proves nothing anyway. After electroplating, parts are checked with a ring made for a class that accounts for the coating, or they are measured before the process.
Set contents and storage
Set contents and gauge storage
INCLUDED IN THE SET:
| ✓ | GO ring gauge M20x2.5 6g |
| ✓ | NO-GO ring gauge M20x2.5 6g |
| ✓ | NO-GO ring gauge marked with a red dot or groove |
STORAGE:
| ✓ | Store in a dry place, in its case |
| ✓ | Protection against dropping — impact alters the thread profile |
| ✓ | Apply rust-preventive oil periodically |
Frequently asked questions
Each checks a different tolerance limit. The GO ring rejects an oversized thread — one that will not fit into a nut. The NO-GO ring rejects a thread turned undersize, which will screw in freely but will not carry the load. A single GO ring is used only for in-process inspection, not acceptance inspection.
Yes, at a typical thickness. The class 6g allowance is 0.042 mm on the pitch diameter, and the coating increases this dimension by approximately four times its thickness. A layer up to approximately 10 µm thick fits within the allowance. Thicker coatings, such as hot-dip galvanizing, require a class 8g thread or an oversize-tapped nut in accordance with separate guidelines.
In the position of the tolerance zone. Both have the same width of 0.170 mm, but 6h starts at the basic size of 18.376 mm, while 6g lies 0.042 mm lower. A 6h thread is used where there will be no coating and fit accuracy matters. A gauge for one class cannot replace the other — the upper limits differ by the full allowance.
No. The fine pitch of 1.5 mm gives a different pitch diameter — 19.026 mm instead of 18.376 mm. A coarse-pitch ring will not screw onto such a thread beyond the first turn, and attempting to tighten it will damage both components. The fine thread version requires a separate set, marked with the full M20x1.5 designation.
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