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Thread plug gauge M20 6H
121323
€29.13 tax inc.
Net price: €23.68
Description
THREADM20x2.5
TOLERANCE6H
STANDARDISO 1502
MATERIALTool steel
M20x2.5 class 6H thread plug gauge
Double-ended gauge for metric coarse threads 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 | 6H |
| Pitch diameter D2 | 18.376 – 18.600 mm |
| Minor diameter D1 | 17.294 – 17.744 mm |
| Gauge type | Double-ended GO / NO-GO |
| Material | Tool steel, hardened |
| Hardness | 58-62 HRC |
| Standard | PN-ISO 1502:1998 |
| Manufacturer | Varel |
1
GO end — longer thread with a full profile. It is screwed in by hand through the full length of the hole.
2
Handle engraved with M20-6H — the size and tolerance class of the hole being checked.
3
Red ring — identifies the NO-GO end.
4
NO-GO end — shorter thread with a truncated profile. It checks only the pitch diameter, so it is allowed to enter by 2 turns.
M20x2.5 thread plug gauge checks the M20 coarse thread — the one implied by the designation “M20" alone, with no pitch specified. The GO end must screw in by hand over the full thread length. The NO-GO end may engage by no more than two turns, counted when unscrewing the gauge. Meeting both conditions means the thread complies with class 6H.
The 2.5 mm pitch is the basic pitch for this diameter per ISO 261, which is precisely why it is omitted from the designation. The store also lists an M20x1.5 gauge — the fine thread version — and these two gauges are not interchangeable in either direction. The one-millimeter pitch difference results in a different pitch diameter: 18.376 mm versus 19.026 mm. The M20x2.5 thread is used where joints need to be quick to assemble and resistant to dirt in the threads: anchor bolts, flanges, machine frames and welded structures. The dimensions are calculated directly from the profile definition: D2 = 20 − 0.6495 × 2.5 = 18.376 mm, and the minor diameter D1 = 20 − 1.0825 × 2.5 = 17.294 mm.
Operating principle: GO and NO-GO ends
Internal metric thread inspection per ISO 1502
1
The GO end is screwed in by hand, without force, through the full length of the M20 x2.5 thread.
2
The NO-GO end stops after no more than 2 turns — turns are counted when unscrewing.
3
The GO end passed, the NO-GO end stopped — the hole is within class 6H.
Thread inspection in 3 steps
1 GO end (passing)
Screwed in by hand — enters through the full length
2 NO-GO end
Screwed in by hand — no more than 2 turns
✓ Acceptable thread
Class 6H confirmed
Turns are counted when unscrewing the gauge, not when screwing it in — this is a requirement of the standard, not a shop-floor convention. If the thread in the part has three threads or fewer, the NO-GO end must not pass all the way through it.
GO end (passing)
Checks the thread as a whole — pitch diameter, pitch and thread angle simultaneously. The standard calls this Taylor's principle: the GO end assesses the thread as a whole. It corresponds to the lower limit of 18.376 mm, which is the basic size, because a class with the letter H has zero lower deviation.
NO-GO end
Checks only the pitch diameter at the upper limit of 18.600 mm. It has a truncated profile and a shorter threaded section — hence the two permissible turns. Screwing it in with force or a wrench damages the gauge and biases the result in favor of the part.
✓ Thread acceptable — GO enters the full length, NO-GO stops.
✗ Thread too tight — GO does not enter the hole.
✗ Thread too loose — NO-GO enters more than 2 turns.
Internal metric thread tolerance classes
Why 6H rather than 4H, 5H, or 7H
Comparison of classes 4H, 5H, 6H and 7H
Basic pitch diameter D2 for the M20x2.5 thread is 18.376 mm. In classes with the letter H, the lower deviation is zero, so the basic size is also the smallest permissible size, and the tolerance zone extends upward.
| Class | D2 nominal [mm] | D2 permissible [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 4H | 18.376 | 18.376 – 18.516 mm | 0.140 mm | Guiding connections, precision engineering | |
| 5H | 18.376 | 18.376 – 18.556 mm | 0.180 mm | Reduced clearance, fitted threads | |
| 6H | 18.376 | 18.376 – 18.600 mm | 0.224 mm | Standard — commercial nut, machine tap | THIS PRODUCT |
| 7H | 18.376 | 18.376 – 18.656 mm | 0.280 mm | Allowance for electroplating, rough assembly |
Class 6H corresponds to a regular nut and a standard machine tap; it is selected to mate with class 6g screws. A separate case is 6G — this does not change the zone width, but shifts the entire zone upward by 0.042 mm to accommodate electroplating applied after tapping.
Tap drill and measurement limits
What a thread gauge cannot assess
The tap drill hole for an M20x2.5 thread is drilled with a 17.5 mm drill bit — the size follows a simple rule: nominal diameter minus pitch. This gives approximately 77 % thread engagement, the recommended value for structural steel. The minor diameter of the finished class 6H thread lies within 17.294 – 17.744 mm and is inspected separately with a plain gauge, not a thread gauge.
The gauge will not determine hole perpendicularity or thread depth in a blind hole. It determines only whether the thread complies with class 6H.
▲
M20x2.5 and M20x1.5 are two different threads
This gauge fits only the coarse pitch of 2.5 mm. The M20x1.5 thread requires a separate gauge — its pitch diameter is 19.026 mm, which is 0.65 mm larger. The simplest way to identify the pitch is with a thread pitch gauge, or by measuring the distance across ten thread pitches and dividing it by ten.
Set contents and storage
Set contents and gauge storage
INCLUDED:
| ✓ | M20x2.5 6H thread plug gauge, double-ended GO / NO-GO |
| ✓ | Size and class marking on the body |
| ✓ | NO-GO end marked with a red ring |
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
In pitch, and consequently in all thread dimensions. M20 without a specified pitch denotes a coarse pitch of 2.5 mm and a pitch diameter of 18.376 mm. M20x1.5 is the fine thread version with a pitch diameter of 19.026 mm. Neither the screws nor the taps are interchangeable, and each thread requires its own gauge.
A 17.5 mm drill bit. The rule is simple: nominal diameter minus pitch, or 20 − 2.5. This gives approximately 77 % thread engagement — increasing it brings no strength benefit but significantly increases tapping torque and the risk of tap breakage. The finished thread minor diameter should be within 17.294 – 17.744 mm.
No more than two, counted when unscrewing the gauge. This is specified in ISO 1502, clause 7.1. If the thread in the part has three threads or fewer, a stricter criterion applies: the NO-GO end must not pass all the way through the thread. The three-turn rule specified for inch gauges comes from a different standard and does not apply here.
Standard class 6g screws — this is the default mating combination in the metric system. The pitch diameter of an electroplated zinc screw increases after coating, so a class 6G or 7H internal thread is selected for it. Changing the class in the opposite direction, meaning 6g in a 4H hole, causes the joint to jam.
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