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Description
THREAD3/4"-10 UNC
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNC 3/4-10 thread plug gauge — hole inspection
From tap drill to gauge
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch thread UNC |
| Thread size | 3/4-10 |
| Threads per inch | 10 TPI |
| Thread pitch | 2.540 mm |
| Nominal diameter | 19.050 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 17.399 – 17.595 mm |
| Hole minor diameter D1 | 16.299 – 16.833 mm |
| Gauge type | Double-ended: GO + NO-GO |
| Material | Tool steel, hardened |
| Hardness | 58-62 HRC |
| Standard | ASME B1.1 / ASME B1.2 |
| 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 3/4-10 UNC-2B — size and tolerance class of the hole being inspected.
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 may engage no more than 3 turns.
Thread plug gauge UNC 3/4-10 checks the internal thread after tapping. The GO end must screw in by hand to the full depth, and the NO-GO end no more than three full turns — as specified by ASME B1.2 for inch threads, unlike metric ISO 1502, which limits engagement to two turns.
Three-quarters of an inch is one of two sizes that structural bolt distributors keep permanently in stock — alongside seven-eighths. ASTM F3125 covers its full range of lengths, as does the foundation anchor standard ASTM F1554. In practice, this means that the 3/4-10 internal thread is found wherever an American steel structure meets its foundation.
Hole before tapping — size 3/4-10
Standard-based criterion instead of percentage of thread height
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 21/32 = 16.669 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 16.700 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 16.299 – 16.833 mm | ASME B1.1 | the hole must fall within this range |
The nearest metric thread is M20, with a diameter of 20.000 mm — it differs from 3/4 inch, or 19.050 mm, by 0.950 mm. This is one of the more noticeable differences in the entire comparison, and a caliper will show it. The pairing with M18 is more difficult: 18.000 mm versus 19.050 mm, with a pitch of 2.500 mm for M18 and 2.540 mm for the inch thread — nearly identical pitches.
The pitch is derived from the designation: 10 threads per inch means 25.4 divided by 10, or 2.540 mm. The basic pitch diameter is 19.050 − 0.6495 × 2.540 = 17.400 mm. This value is also the lower limit for class 2B, because an internal thread has no allowance.
Thread inspection after tapping
Three steps and the acceptance criterion per ASME B1.2
1
The GO end is screwed in by hand, without force, through the full length of the 3/4-10 UNC thread.
2
The NO-GO end stops within 3 full turns, without forcing.
3
The GO end passed and the NO-GO end stopped — the hole meets class 2B.
Thread inspection in 3 steps
1 GO end (passing)
Screwed in by hand — enters to the full depth
2 NO-GO end
Screwed in by hand — no more than 3 full turns
✓ Acceptable thread
Class 2B confirmed
At a pitch of 2.540 mm, three turns of the NO-GO end equal a depth of 7.620 mm. For a thin nut, the limit is reduced by agreement between the parties.
GO end (passing)
Enters only a hole correctly produced along its entire length — it corresponds to the minimum permissible pitch diameter of 17.399 mm. Resistance during insertion indicates a thread that is too tight or not tapped to full depth.
NO-GO end
Stops in a correctly produced hole — it corresponds to the maximum permissible pitch diameter of 17.595 mm. If it enters more than three turns, the thread is too loose. The body may be stamped HI.
✓ Thread acceptable — GO enters the full length, NO-GO stops.
✗ Thread too tight — GO does not enter the hole.
✗ Thread too loose — the NO-GO end enters more than 3 turns.
Three internal thread classes — 3/4-10
How 1B, 2B, and 3B differ
Comparison of internal thread classes
All three classes start at the same point — the basic pitch diameter of 17.399 mm. They differ only in the upper limit, because an internal thread has no allowance.
| Class | D2 min [mm] | D2 max [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1B | 17.399 | 17.691 | 0.292 mm | Field assembly, threads exposed to contamination | |
| 2B | 17.399 | 17.595 | 0.196 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 17.399 | 17.544 | 0.145 mm | Joints requiring repeatability |
A class 2B hole mates with class 1A, 2A, and 3A bolts — mixing classes is permitted. Class 2B is the default: unless the drawing specifies otherwise, this is the intended class.
▲
A pitch of 2.540 mm looks like the metric 2.500 mm pitch
The 3/4-10 UNC thread has a pitch of 2.540 mm, while metric M18 has a pitch of 2.500 mm. The difference of 0.040 mm per turn is undetectable with a thread pitch gauge; it only becomes apparent after a dozen or more turns as an accumulating offset. For this size, diameter alone is decisive: 19.050 versus 18.000 mm.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 17.399 – 17.595 mm |
| ▸ | Checks profile and pitch together, using the GO end |
| ▸ | Detects an oversized thread, using the NO-GO end |
WHAT IT CANNOT MEASURE:
| ▸ | Hole minor diameter — this requires a separate plain plug gauge |
| ▸ | Thread depth in a blind hole |
| ▸ | Thread perpendicularity to the end face |
Frequently asked questions
In diameter and pitch. Three-quarters of an inch is 19.050 mm versus 20.000 mm for M20, and the pitches are 2.540 and 2.500 mm. The diameter difference of 0.950 mm is large enough to detect with a caliper. Confusion with M18, where the pitches are nearly identical, is more dangerous.
For the thread — yes. The gauge will confirm that the pitch diameter meets class 2B. Keep in mind that an anchor nut is screwed onto a thread covered with concrete dust; the thread must be cleaned before measurement, otherwise the GO end will stop on contamination rather than a thread defect.
No, ASME B1.1 eliminated this criterion. A different requirement applies: the drilled hole, including any oversize caused by drilling, must fall between the lower and upper minor diameter limits for the class. Percentage of thread height depends on the reference used and means different things in different sources.
ASME B1.2, section 4.2.1, specifies this for inch threads. Metric ISO 1502 permits two turns — it is a different standard covering a different thread. In practice, gauge manufacturers recommend no more than one and a half turns for thin walls and ductile materials.
Product accessories
Thread ring gauge UNC 3/4-10 2A GO NO-GO
Reference: 15408 In stock < 10 items
€50.00
€61.50 brutto
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