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Description
THREAD1"-12 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 1-12 thread plug gauge — hole inspection
From tap drill to gauge
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1-12 |
| Threads per inch | 12 TPI |
| Thread pitch | 2.117 mm |
| Nominal diameter | 25.400 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 24.026 – 24.219 mm |
| Hole minor diameter D1 | 23.109 – 23.569 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 1-12 UNF-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.
UNF 1-12 thread plug gauge 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.
The one-inch fine thread starts the twelve-thread-per-inch family. ASME B1.1 specifies 12 threads per inch as the only fine thread for diameters from one inch to one and a half inches — the same pitch is therefore repeated across five successive diameters. The 1-12 size itself is not a hydraulic size: the JIC and ORB systems go directly from 7/8-14 to 1 1/16-12.
Hole before tapping — size 1-12
Standard-based criterion instead of percentage of thread height
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 59/64 = 23.416 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 23.400 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 23.109 – 23.569 mm | ASME B1.1 | the hole must fall within this range |
At the same diameter of 25.400 mm, the coarse UNC series has 8 threads per inch and a pitch of 3.175 mm, while the fine UNF series has 12 threads per inch and a pitch of 2.117 mm. Its metric neighbor is M24, with a diameter of 24.000 mm and a pitch of 3.000 mm in the standard version or 2.000 mm in the fine version — the latter pair can be confusing because the pitches are close.
The pitch is derived from the designation: 12 threads per inch means 25.4 divided by 12, or 2.117 mm. The basic pitch diameter is 25.400 − 0.6495 × 2.117 = 24.025 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
Screw the GO end in by hand, without force, through the full length of the 1-12 UNF 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 2.117 mm, three turns of the NO-GO end equal a depth of 6.351 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 24.026 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 24.219 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 — 1-12
How 1B, 2B, and 3B differ
Comparison of internal thread classes
All three classes start at the same point — the basic pitch diameter of 24.026 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 | 24.026 | 24.315 | 0.290 mm | Field assembly, threads exposed to contamination | |
| 2B | 24.026 | 24.219 | 0.193 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 24.026 | 24.171 | 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.
▲
This is not a cylinder piston rod thread
For a one-inch-diameter piston rod, cylinder manufacturers use the thread 1-14 UNS, not 1-12 UNF. A difference of two threads per inch is enough to prevent the rod end from screwing on. If the 1-12 gauge will not screw into the piston rod thread, it is most likely a 1-14 thread.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 24.026 – 24.219 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
No. JIC and ORB systems skip this size — after 7/8-14, or dash -10, the next size is 1 1/16-12, designated dash -12. Exactly one inch with 12 threads per inch primarily appears in the UNF table for general-purpose bolts and threaded rods.
This is specified by the UNF series table in ASME B1.1: for diameters from 1 to 1 1/2 inches, the only fine thread has 12 threads per inch. Above 1 1/2 inches, the same pitch is designated as the 12-UN constant-pitch series, not UNF — the series name changes, not the geometry.
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.
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