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Description
THREAD1 1/2"-12 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 1.1/2-12 thread plug gauge — GO and NO-GO ends
What each end of the gauge detects
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1.1/2-12 |
| Threads per inch | 12 TPI |
| Thread pitch | 2.117 mm |
| Nominal diameter | 38.100 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 36.726 – 36.937 mm |
| Hole minor diameter D1 | 35.809 – 36.269 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 1/2-12 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 1.1/2-12 thread plug gauge checks an internal thread against both limits at once. The GO end corresponds to maximum material, the NO-GO end to minimum material. The acceptance criterion comes from ASME B1.2: the NO-GO end may enter no more than three full turns.
One and a half inches with twelve threads per inch is the last size in the UNF series table — above this diameter, the same pitch is designated as the 12-UN constant-pitch series. It is also a typical clevis and piston rod end thread in large hydraulic cylinders built to the NFPA standard.
What the GO end detects and what the NO-GO end detects
Two limits of the tolerance zone per ASME B1.2
1
The GO end is screwed in by hand, without force, through the full length of the 1 1/2-12 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
The GO end alone will accept a hole that is too loose; the NO-GO end alone will accept one that is too tight. Only both ends together bound the 0.211 mm tolerance zone.
GO end (passing)
Checks all thread characteristics at once and stops at 36.726 mm — the maximum material limit. It does not distinguish which characteristic has failed: profile, pitch, and diameter errors produce the same symptom.
NO-GO end
Checks only the functional diameter at the minimum material limit, which is 36.937 mm. It may enter up to three full turns. The body may be stamped HI — the historical designation for the NO-GO end.
✓ 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.
Class 2B tolerance zone for 1.1/2-12
Comparison with classes 1B and 3B
Comparison of internal thread classes
All three classes start at the same point — the basic pitch diameter of 36.726 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 | 36.726 | 37.043 | 0.318 mm | Field assembly, threads exposed to contamination | |
| 2B | 36.726 | 36.937 | 0.211 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 36.726 | 36.886 | 0.160 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.
Tap drill for 1.1/2-12
Class 2B minor diameter range
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 1 27/64 = 36.116 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 36.000 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 35.809 – 36.269 mm | ASME B1.1 | the hole must fall within this range |
The twelve-thread family from one inch to one and a half inches has identical pitches 2.117 mm across five different diameters. At this size, terminology creates another potential source of confusion: the 1 9/16-12 thread looks like a continuation of the same series, but formally belongs to the 12-UN series, not UNF. The geometry is the same; the name is different.
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 38.100 − 0.6495 × 2.117 = 36.725 mm. This value is also the lower limit for class 2B, because an internal thread has no allowance.
▲
Above one and a half inches, it is a different series
The UNF table in ASME B1.1 ends at 1 1/2-12. Larger sizes with the same pitch — 1 9/16-12, 1 5/8-12, and beyond — belong to the 12-UN constant-pitch series. When ordering a gauge above one and a half inches, specify 12-UN, because the supplier will not find that size under UNF.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 36.726 – 36.937 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
This is where the UNF table in ASME B1.1 ends. For diameters from 1 to 1 1/2 inches, the only fine thread has 12 threads per inch, and these sizes are designated UNF. Above one and a half inches, the same pitch continues under the 12-UN constant-pitch series designation — the geometry does not change, only the series designation.
Only by diameter. The pitch is identical at 2.117 mm, so a thread pitch gauge cannot distinguish them. A caliper measurement does: 34.925 mm is 1 3/8 inches, and 38.100 mm is 1 1/2 inches. The 3.175 mm difference is substantial, but it must be measured rather than judged visually.
First check that there is no chip in the hole and that the first thread has not been rolled over. If the thread is clean but the plug still will not enter, the hole is too tight — usually because the drill was too small or the tap was dull and displaced material instead of cutting it. This thread can still be reworked with a finishing tap.
Each end bounds the tolerance zone on a different side. The GO end confirms that the thread is not too tight, but it will also enter a hole that is too loose. The NO-GO end works the other way around. Checking with only one end leaves half the tolerance unchecked, which is the most common error during thread acceptance inspection.
Product accessories
Thread ring gauge UNF 1.1/2-12 2A GO NO-GO
Reference: 15424 In stock < 10 items
€89.47
€110.05 brutto
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