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Description
THREAD1 1/2"-12 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
Thread ring gauge UNF 1.1/2-12—where this thread is used
Limit gauging per ASME B1.2
| 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 | 2A |
| Pitch diameter d2 | 36.515 – 36.678 mm |
| Gauge type | Set: GO ring + NO-GO ring |
| Material | Tool steel, hardened |
| Hardness | 58-62 HRC |
| Standard | ASME B1.1 / ASME B1.2 |
| Manufacturer | Varel |
T
GO ring gauge — screwed on by hand along the full bolt thread.
Z
NO-GO ring with a groove in the knurled surface — stops after no more than 3 turns.
The 1 1/2-12 UNF-2A engraving on the face of both ring gauges indicates the size and tolerance class of the bolt being inspected.
UNF 1.1/2-12 thread ring gauge checks an external thread—on a bolt, fitting end, or stud. The GO ring gauge must thread on by hand over the full thread length; the NO-GO ring gauge must advance no more than three full turns per ASME B1.2.
One and a half inches with twelve threads per inch is the last entry in the UNF series table in ASME B1.1 — above this diameter, the same pitch continues as the constant-pitch 12-UN series. It is also a typical piston rod end and clevis thread in large hydraulic cylinders built to the NFPA standard.
Identifying the thread before measurement
Caliper, thread pitch gauge, and thread gauge
The twelve-thread family from one inch to one and a half inches has identical pitches 2.117 mm across five different diameters, so a thread pitch gauge cannot distinguish them. Measurement is decisive: 34.925 mm is 1 3/8 inches, and 38.100 mm is 1 1/2. There is also a naming pitfall — 1 9/16-12 looks like a continuation of the series, but formally it is already 12-UN.
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. Class 2A ring gauges check it within the range 36.515 – 36.678 mm, below the basic value by an allowance of 0.048 mm.
Classes 1A, 2A, and 3A—size 1.1/2-12
Which class to use for each application
Comparison of external thread classes
The basic pitch diameter for this thread is 36.725 mm. Classes 1A and 2A have an allowance of 0.048 mm, so their upper limit lies below the basic value. Class 3A has no allowance.
| Class | Allowance [mm] | d2 permissible [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1A | 0.048 | 36.434 – 36.678 mm | 0.244 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.048 | 36.515 – 36.678 mm | 0.163 mm | Commercial standard — bolts and threaded studs | THIS PRODUCT |
| 3A | 0.000 | 36.604 – 36.726 mm | 0.122 mm | Aviation, precision engineering |
Class 2A is the commercial standard — it covers the vast majority of inch bolts and mates with class 1B, 2B, and 3B internal threads. Class 3A is used where tightening torque must be repeatable from part to part.
GO/NO-GO inspection of external threads
Three steps per ASME B1.2
1
The GO ring gauge is threaded by hand, without force, over the full thread of the 1 1/2-12 UNF bolt.
2
The NO-GO ring stops after no more than 3 full turns, without forcing.
3
The GO ring passed and the NO-GO ring stopped — the bolt meets class 2A.
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 3 full turns
✓ Acceptable thread
Class 2A confirmed
At a pitch of 2.117 mm, the three permissible turns of the NO-GO ring gauge equal 6.351 mm. For a short thread, the limit must be reduced.
GO ring (GO gauge)
Represents the maximum material condition—36.678 mm. It must pass over the full thread length without resistance; resistance indicates an oversized thread or a damaged thread form.
NO-GO ring (NO-GO gauge)
Represents the minimum material condition—36.515 mm. It may advance up to three full turns, which equals 6.351 mm at this pitch. The body may be stamped LO.
✓ Thread acceptable — GO screws onto the full thread, NO-GO stops.
✗ Thread oversized — GO does not screw onto the bolt.
✗ Thread undersized — the NO-GO ring screws on more than 3 turns.
▲
Above one and a half inches, it is a different series
The UNF table 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 constant-pitch 12-UN series. The geometry is the same, but the series designation differs, so when ordering a gauge above one and a half inches, specify 12-UN; the supplier will not find this size under UNF.
Inspection scope and storage
What the gauge confirms and what it cannot measure
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 1.1/2-12 2A |
| ▸ | NO-GO (reject) ring gauge UNF 1.1/2-12 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT IT CANNOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 37.762 – 38.052 mm |
| ▸ | Stud straightness and runout |
| ▸ | Thread flank roughness |
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 constant-pitch 12-UN series designation — unchanged geometry, a different designation.
By diameter alone, because the pitch is identical at 2.117 mm. Measurement with a caliper is decisive: 34.925 mm is 1 3/8 inches, and 38.100 mm is 1 1/2 inches. The 3.175 mm difference is clear in measurement but completely invisible on a thread pitch gauge.
No. The GO ring gauge will detect an oversized thread, but it will also screw onto an undersized thread. Only the NO-GO ring gauge checks the other limit of the tolerance zone. Inspection with a single ring gauge is the most common mistake in external thread acceptance inspection.
No. Ring gauges assess pitch diameter and thread form. The major diameter is checked with a separate plain gauge—the permissible range for class 2A in this size is given in the section on inspection scope.
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