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Description
THREAD1/2"-13 UNC
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
UNC 1/2-13 thread ring gauge — GO and NO-GO set
Allowance and class 2A tolerance zone
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch thread UNC |
| Thread size | 1/2-13 |
| Threads per inch | 13 TPI |
| Thread pitch | 1.954 mm |
| Nominal diameter | 12.700 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 11.265 – 11.392 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 1/2-13 UNC-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNC 1/2-13 thread ring gauge checks an external thread against the two limits of the class 2A tolerance zone. The GO ring gauge corresponds to maximum material condition, and the NO-GO ring gauge to minimum material condition. The acceptance criterion comes from ASME B1.2: the NO-GO side must advance no more than three full turns.
Half an inch is the lower limit of ASTM F3125, which covers heavy hex structural bolts for butt joints in steel structures — the standard does not cover smaller diameters at all. The American building code specifies the same dimension as the minimum diameter of an anchor securing a sill plate to a foundation.
What the GO ring gauge detects and what the NO-GO ring gauge detects
The two limits of the class 2A tolerance zone
1
The GO ring is screwed on by hand, without force, over the entire thread of the 1/2-13 UNC 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
The difference between the ring gauges is the full class 2A tolerance zone, or 0.127 mm. A thread falling between them complies with the standard.
GO ring (GO gauge)
It is at the upper limit of the class 2A tolerance zone, or 11.392 mm — the basic size minus the allowance of 0.038 mm. It checks all thread features together.
NO-GO ring (NO-GO gauge)
It is at the lower limit of the zone, or 11.265 mm. The difference between the ring gauges is the full class 2A tolerance zone, 0.127 mm. The body may be stamped LO — the historical designation for the NO-GO side.
✓ 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.
Allowance and tolerance zone — 1/2-13
Why 2A starts below the basic size
Comparison of external thread classes
The basic pitch diameter for this thread is 11.431 mm. Classes 1A and 2A have an allowance of 0.038 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 | 0.038 | 11.204 – 11.392 mm | 0.188 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.038 | 11.265 – 11.392 mm | 0.127 mm | Commercial standard — bolts and threaded studs | THIS PRODUCT |
| 3A | 0.000 | 11.336 – 11.430 mm | 0.094 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.
Inch and metric threads side by side
Why substitution is not an option
The closest metric thread is M12, with a diameter of 12.000 mm versus 12.700 mm for half an inch — a difference of 0.700 mm. The pitches are 1.750 and 1.954 mm. The second pitfall is the fine series 1/2-20, with the same nominal diameter and a pitch of 1.270 mm.
The pitch is derived from the designation: 13 threads per inch means 25.4 divided by 13, or 1.954 mm. The basic pitch diameter is 12.700 − 0.6495 × 1.954 = 11.431 mm. Class 2A ring gauges check it within the range 11.265 – 11.392 mm, below the basic value by an allowance of 0.038 mm.
▲
The zinc coating fits within the allowance
Structural bolts are usually zinc coated, and class 2A dimensions apply to the condition before coating — the allowance is precisely the space provided for the coating. A 2A ring gauge may not screw onto a hot-dip galvanized bolt even if the thread is correct. The thread is inspected separately after coating, using an oversized gauge.
Inspection scope and storage
What the gauge confirms and what it cannot measure
INCLUDED IN THE SET:
| ▸ | GO ring (GO gauge) UNC 1/2-13 2A |
| ▸ | NO-GO ring (NO-GO gauge) UNC 1/2-13 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT IT CANNOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 12.385 – 12.662 mm |
| ▸ | Stud straightness and runout |
| ▸ | Thread flank roughness |
Frequently asked questions
No. Half an inch is 12.700 mm, while M12 is 12.000 mm; the pitches are 1.954 and 1.750 mm. The metric bolt will fit loosely into the inch nut and will not carry the calculated load, while the reverse combination will not fit at all. When repairing imported structures, a replacement must be found in the inch system.
Not necessarily. Class 2A describes the thread before coating, and the zinc coating adds several tens of micrometers to each surface. A hot-dip galvanized bolt may therefore fall outside the 2A tolerance zone even though the thread itself was made correctly. A separate gauge is used for inspection after coating.
It is an intentional reduction in the external thread size relative to the basic size, present in classes 1A and 2A. It provides space for electroplating and makes dirty threads easier to assemble. Class 3A has no allowance, so its upper limit reaches the basic value. Internal threads always have zero allowance.
No, the classes can be combined freely. A class 2A external thread mates with 1B, 2B and 3B threaded holes — the standard imposes no restriction on this. The class describes the width of the tolerance zone for each component separately, not a matched pair.
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