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Description
THREAD1/2"-20 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
Thread ring gauge UNF 1/2-20—where this thread is used
Limit gauging per ASME B1.2
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1/2-20 |
| Threads per inch | 20 TPI |
| Thread pitch | 1.270 mm |
| Nominal diameter | 12.700 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 11.732 – 11.841 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 1/2-20 UNF-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNF 1/2-20 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.
Every automotive repair shop knows this thread: it is used on the oil drain plug in millions of General Motors, Ford, and Chrysler vehicles. The same size is used on the JIC dash -5 hydraulic fitting, the AN8 aircraft bolt, and the fitting for a brake line with a diameter of 5/16 inch.
Identifying the thread before measurement
Caliper, thread pitch gauge, and thread gauge
At the same half-inch diameter, the coarse series has 13 threads per inch and a pitch of 1.954 mm, while the fine series has 20 threads per inch and a pitch of 1.270 mm. The real risk, however, is the metric M12: drain plugs in European cars are metric and easy to mix up in stock, and the diameter difference is 0.700 mm.
The pitch is derived from the designation: 20 threads per inch means 25.4 divided by 20, or 1.270 mm. The basic pitch diameter is 12.700 − 0.6495 × 1.270 = 11.875 mm. Class 2A ring gauges check it within the range 11.732 – 11.841 mm, below the basic value by an allowance of 0.033 mm.
Classes 1A, 2A, and 3A—size 1/2-20
Which class to use for each application
Comparison of external thread classes
The basic pitch diameter for this thread is 11.875 mm. Classes 1A and 2A have an allowance of 0.033 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 | 0.033 | 11.679 – 11.841 mm | 0.163 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.033 | 11.732 – 11.841 mm | 0.109 mm | Commercial standard — bolts and threaded studs | THIS PRODUCT |
| 3A | 0.000 | 11.793 – 11.874 mm | 0.081 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 screws on by hand, without force, over the full thread length of the 1/2-20 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 1.270 mm, the three permissible turns of the NO-GO ring gauge equal 3.810 mm. For a short thread, the limit must be reduced.
GO ring (GO gauge)
Represents the maximum material condition—11.841 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—11.732 mm. It may advance up to three full turns, which equals 3.810 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.
▲
An oversized plug is not a thread repair
A drain plug with an oversized thread addresses the symptom, not the cause—when screwed in, it cuts a new thread in the oil pan and shortens the service life of the next repair. The ring gauge shows whether the plug itself still meets class 2A requirements or whether its thread is worn. Sometimes replacing the plug is sufficient and the oil pan does not need replacement.
Inspection scope and storage
What the gauge confirms and what it cannot measure
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 1/2-20 2A |
| ▸ | NO-GO (reject) ring gauge UNF 1/2-20 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT IT CANNOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 12.461 – 12.667 mm |
| ▸ | Stud straightness and runout |
| ▸ | Thread flank roughness |
Frequently asked questions
In many American cars, yes—it is one of the most common plug sizes in GM, Ford, and Chrysler vehicles. Manufacturers also use metric M12 and M14 plugs, so check the thread by measurement before ordering parts, rather than relying on a model-year catalog.
First, check the plug with a ring gauge. If its thread falls outside class 2A, a new plug is sufficient. If the plug is good but the connection still will not hold, the hole in the oil pan is worn and a thread repair insert is needed. This sequence avoids the most expensive part of the repair.
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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| Courier GLS | Cash on delivery | 17,26 zł gross | Lead time 1 working day |
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