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Description
THREAD3/4"-16 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
UNF 3/4-16 thread ring gauge — GO and NO-GO set
Two ring gauges for external inch threads
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 3/4-16 |
| Threads per inch | 16 TPI |
| Thread pitch | 1.587 mm |
| Nominal diameter | 19.050 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 17.854 – 17.981 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 3/4-16 UNF-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNF 3/4-16 thread ring gauge checks an external thread — on a bolt, stud, or threaded shaft. The GO ring gauge must screw on by hand over the full thread length, while the NO-GO ring gauge may advance no more than three full turns per ASME B1.2. The metric standard ISO 1502 permits two threads and does not apply here.
The three-quarter-inch fine thread is used on the JIC dash -8 hydraulic fitting—one of the most popular sizes in hydraulic power systems. An ORB O-ring boss port of the same size has the same thread, and in aviation it corresponds to the AN12 bolt.
GO/NO-GO inspection: GO and NO-GO ring gauges
Thread acceptance criterion per ASME B1.2
1
The GO ring gauge screws on by hand, without force, over the full thread length of the 3/4-16 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
Both conditions must be met simultaneously. At a pitch of 1.587 mm, the three permitted turns of the NO-GO ring gauge give 4.761 mm of engagement.
GO ring (GO gauge)
Represents the maximum material condition — the largest permissible pitch diameter of 17.981 mm. Checks thread form, pitch, and diameter simultaneously, because an error in any of these increases the functional thread diameter.
NO-GO ring (NO-GO gauge)
Represents the least material condition — the smallest permissible pitch diameter of 17.854 mm. Detects an undersized thread that will assemble easily but will not maintain tension. 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.
Threads commonly confused with this one
The decisive measurement
At the same diameter of 19.050 mm, the coarse series has 10 threads per inch and a pitch of 2.540 mm, while the fine series has 16 threads per inch and a pitch of 1.587 mm. The neighboring metric size is M20, with a diameter of 20.000 mm; the difference is 0.950 mm, but it is not visible on a fitting end covered in hydraulic oil.
The pitch is derived from the designation: 16 threads per inch means 25.4 divided by 16, or 1.587 mm. The basic pitch diameter is 19.050 − 0.6495 × 1.587 = 18.019 mm. Class 2A ring gauges check it within the range 17.854 – 17.981 mm, below the basic value by an allowance of 0.038 mm.
Classes 1A, 2A, and 3A for size 3/4-16
Why this set is class 2A
Comparison of external thread classes
The basic pitch diameter for this thread is 18.019 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 | 17.790 – 17.981 mm | 0.190 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.038 | 17.854 – 17.981 mm | 0.127 mm | Commercial standard — bolts and threaded studs | THIS PRODUCT |
| 3A | 0.000 | 17.922 – 18.019 mm | 0.097 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.
▲
The cone seals, the thread clamps
In a JIC fitting, the seal is formed at the 37° cone, while the thread only generates the clamping force. Ring gauges confirm that the thread meets class 2A requirements but do not assess a scratched or dented cone—the most common cause of leakage. Even with a good thread, a fitting end with a damaged cone must still be replaced.
Inspection scope and storage
What the gauge confirms and what it cannot measure
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 3/4-16 2A |
| ▸ | NO-GO (reject) ring gauge UNF 3/4-16 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT IT CANNOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 18.773 – 19.012 mm |
| ▸ | Stud straightness and runout |
| ▸ | Thread flank roughness |
Frequently asked questions
Yes. In the JIC 37° system, dash -8 has a 3/4-16 UNF thread; the same thread is used in an ORB O-ring boss port of this size. However, the dash number describes the tube diameter, which is half an inch, not the thread diameter—specify the thread size when ordering.
The NO-GO ring gauge provides the answer. A thread stretched by overtightening with a wrench screws together easily but cannot maintain the clamping force on the cone—the connection leaks during pressure surges. The GO ring gauge will not detect this because the thread is undersized, not oversized.
No. The GO ring gauge only confirms that the thread is not oversized — it will also screw onto an undersized thread that is below the lower tolerance limit. Only the NO-GO ring gauge checks the other limit. Inspection with a single ring gauge leaves half the tolerance unchecked.
Ring gauges are verified with setting gauges — plugs made to tighter tolerances in class W. The standard specifies neither a time interval nor a number of measurements: the frequency is set by the user's quality system, because wear depends on the intensity of use and operating conditions.
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