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Description
THREAD5/16"-24 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
UNF 5/16-24 thread ring gauge — GO and NO-GO set
Hydraulic fitting and aircraft bolt thread per ASME B1.1
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 5/16-24 |
| Threads per inch | 24 TPI |
| Thread pitch | 1.058 mm |
| Nominal diameter | 7.938 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 7.127 – 7.221 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 5/16-24 UNF-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNF 5/16-24 thread ring gauge checks an external thread in a size known by another name in hydraulics. It is the connection thread of a JIC 37° flare fitting in dash size -2, for 1/8-inch tubing, and also the thread of an ORB O-ring boss port of the same size. In aviation, the AN5 bolt uses the same thread. The acceptance criterion follows the inch system: the GO ring gauge must pass over the full thread length, and the NO-GO ring gauge may advance no more than three full turns per ASME B1.2.
This has one practical consequence for the workshop. A JIC fitting seals at the 37° cone, not at the thread — the thread simply draws the mating cones together. A worn or stretched thread will not directly cause leakage, but it will fail to maintain clamping force, and the connection will begin to leak during pressure surges. This is why the thread is checked when reconditioning hydraulic lines, even though the seal is formed elsewhere.
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 5/16-24 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 1.058 mm, the three permitted turns of the NO-GO ring gauge correspond to 3.2 mm of engagement. The thread on a hydraulic hose fitting may be only slightly longer, so the limit sometimes needs to be reduced.
GO ring (GO gauge)
Represents the maximum material condition — the largest permissible pitch diameter of 7.221 mm. Checks thread form, pitch, and diameter simultaneously, because an error in any of these increases the functional thread diameter. On hydraulic fittings, this ring gauge detects threads deformed by overtightening with a wrench.
NO-GO ring (NO-GO gauge)
Represents the least material condition — the smallest permissible pitch diameter of 7.127 mm. Detects an undersized thread that will assemble but cannot sustain the torque required to clamp the cone. 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.
Pitch of 1.058 mm alongside a metric pitch of 1.00 mm
The closest pair in the entire inch thread comparison
| DECISIVE MEASUREMENT | |||
|---|---|---|---|
| Thread | Nominal diameter | Pitch | Threads per 25 mm |
| 5/16-24 UNF | 7.938 mm | 1.058 mm | 23.6 |
| 5/16-18 UNC | 7.938 mm | 1.411 mm | 17.7 |
| M8 × 1 | 8.000 mm | 1.000 mm | 25.0 |
The pair consisting of 5/16-24 UNF and M8 with a pitch of 1.0 mm is the most difficult case in the entire inch thread family. The diameters differ by 0.062 mm, and the pitches by only 0.058 mm. A metric 1.0 mm thread pitch gauge placed against a 5/16-24 thread appears to fit — the mismatch becomes apparent only after a dozen or so threads. Over a length of 25 mm, the inch thread has 23.6 threads, while the metric thread has exactly 25.
Pitch is derived from the designation: 24 threads per inch means 25.4 divided by 24, or 1.058 mm. The nominal pitch diameter is therefore 7.938 − 0.6495 × 1.058 = 7.250 mm. Class 2A ring gauges check it within the range 7.127 – 7.221 mm, below the nominal value, because an external thread of this class has an allowance of 0.028 mm.
Classes 1A, 2A, and 3A for size 5/16-24
Why class 2A is used for hydraulic fittings
Comparison of external thread classes
The nominal pitch diameter of the 5/16"-24 UNF thread is 7.250 mm. Classes 1A and 2A have an allowance of 0.028 mm; class 3A has none and starts at the nominal size.
| Class | Allowance [mm] | d2 permissible [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1A | 0.028 | 7.082 – 7.221 mm | 0.140 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.028 | 7.127 – 7.221 mm | 0.094 mm | Hydraulic fittings, hardware, catalog bolts | THIS PRODUCT |
| 3A | 0.000 | 7.181 – 7.249 mm | 0.069 mm | Aviation, precision engineering |
Class 2A is standard in hydraulics because fittings must be assembled repeatedly, and slight clearance protects the thread against galling. Class 3A is used for aircraft components, where tightening torque repeatability matters. Class 1A has no practical application at this size — its 0.140 mm tolerance zone is too wide for a pressurized connection.
▲
A ring gauge will not check the 37° cone
In a JIC fitting, the cone provides the seal, not the thread. A ring gauge set confirms that the thread meets class 2A, but will not detect a scratched, dented, or misaligned cone — the most common cause of leakage. The cone is assessed visually and with an angle gauge. A conforming thread with a damaged cone still means the fitting must be replaced.
Set contents and inspection scope
Set contents and scope of ring gauge inspection
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 5/16-24 2A |
| ▸ | NO-GO (reject) ring gauge UNF 5/16-24 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT THEY DO NOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 7.727 – 7.910 mm and is measured with a plain gauge |
| ▸ | The 37° sealing cone |
| ▸ | Condition of the groove and O-ring in the ORB port |
Frequently asked questions
Yes. In the JIC 37° system per SAE J514, dash size -2, intended for 1/8-inch tubing, uses the 5/16-24 UNF thread. The same thread is used in an ORB O-ring boss port of size -2, allowing the ORB port to accept an adapter for a JIC fitting of the same size. This is the smallest standard size in both families.
No. The pitches differ by 0.058 mm, so a metric 1.0 mm thread pitch gauge placed against a 5/16-24 thread appears to fit. The difference only accumulates over a longer distance: over 25 mm, the inch thread has 23.6 threads, while M8 with a pitch of 1.0 mm has exactly 25. An inch thread pitch gauge or counting threads over a longer length provides a definitive identification.
The thread generates the force that presses the mating cones together. A thread stretched or deformed by overtightening with a wrench will assemble without resistance but will not maintain clamping force — the connection will begin to leak during pressure surges, even if the cone is smooth. The NO-GO ring gauge detects these threads because it advances more than three turns on them.
Class 2A has an allowance of 0.028 mm and a tolerance zone of 0.094 mm; class 3A has zero allowance and a tolerance zone of 0.069 mm. Class 2A is used for hydraulic fittings because slight clearance protects the thread during repeated assembly. Class 3A is used in aviation and wherever tightening torque must be repeatable from part to part.
Product accessories
UNF 5/16-24 HSS DIN 22568 Cztool tooling
Reference: 0000005623 In stock > 10 items
€6.84
€8.42 brutto
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