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Description
THREAD9/16"-18 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
UNF 9/16-18 thread ring gauge — GO and NO-GO set
Allowance and class 2A tolerance zone
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 9/16-18 |
| Threads per inch | 18 TPI |
| Thread pitch | 1.411 mm |
| Nominal diameter | 14.287 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 13.221 – 13.335 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 9/16-18 UNF-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNF 9/16-18 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.
The nine-sixteenths-inch fine thread is used on the JIC dash -6 hydraulic fitting and an ORB O-ring boss port of the same size—one of the more common sizes in mobile hydraulics. The same thread is used on wheel studs in heavy-duty American pickup trucks and on the AN9 aircraft bolt.
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 gauge screws on by hand, without force, over the full thread length of the 9/16-18 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
The difference between the ring gauges is the full class 2A tolerance zone, or 0.114 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 13.335 mm — the basic size minus the allowance of 0.036 mm. It checks all thread features together.
NO-GO ring (NO-GO gauge)
It is at the lower limit of the zone, or 13.221 mm. The difference between the ring gauges is the full class 2A tolerance zone, 0.114 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 — 9/16-18
Why 2A starts below the basic size
Comparison of external thread classes
The basic pitch diameter for this thread is 13.371 mm. Classes 1A and 2A have an allowance of 0.036 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 | 0.036 | 13.162 – 13.335 mm | 0.173 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.036 | 13.221 – 13.335 mm | 0.114 mm | Commercial standard — bolts and threaded studs | THIS PRODUCT |
| 3A | 0.000 | 13.284 – 13.371 mm | 0.086 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 adjacent size 5/8-18 has identical pitches 1.411 mm and differs only in diameter—15.875 versus 14.288 mm. A thread pitch gauge will give the same result for both. Another potential mix-up is the coarse 9/16-12 thread with the same nominal diameter and a pitch of 2.117 mm.
The pitch is derived from the designation: 18 threads per inch means 25.4 divided by 18, or 1.411 mm. The basic pitch diameter is 14.287 − 0.6495 × 1.411 = 13.371 mm. Class 2A ring gauges check it within the range 13.221 – 13.335 mm, below the basic value by an allowance of 0.036 mm.
▲
A wheel stud operates differently from a bolt
A wheel stud carries variable loads and is tightened with an impact wrench at every tire change. Its thread deteriorates from the nut end and stretches rather than wearing away. The GO ring gauge will not reject such a stud—only the NO-GO gauge advances more than three turns. A stud that passes only one of the checks must be replaced.
Inspection scope and storage
What the gauge confirms and what it cannot measure
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 9/16-18 2A |
| ▸ | NO-GO (reject) ring gauge UNF 9/16-18 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT IT CANNOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 14.031 – 14.252 mm |
| ▸ | Stud straightness and runout |
| ▸ | Thread flank roughness |
Frequently asked questions
Only by diameter, because both have the same pitch of 1.411 mm. A thread pitch gauge will not distinguish them. A caliper measurement does: 14.288 mm is 9/16 inch, and 15.875 mm is 5/8 inch. The difference of 1.588 mm is clear, but you need to use a caliper.
Yes, if the stud has a 9/16-18 UNF thread, as found on heavy-duty American pickup trucks. The ring gauges assess the thread but will not detect a bent shank or a cracked notch under the head. A stud tightened with an impact wrench should be checked with both gauges because it deteriorates by stretching rather than abrasion.
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.
Product accessories
UNF 9/16-18 HSS DIN 22568 buttonhole Poland
Reference: 0000007398 In stock > 10 items
€14.21
€17.48 brutto
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