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Description
THREAD5/8"-18 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
Thread ring gauge UNF 5/8-18—where this thread is used
Limit gauging per ASME B1.2
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 5/8-18 |
| Threads per inch | 18 TPI |
| Thread pitch | 1.411 mm |
| Nominal diameter | 15.875 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 14.803 – 14.922 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 5/8-18 UNF-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNF 5/8-18 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.
This size stands out for what it lacks: as the only size in this shipment is used in neither JIC fittings nor ORB ports —hydraulic systems skip it. It is used in automotive applications, however: on fittings for brake lines with a diameter of 3/8 inch, wheel studs in heavier vehicles and tractors, and the AN10 aircraft bolt.
Identifying the thread before measurement
Caliper, thread pitch gauge, and thread gauge
The adjacent size 9/16-18 has the same pitch 1.411 mm and differs only in diameter. Another potential mix-up is the coarse 5/8-11 thread with the same nominal diameter and a pitch of 2.309 mm. A third, metric possibility is M16—16.000 mm versus 15.875 mm, a difference of 0.125 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 15.875 − 0.6495 × 1.411 = 14.958 mm. Class 2A ring gauges check it within the range 14.803 – 14.922 mm, below the basic value by an allowance of 0.036 mm.
Classes 1A, 2A, and 3A—size 5/8-18
Which class to use for each application
Comparison of external thread classes
The basic pitch diameter for this thread is 14.958 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 | 14.745 – 14.922 mm | 0.178 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.036 | 14.803 – 14.922 mm | 0.119 mm | Commercial standard — bolts and threaded studs | THIS PRODUCT |
| 3A | 0.000 | 14.869 – 14.958 mm | 0.089 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 5/8-18 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
At a pitch of 1.411 mm, the three permissible turns of the NO-GO ring gauge equal 4.233 mm. For a short thread, the limit must be reduced.
GO ring (GO gauge)
Represents the maximum material condition—14.922 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—14.803 mm. It may advance up to three full turns, which equals 4.233 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.
▲
Absence from hydraulics can be a clue
A 5/8-18 UNF thread in a machine is not a hydraulic connection—the JIC and ORB systems skip this size. It is found in brake components, wheel studs, or aircraft hardware instead. This knowledge saves time when identifying an unmarked part.
Inspection scope and storage
What the gauge confirms and what it cannot measure
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 5/8-18 2A |
| ▸ | NO-GO (reject) ring gauge UNF 5/8-18 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT IT CANNOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 15.618 – 15.839 mm |
| ▸ | Stud straightness and runout |
| ▸ | Thread flank roughness |
Frequently asked questions
SAE tables skip this size: dash -6 is 9/16-18, and dash -8 is 3/4-16. The five-eighths-inch fine thread was not included in either the JIC or ORB system. This thread is most often found in brake components or wheel studs.
Only by diameter—the pitch is identical at 1.411 mm. A caliper measurement distinguishes them: 15.875 mm is 5/8 inch, and 14.288 mm is 9/16 inch. A thread pitch gauge is useless for this pair because it will show eighteen threads per inch in both cases.
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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Shipping
All orders are delivered by courier company GLS. For orders exceeding 369 PLN gross shipping is FREE
| Courier GLS | Przelew24 / Prepayment | 14,76 zł gross | Delivery time 1 working day |
| Courier GLS | Cash on delivery | 17,26 zł gross | Lead time 1 working day |
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