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Description
THREAD5/8"-18 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 5/8-18 thread plug gauge — port inspection
Class 2B internal thread per ASME B1.1
| 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 | 2B |
| Pitch diameter D2 | 14.958 – 15.110 mm |
| Hole minor diameter D1 | 14.348 – 14.671 mm |
| Gauge type | Double-ended: GO + NO-GO |
| Material | Tool steel, hardened |
| Hardness | 58-62 HRC |
| Standard | ASME B1.1 / ASME B1.2 |
| Manufacturer | Varel |
1
GO end — longer thread with a full profile. It is screwed in by hand through the full length of the hole.
2
Handle engraved with 5/8-18 UNF-2B — size and tolerance class of the hole being inspected.
3
Red ring — identifies the NO-GO end.
4
NO-GO end — shorter thread with a truncated profile. It checks only the pitch diameter, so it may engage no more than 3 turns.
UNF 5/8-18 thread plug gauge checks an internal thread — a port or a tapped hole in a body. The GO end must screw in by hand to the full thread depth; the NO-GO end may enter no more than three full turns per ASME B1.2.
This size stands out in the entire shipment for what it lacks: it is the only size examined that is used in neither JIC fittings nor ORB ports. Hydraulic systems skip it. It is used in automotive applications, however — for a 3/8-inch brake line connection, wheel studs in heavier vehicles and tractors, and the AN10 aircraft bolt.
Pre-drilled hole for 5/8-18 thread
Drill selection based on the D1 range
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 37/64 = 14.684 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 14.500 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 14.348 – 14.671 mm | ASME B1.1 | the hole must fall within this range |
The adjacent size 9/16-18 has the same pitch of 1.411 mm and differs only in diameter — 14.288 versus 15.875 mm. That is the first pitfall. The second is the coarse 5/8-11 thread with the same nominal diameter and a pitch of 2.309 mm. The third, metric pitfall is M16, with a diameter of 16.000 mm, which differs from 5/8 inch by only 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. This value is also the lower limit for class 2B, because an internal thread has no allowance.
Classes 1B, 2B, and 3B — size 5/8-18
Which class to use for each application
Comparison of internal thread classes
All three classes start at the same point — the basic pitch diameter of 14.958 mm. They differ only in the upper limit, because an internal thread has no allowance.
| Class | D2 min [mm] | D2 max [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1B | 14.958 | 15.189 | 0.231 mm | Field assembly, threads exposed to contamination | |
| 2B | 14.958 | 15.110 | 0.152 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 14.958 | 15.072 | 0.114 mm | Joints requiring repeatability |
A class 2B hole mates with class 1A, 2A, and 3A bolts — mixing classes is permitted. Class 2B is the default: unless the drawing specifies otherwise, this is the intended class.
Port inspection in three steps
Internal thread acceptance criterion according to ASME B1.2
1
The GO end is screwed in by hand, without force, along the full length of the 5/8-18 UNF thread.
2
The NO-GO end stops within 3 full turns, without forcing.
3
The GO end passed and the NO-GO end stopped — the hole meets class 2B.
Thread inspection in 3 steps
1 GO end (passing)
Screwed in by hand — enters to the full depth
2 NO-GO end
Screwed in by hand — no more than 3 full turns
✓ Acceptable thread
Class 2B confirmed
At a pitch of 1.411 mm, the three permitted turns correspond to a depth of 4.233 mm — in a shallow port, that is already the entire thread.
GO end (passing)
Represents the maximum material condition of the port — the smallest permissible pitch diameter of 14.958 mm. In a port, it most often stops at the first thread rolled over by a cross-threaded component.
NO-GO end
Represents the minimum material condition — the largest permissible pitch diameter of 15.110 mm. Detects a port with stripped threads in which the mating component cannot be fully tightened. The body may be stamped HI.
✓ Thread acceptable — GO enters the full length, NO-GO stops.
✗ Thread too tight — GO does not enter the hole.
✗ Thread too loose — the NO-GO end enters more than 3 turns.
▲
The inch drill does not fit within the range
The 37/64-inch drill measures 14.684 mm, while the upper minor diameter limit for class 2B is 14.671 mm. As with 9/16-18, the American chart takes you outside the tolerance here. The correct choice is a metric drill of 14.500 mm, which leaves a 0.171 mm allowance for drill oversize.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 14.958 – 15.110 mm |
| ▸ | Checks profile and pitch together, using the GO end |
| ▸ | Detects an oversized thread, using the NO-GO end |
WHAT IT CANNOT MEASURE:
| ▸ | Hole minor diameter — this requires a separate plain plug gauge |
| ▸ | Thread depth in a blind hole |
| ▸ | Thread perpendicularity to the end face |
Frequently asked questions
The JIC and ORB systems skip this size: dash -8 is 3/4-16, while dash -6 is 9/16-18. The five-eighths-inch fine thread was not included in the SAE tables. This thread on a machine is not a hydraulic connection — it is most likely a brake component or a wheel stud.
With a 14.500 mm metric drill. The 37/64-inch drill measures 14.684 mm and exceeds the upper minor diameter limit for class 2B of 14.671 mm — the thread would be too shallow even before drill oversize is taken into account.
No, only the thread. It does not assess the face, recess, or blind hole depth — yet these features determine leak tightness just as often as the thread itself. A good thread with a damaged face still means the connection needs repair.
These are two different standards: inch threads are governed by ASME B1.2, metric threads by ISO 1502. The inch criterion is more lenient on paper, but with a fine pitch, three turns still amount to only a few millimeters of depth.
Product accessories
Hand tap UNF 5/8-18 DIN 2181/2 HSS 2B
Reference: 0000006470 In stock > 10 items
€15.79
€19.42 brutto
Thread ring gauge UNF 5/8-18 2A GO NO-GO
Reference: 15417 In stock < 10 items
€57.89
€71.21 brutto
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| 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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