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Description
THREAD9/16"-18 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 9/16-18 thread plug gauge — GO and NO-GO ends
What each end of the gauge detects
| 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 | 2B |
| Pitch diameter D2 | 13.371 – 13.520 mm |
| Hole minor diameter D1 | 12.761 – 13.084 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 9/16-18 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 9/16-18 thread plug gauge checks an internal thread against both limits at once. The GO end corresponds to maximum material, the NO-GO end to minimum material. The acceptance criterion comes from ASME B1.2: the NO-GO end may enter no more than three full turns.
The nine-sixteenths-inch fine thread is used on a dash -6 JIC hydraulic fitting and an ORB O-ring port of the same size — one of the more commonly used sizes in mobile hydraulics. The same thread is used on wheel studs in heavy American pickups and on the AN9 aircraft bolt.
What the GO end detects and what the NO-GO end detects
Two limits of the tolerance zone per ASME B1.2
1
The GO end is screwed in by hand, without force, along the full length of the 9/16-18 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
The GO end alone will accept a hole that is too loose; the NO-GO end alone will accept one that is too tight. Only both ends together bound the 0.150 mm tolerance zone.
GO end (passing)
Checks all thread characteristics at once and stops at 13.371 mm — the maximum material limit. It does not distinguish which characteristic has failed: profile, pitch, and diameter errors produce the same symptom.
NO-GO end
Checks only the functional diameter at the minimum material limit, which is 13.520 mm. It may enter up to three full turns. The body may be stamped HI — the historical designation for the NO-GO end.
✓ 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.
Class 2B tolerance zone for 9/16-18
Comparison with classes 1B and 3B
Comparison of internal thread classes
All three classes start at the same point — the basic pitch diameter of 13.371 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 | 13.371 | 13.597 | 0.226 mm | Field assembly, threads exposed to contamination | |
| 2B | 13.371 | 13.520 | 0.150 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 13.371 | 13.482 | 0.112 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.
Tap drill for 9/16-18
Class 2B minor diameter range
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 33/64 = 13.097 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 13.000 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 12.761 – 13.084 mm | ASME B1.1 | the hole must fall within this range |
Here, confusion with the adjacent fine thread size is more dangerous than confusion with the coarse thread. The 9/16-18 and 5/8-18 threads have identical pitches of 1.411 mm, while their diameters differ by one-sixteenth of an inch, or 1.588 mm. A thread pitch gauge gives the same reading for both, and the difference only becomes apparent with a caliper.
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. This value is also the lower limit for class 2B, because an internal thread has no allowance.
▲
The inch drill falls outside the tolerance
The 33/64-inch drill measures 13.097 mm, while the upper minor diameter limit for class 2B is 13.084 mm. The inch drill therefore falls outside the range even before drill oversize is taken into account and will produce a thread that is too shallow. A 13.000 mm metric drill is used for this size — one of the few cases in which an American drill chart leads you astray.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 13.371 – 13.520 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
Only a 13.000 mm metric drill. The 33/64-inch drill measures 13.097 mm and exceeds the upper minor diameter limit for class 2B, which is 13.084 mm — the thread will be too shallow. This is one of the few sizes for which an American drill chart does not work in practice.
Only by diameter, because both have the same pitch of 1.411 mm. A thread pitch gauge gives the same reading. A caliper measurement settles it: 14.288 mm is 9/16 inch, while 15.875 mm is 5/8 inch. The 1.588 mm difference is clear, but you need to use a caliper.
First check that there is no chip in the hole and that the first thread has not been rolled over. If the thread is clean but the plug still will not enter, the hole is too tight — usually because the drill was too small or the tap was dull and displaced material instead of cutting it. This thread can still be reworked with a finishing tap.
Each end bounds the tolerance zone on a different side. The GO end confirms that the thread is not too tight, but it will also enter a hole that is too loose. The NO-GO end works the other way around. Checking with only one end leaves half the tolerance unchecked, which is the most common error during thread acceptance inspection.
Product accessories
Hand tap UNF 9/16-18 DIN 2181/2 HSS
Reference: 0000005744 In stock < 10 items
€12.63
€15.54 brutto
Thread ring gauge UNF 9/16-18 2A GO NO-GO
Reference: 15416 In stock < 10 items
€52.63
€64.74 brutto
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