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Description
THREAD5/16"-24 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 5/16-24 thread plug gauge — GO and NO-GO ends
Inspection of an ORB dash -2 hydraulic port
| 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 | 2B |
| Pitch diameter D2 | 7.249 – 7.371 mm |
| Hole minor diameter D1 | 6.792 – 7.038 mm |
| Gauge type | Double-ended: GO + NO-GO |
| Material | Tool steel, hardened |
| Hardness | 58-62 HRC |
| Standard | ASME B1.1 / ASME B1.2 |
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 5/16-24 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 5/16-24 thread plug gauge checks an internal thread, meaning a hole or port. The GO end must screw in by hand to the full thread depth, and the NO-GO end no more than three full turns per ASME B1.2 — the metric standard ISO 1502 permits two turns and does not apply here.
An internal thread of this size is primarily found in a hydraulic port with an O-ring seal, designated ORB dash -2 — the smallest in this family. The port is machined in the body of a directional control valve, pump, or control manifold, and accepts a fitting sealed by an O-ring against the face. A hole for an AN5 aircraft bolt has the same thread. Both applications share one feature: the port is part of the body, so repairing it costs many times more than replacing the component screwed into it.
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 5/16-24 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 1.058 mm, the three permissible turns of the NO-GO end correspond to a depth of 3.2 mm. An ORB port may be shallow, in which case the limit must be reduced by agreement between the parties.
GO end (passing)
Represents the maximum material condition of the hole — the minimum permissible pitch diameter of 7.249 mm. It checks thread form, pitch, and diameter simultaneously. In a hydraulic port, it most often stops on a thread turn deformed by a cross-threaded fitting.
NO-GO end
Represents the minimum material condition — the maximum permissible pitch diameter of 7.371 mm. It detects a port with worn, oversized threads where the fitting cannot be tightened fully and the O-ring cannot be pressed against the face. 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.
Tap drill for 5/16-24 and the hole tolerance range
How to determine the hole size to drill
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | I = 6.909 mm | letter-size drill chart | difficult to obtain in Poland |
| Metric drill bit | 6.900 mm | selected to fit the D1 range | recommended, widely available |
| Class 2B minor diameter D1 | 6.792 – 7.038 mm | ASME B1.1 | the hole must fall within this range |
The minor diameter range is 0.246 mm, and a 6.900 mm drill bit leaves a margin of 0.138 mm to its upper limit. This is more forgiving than 1/4-28, where the margin is only 0.080 mm. The inch-series letter I drill bit measures 6.909 mm and is practically identical to the metric bit — there is no point in importing it.
The pitch diameter follows from the designation. The notation 5/16-24 means a diameter of 5/16 inch and 24 threads per inch of length. The pitch is 25.4 divided by 24, or 1.058 mm. The basic pitch diameter is 7.938 − 0.6495 × 1.058 = 7.249 mm and is also the lower limit of class 2B. An internal thread has no allowance, so the tolerance zone starts exactly at this value.
Classes 1B, 2B, and 3B for size 5/16-24
Which class to use for a hydraulic port
Comparison of internal thread classes
The three classes start at the same point — the basic pitch diameter of 7.249 mm — and differ only in the upper limit.
| Class | D2 min [mm] | D2 max [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1B | 7.249 | 7.429 | 0.180 mm | Field assembly, threads exposed to contamination | |
| 2B | 7.249 | 7.371 | 0.122 mm | Commercial standard — hydraulic ports, nuts | THIS PRODUCT |
| 3B | 7.249 | 7.341 | 0.092 mm | Aviation, precision engineering |
Hydraulic ports are made to class 2B because the fitting is installed and removed throughout the machine's service life. Class 3B is used for aircraft components. Class 1B has no application in this size — its tolerance range of 0.180 mm is too wide for a pressurized port.
▲
The gauge cannot fully assess an ORB port
In an O-ring port, the seal is provided by an O-ring pressed against the face, rather than the thread. The plug will confirm that the thread meets class 2B, but it will not detect a scratched face, a recess that is too shallow, or a sharp edge that will cut the O-ring during assembly. These features are assessed separately.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | The port pitch diameter within the range of 7.249 – 7.371 mm |
| ▸ | Thread profile and pitch together, using the GO end |
| ▸ | That the port thread is not worn oversize, using the NO-GO end |
WHAT IT CANNOT MEASURE:
| ▸ | The face and the O-ring recess |
| ▸ | Hole minor diameter — this requires a plain plug gauge |
| ▸ | Perpendicularity of the port to the body face |
Frequently asked questions
Yes, as far as the thread is concerned. The dash -2 O-ring port has a 5/16-24 UNF thread, and that is what this plug checks. It does not assess the face or the recess where the O-ring seats — which is where the seal is formed. A good thread with a scratched face still means leakage.
A 6.900 mm drill bit. The hole, including oversize caused by drilling, must fall between 6.792 and 7.038 mm — 6.900 mm falls within this range with a margin of 0.138 mm to the upper limit. American tables specify a letter I drill bit with a diameter of 6.909 mm, practically identical to the metric bit, so there is no reason to import it.
The port threads are probably stripped. The GO end will screw in without resistance because the thread is looser, not tighter. The NO-GO end provides the indication — it will enter more than three full turns. This thread will not withstand the tightening torque, so the O-ring will not be pressed against the face and the connection will leak during pressure surges.
This is the most difficult pair in the entire inch thread system: the diameters differ by 0.062 mm and the pitches by 0.058 mm. A 1.0 mm metric thread pitch gauge placed against a 5/16-24 thread appears to fit. The discrepancy becomes apparent only over a longer length — over 25 mm, the inch thread has 23.6 turns, while M8 has exactly 25. A gauge is the most reliable option in a hole.
Product accessories
Thread ring gauge UNF 5/16-24 2A GO NO-GO
Reference: 15412 In stock < 10 items
€50.00
€61.50 brutto
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