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Description
THREAD5/16"-18 UNC
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
Thread plug gauge UNC 5/16-18 — GO and NO-GO ends
Internal inch thread inspection according to ASME B1.1
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch thread UNC |
| Thread size | 5/16-18 |
| Threads per inch | 18 TPI |
| Thread pitch | 1.411 mm |
| Nominal diameter | 7.938 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 7.021 – 7.155 mm |
| Hole minor diameter D1 | 6.411 – 6.734 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 5/16-18 UNC-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.
Thread plug gauge UNC 5/16-18 checks an internal thread — a tapped hole or nut. The GO end must screw in by hand through the full thread depth. The NO-GO end may enter no more than three full turns, as specified by ASME B1.2 for inch threads. The metric standard ISO 1502 specifies two threads and does not apply here.
In the workshop, this thread is most commonly found in T-slot nuts for milling machine and drill press tables — nuts for slot widths of 3/8, 7/16, and 1/2 inch are made with the 5/16-18 thread. The threaded hole in a T-slot nut carries the entire workpiece clamping force, so its thread is no minor detail. Another typical application is tapped holes in tooling imported from overseas, where thread restoration costs less than replacing the body.
Hole inspection in three steps
Internal thread acceptance criterion according to ASME B1.2
1
The GO end is screwed in by hand, without force, through the full length of the 5/16-18 UNC 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 of the NO-GO end give a penetration depth of 4.2 mm. In a nut 8 mm high, this leaves less than half the thread to assess — the limit must then be reduced.
GO end (passing)
Represents the hole's maximum material condition, meaning the minimum permissible pitch diameter of 7.021 mm. Checks profile, pitch, and diameter simultaneously — any error in any of these features reduces the functional diameter and stops the plug. Resistance during insertion indicates an undersized or incompletely tapped thread.
NO-GO end
Represents the least material condition — the maximum permissible pitch diameter of 7.155 mm. Detects an oversized threaded hole in which a bolt will loosen. The body may be stamped HI — the historical designation for the NO-GO end of an internal thread gauge.
✓ 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 5/16-18 and how its diameter is determined
What the gauge will indicate if the predrilled hole was incorrect
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | F = 6.530 mm | letter-size drill chart | difficult to obtain in Poland |
| Class 2B minor diameter D1 | 6.411 – 6.734 mm | ASME B1.1 | the hole must fall within this range |
| Margin to the upper limit | 0.204 mm | calculation | margin for drill oversize |
The standard no longer requires drill selection based on percentage thread height. The criterion is different and simpler: the drilled hole — including any drill oversize — must fall between 6.411 and 6.734 mm. A letter-size F drill bit measures 6.530 mm and falls within this range with a margin of 0.204 mm to the upper limit. A standard 6.500 mm metric drill bit also fits, so a Polish workshop has no reason to seek an inch drill bit.
How the pitch diameter is derived. The designation 5/16-18 means a diameter of 5/16 inch and 18 threads per inch. The pitch is 25.4 divided by 18, or 1.411 mm. The basic pitch diameter is 7.938 − 0.6495 × 1.411 = 7.021 mm and is also the lower limit for class 2B. An internal thread has no allowance — its tolerance zone starts exactly at the basic dimension and extends upward.
Classes 1B, 2B, and 3B for size 5/16-18
Why this gauge is class 2B
Comparison of internal thread classes
All three classes start at the same point — the basic pitch diameter of 7.021 mm. They differ only in the upper limit, because an internal thread has no allowance. The higher the class number, the narrower the tolerance zone.
| Class | D2 min [mm] | D2 max [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1B | 7.021 | 7.221 | 0.200 mm | Field assembly, threads exposed to contamination | |
| 2B | 7.021 | 7.155 | 0.135 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 7.021 | 7.120 | 0.099 mm | Joints requiring repeatability |
A class 2B hole mates with class 1A, 2A, and 3A bolts — mixing classes is permitted. Remember the difference from an external thread: its tolerance zone is shifted downward by the allowance, whereas here it always starts at the basic dimension. This is why the GO thread plug gauge has exactly the nominal dimension.
▲
Class 2B is not the same as 2A
The letter determines whether the gauge is intended for a hole or a bolt. B denotes an internal thread, which is the thread this plug gauge is intended for; A denotes an external thread, which requires a thread ring gauge. Ordering a "2A thread plug gauge" makes no sense, yet such orders are placed — a plug gauge never checks a bolt.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Hole pitch diameter within 7.021 – 7.155 mm |
| ▸ | Thread profile and pitch together, using the GO end |
| ▸ | That the thread is not oversized, 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
A 6.500 mm drill bit. The standard requires the hole, including drill oversize, to fall between 6.411 and 6.734 mm — a 6.500 mm metric drill bit fits within this range with a margin. American charts specify a letter-size F drill bit with a diameter of 6.530 mm, practically identical but difficult to obtain in Poland. Percentage thread height is not a criterion — ASME B1.1 eliminated it.
No. The diameters differ by 0.062 mm and the pitches by 0.161 mm — 5/16 inch means 7.938 mm with a pitch of 1.411 mm, while M8 means 8.000 mm with a pitch of 1.25 mm. Forcing the plug into an M8 thread will destroy both components. Both threads do have the same thread angle of 60°, but that is their only shared feature.
First check for a chip in the hole and for a rolled-over first thread. If the thread is clean and the plug still will not enter, the hole is undersized — most often due to a drill bit that is too small or a dull tap that displaces material instead of cutting it. Such a thread can still be reworked with a finishing tap.
Two different standards apply. Inch threads are inspected according to ASME B1.2, section 4.2.1, which permits three full turns of the NO-GO end. Metric threads are covered by ISO 1502, with a limit of two threads. At a pitch of 1.411 mm, three turns correspond to a depth of 4.2 mm — for a thin nut, the limit must be reduced by agreement between the parties.
Product accessories
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