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Description
THREAD1/4"-28 UNF
TOLERANCE2A
STANDARDASME B1.1
MATERIALTool steel
UNF 1/4-28 thread ring gauge — GO and NO-GO set
Inch fine thread inspection per ASME B1.1
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1/4-28 |
| Threads per inch | 28 TPI |
| Thread pitch | 0.907 mm |
| Nominal diameter | 6.350 mm |
| Tolerance class | 2A |
| Pitch diameter d2 | 5.652 – 5.735 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 1/4-28 UNF-2A engraving on the face of both rings indicates the size and tolerance class of the bolt being inspected.
UNF 1/4-28 thread ring gauge checks an external thread in the smallest size in this shipment. The GO ring must screw on by hand over the full thread length. The NO-GO ring may advance no more than three full turns — this is specified by ASME B1.2 for inch threads, unlike the metric standard ISO 1502, which specifies two turns.
This size has two applications in which it is effectively standard. The first is grease fittings: the 1/4-28 UNF straight thread per SAE J534 is the most common grease fitting connection on American machinery, found wherever a bearing is lubricated with a grease gun. The second is aviation — an AN4 bolt has a shank diameter of 1/4 inch and this exact thread. Both cases share one feature: the component is small, repeatedly screwed in, and exposed to vibration, so the fine pitch is not a designer's whim.
GO/NO-GO inspection of fine threads
Three steps and the acceptance criterion per ASME B1.2
1
The GO ring gauge screws on by hand, without force, over the full thread length of the 1/4-28 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 28 threads per inch, three turns of the NO-GO ring gauge mean just 2.7 mm of engagement — the least in the entire inch series. In practice, this makes the criterion stricter than it looks on paper.
GO ring (GO gauge)
Represents the maximum material condition, meaning the largest permissible pitch diameter of 5.735 mm. Checks thread form, pitch, and diameter simultaneously — an error in any of these features increases the functional diameter and stops the ring gauge. With a fine pitch, the ring gauge is short, so resistance is felt immediately.
NO-GO ring (NO-GO gauge)
Represents the least material condition — the smallest permissible pitch diameter of 5.652 mm. The three permitted turns correspond to 2.7 mm of thread length. The body may be stamped LO, the historical designation for the NO-GO side of an external thread gauge.
✓ 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.
The narrowest tolerance zone in the entire inch series
Classes 1A, 2A, and 3A for size 1/4-28
Comparison of external thread classes
The nominal pitch diameter of the 1/4"-28 UNF thread is 5.761 mm. Classes 1A and 2A have an allowance of 0.025 mm — the smallest in the entire shipment, because allowance increases with diameter and pitch. Class 3A has no allowance.
| Class | Allowance [mm] | d2 permissible [mm] | Tolerance zone | Application | Status |
|---|---|---|---|---|---|
| 1A | 0.025 | 5.608 – 5.735 mm | 0.127 mm | Field assembly, threads exposed to impact damage | |
| 2A | 0.025 | 5.652 – 5.735 mm | 0.084 mm | Commercial standard for bolts and grease fittings | THIS PRODUCT |
| 3A | 0.000 | 5.697 – 5.761 mm | 0.064 mm | Aviation, precision engineering |
The entire class 2A tolerance zone spans 0.084 mm — less than the thickness of a hair. At this size, dirt, a metal chip, or a damaged first thread can therefore stop the GO ring gauge on a conforming thread. At the 1-inch size, the same tolerance zone spans 0.173 mm, and minor contamination makes no difference.
▲
Cleaning the thread before measurement
With a tolerance zone of 0.084 mm, residual grease containing metal chips is enough to stop the GO ring gauge halfway along the thread. Clean the thread and repeat the check before rejecting the part. Never tighten the gauge with a wrench or pliers — forcing it on damages both the thread and the ring gauge, and the ring gauge is more expensive.
Fine thread 1/4-28 alongside coarse thread 1/4-20 and M6
Three threads with similar diameters, none interchangeable
Nominal diameter does not distinguish UNF from UNC. Both are 1/4 inch, or 6.350 mm — only the thread count differs: 28 in the fine series and 20 in the coarse series. Pitch is obtained by simple division: 25.4 divided by 28 is 0.907 mm, and 25.4 divided by 20 is 1.270 mm. Over a length of 25 mm, this gives 28 threads versus just under 20 — a difference visible to the naked eye when a reference is available.
A third thread that can be confused with these is M6, with a pitch of 1.0 mm and a diameter of 6.000 mm. Its diameter differs from 1/4 inch by 0.350 mm, more than the difference between 5/16 inch and M8, but still small enough for someone to attempt assembly when the thread is dirty. An M6 bolt screwed into a 1/4-28 thread will strip the threads of both components after two turns.
Set contents and inspection scope
Set contents and scope of ring gauge inspection
INCLUDED IN THE SET:
| ▸ | GO (pass) ring gauge UNF 1/4-28 2A |
| ▸ | NO-GO (reject) ring gauge UNF 1/4-28 2A |
| ▸ | NO-GO ring marked with a groove or dot |
WHAT THEY DO NOT MEASURE:
| ▸ | Thread major diameter — for 2A, this is 6.159 – 6.325 mm and is measured with a plain gauge |
| ▸ | Grease fitting sealing cone |
| ▸ | Grease fitting passage clearance and check ball operation |
Frequently asked questions
Yes, if the grease fitting has a straight 1/4-28 UNF thread — the most common version on American machines. The ring gauges check pitch diameter and thread form. They do not assess the sealing cone or whether the passage is clear. Note: some grease fittings have a 1/8-27 NPTF taper thread, for which this set is unsuitable.
Only by thread count, because the nominal diameter is identical at 6.350 mm. The UNF fine series has 28 threads per inch and a pitch of 0.907 mm; the UNC coarse series has 20 threads per inch and a pitch of 1.270 mm. Use an inch thread pitch gauge or count the threads to distinguish them. A caliper cannot distinguish them.
Clean the thread first. The class 2A tolerance zone here is only 0.084 mm, so grease containing metal chips or a damaged first thread will stop the ring gauge on a conforming part. If the ring gauge still stops after cleaning, the thread is oversized — most often because of a worn die or material that has expanded during thread rolling. Do not tighten the gauge with a tool.
These are two different standards. Inch threads are checked according to ASME B1.2, section 5.2.1, which permits three full turns of the NO-GO ring gauge. Metric threads are checked according to ISO 1502, with a limit of two threads. At a pitch of 0.907 mm, three turns mean 2.7 mm — still very little in practice.
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| Courier GLS | Cash on delivery | 17,26 zł gross | Lead time 1 working day |
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