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Description
THREAD1 1/8"-12 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 1.1/8-12 thread plug gauge — port inspection
Class 2B internal thread per ASME B1.1
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1.1/8-12 |
| Threads per inch | 12 TPI |
| Thread pitch | 2.117 mm |
| Nominal diameter | 28.575 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 27.201 – 27.399 mm |
| Hole minor diameter D1 | 26.284 – 26.744 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 1 1/8-12 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 1.1/8-12 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.
One and one-eighth inches belongs to the twelve-thread family, in which ASME B1.1 specifies the same pitch for five consecutive diameters — from one inch to one and a half inches. This particular size is the least common in the family: hydraulic systems skip it, and cylinder catalogs more frequently list the adjacent sizes, 1 1/4 and 1 3/8 inches.
Pre-drilled hole for 1.1/8-12 thread
Drill selection based on the D1 range
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 1 3/64 = 26.591 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 26.500 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 26.284 – 26.744 mm | ASME B1.1 | the hole must fall within this range |
The entire family from one inch to one and a half inches has identical pitches 2.117 mm and differs only in diameter. A thread pitch gauge will show twelve threads per inch for each of the five sizes. Another potential source of confusion is the hydraulic 1 1/16-12 thread with a diameter of 26.988 mm — it differs from 1 1/8 inches by one-sixteenth of an inch, or 1.588 mm.
The pitch is derived from the designation: 12 threads per inch means 25.4 divided by 12, or 2.117 mm. The basic pitch diameter is 28.575 − 0.6495 × 2.117 = 27.200 mm. This value is also the lower limit for class 2B, because an internal thread has no allowance.
Classes 1B, 2B, and 3B — size 1.1/8-12
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 27.201 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 | 27.201 | 27.498 | 0.297 mm | Field assembly, threads exposed to contamination | |
| 2B | 27.201 | 27.399 | 0.198 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 27.201 | 27.351 | 0.150 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, through the full length of the 1 1/8-12 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 2.117 mm, the three permitted turns correspond to a depth of 6.351 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 27.201 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 27.399 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.
▲
This is not a dash -12 ORB port
In ORB O-ring ports, dash -12 uses the thread 1 1/16-12, or 26.988 mm — not 1 1/8 inches. Both have the same pitch and differ in diameter by one and a half millimeters, so ordering the wrong size is easy and results in a gauge that will not enter the port. Measure the diameter before ordering.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 27.201 – 27.399 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
No. In ORB ports, dash -12 uses a 1 1/16-12 thread, or 26.988 mm, not 1 1/8 inches. These two sizes have the same pitch and differ by one-sixteenth of an inch, making them easy to confuse when ordering a gauge.
Use a 26.500 mm drill bit. After drill oversize, the hole should measure between 26.284 and 26.744 mm, so the drill leaves a 0.244 mm margin below the upper limit. American tables specify a fractional-inch drill bit of 1 3/64 inches with a diameter of 26.591 mm.
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
Thread ring gauge UNF 1.1/8-12 2A GO NO-GO
Reference: 15421 In stock < 10 items
€80.26
€98.72 brutto
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