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Description
THREAD1 1/4"-12 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 1.1/4-12 thread plug gauge — port inspection
Class 2B internal thread per ASME B1.1
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1.1/4-12 |
| Threads per inch | 12 TPI |
| Thread pitch | 2.117 mm |
| Nominal diameter | 31.750 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 30.376 – 30.579 mm |
| Hole minor diameter D1 | 29.459 – 29.919 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 1/4-12 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 1.1/4-12 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.
This size is a typical end thread for a hydraulic cylinder piston rod approximately one inch in diameter, and a thread used in mounting clevises and eyes on cylinders built to the NFPA standard. The 1 1/4-12 internal thread is therefore found in an eye, a spherical joint, or a locknut — components assembled and disassembled whenever the cylinder is rebuilt.
Pre-drilled hole for 1.1/4-12 thread
Drill selection based on the D1 range
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 1 11/64 = 29.766 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 29.800 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 29.459 – 29.919 mm | ASME B1.1 | the hole must fall within this range |
The entire family from one inch to one and a half inches has the same pitch 2.117 mm and differs only in diameter. A thread pitch gauge will show the same reading for each of these five sizes. Only a diameter measurement distinguishes them: 31.750 mm is 1 1/4 inches, 28.575 mm is 1 1/8, and 34.925 mm is 1 3/8.
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 31.750 − 0.6495 × 2.117 = 30.375 mm. This value is also the lower limit for class 2B, because an internal thread has no allowance.
Classes 1B, 2B, and 3B — size 1.1/4-12
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 30.376 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 | 30.376 | 30.681 | 0.305 mm | Field assembly, threads exposed to contamination | |
| 2B | 30.376 | 30.579 | 0.203 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 30.376 | 30.528 | 0.152 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 1/4-12 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 30.376 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 30.579 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.
▲
The piston rod thread is subject to fatigue loading
The piston rod end transmits the full cylinder force in a push-pull cycle, so its thread wears differently from that of a bolt tightened once. Thread stripping starts at the first threads near the transition to the shank and remains invisible to the naked eye for a long time. The NO-GO end detects it earlier than any visual inspection.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 30.376 – 30.579 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
Yes, it is one of the typical piston rod end and clevis threads in NFPA-standard cylinders with a piston rod approximately one inch in diameter. Eyes and spherical joints mounted on the piston rod use the same thread. However, this size is not used in hydraulic connections — ORB and JIC skip it.
Only by diameter, because the pitch is identical for all of them at 2.117 mm. A caliper measurement distinguishes them: 1 1/8 inches is 28.575 mm, 1 1/4 is 31.750 mm, and 1 3/8 is 34.925 mm. A thread pitch gauge is useless within this family.
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 1.1/4-12 2A GO NO-GO
Reference: 15422 In stock < 10 items
€92.11
€113.29 brutto
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