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Description
THREAD1/2"-20 UNF
TOLERANCE2B
STANDARDASME B1.1
MATERIALTool steel
UNF 1/2-20 thread plug gauge — port inspection
Class 2B internal thread per ASME B1.1
| TECHNICAL DATA | |
|---|---|
| Thread type | Unified inch fine thread UNF |
| Thread size | 1/2-20 |
| Threads per inch | 20 TPI |
| Thread pitch | 1.270 mm |
| Nominal diameter | 12.700 mm |
| Tolerance class | 2B |
| Pitch diameter D2 | 11.874 – 12.017 mm |
| Hole minor diameter D1 | 11.326 – 11.618 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 1/2-20 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 1/2-20 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.
Every auto repair shop knows this thread, although not everyone knows it is an inch thread: it is used on the oil drain plug in millions of General Motors, Ford, and Chrysler vehicles. The 1/2-20 tapped hole is therefore found in the oil pan, and repairing it is one of the most common thread restoration jobs in automotive service. The same size is used for a dash -5 JIC hydraulic fitting, an AN8 aircraft bolt, and a 5/16-inch brake line connection.
Pre-drilled hole for 1/2-20 thread
Drill selection based on the D1 range
| HOLE BEFORE TAPPING | |||
|---|---|---|---|
| Size | Value | Source | Note |
| Inch drill bit | 29/64 = 11.509 mm | inch drill chart | difficult to obtain in Poland |
| Metric drill bit | 11.500 mm | selected to fit the D1 range | recommended |
| Class 2B minor diameter D1 | 11.326 – 11.618 mm | ASME B1.1 | the hole must fall within this range |
At the same half-inch diameter, the coarse UNC series has 13 threads per inch and the fine UNF series has 20. The pitches are 1.954 and 1.270 mm. The nearest metric thread is M12, with a diameter of 12.000 mm versus 12.700 mm — and this is the real risk, because oil drain plugs in European cars are metric and can easily be mixed up in stock.
The pitch is derived from the designation: 20 threads per inch means 25.4 divided by 20, or 1.270 mm. The basic pitch diameter is 12.700 − 0.6495 × 1.270 = 11.875 mm. This value is also the lower limit for class 2B, because an internal thread has no allowance.
Classes 1B, 2B, and 3B — size 1/2-20
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 11.874 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 | 11.874 | 12.088 | 0.213 mm | Field assembly, threads exposed to contamination | |
| 2B | 11.874 | 12.017 | 0.142 mm | Commercial standard — nuts and tapped holes | THIS PRODUCT |
| 3B | 11.874 | 11.981 | 0.107 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, along the full length of the 1/2-20 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 1.270 mm, the three permitted turns correspond to a depth of 3.810 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 11.874 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 12.017 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.
▲
An oil pan gives you only one second chance
The drain plug thread is in aluminum or thin sheet metal and stops holding after two or three unsuccessful tightening attempts. The gauge shows whether the hole can still be used with the original plug or whether a thread repair insert or an oversized plug is needed. The check takes only a dozen or so seconds, compared with replacing the oil pan.
Inspection scope and storage
What the gauge confirms and what it cannot measure
THE GAUGE CONFIRMS:
| ▸ | Confirms hole pitch diameter within 11.874 – 12.017 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
In many American cars, yes — it is one of the most common plug sizes in GM, Ford, and Chrysler vehicles. Note: manufacturers also use M12 and M14 metric plugs, so measure the thread before ordering parts rather than relying on a model-year catalog.
The NO-GO end provides the indication. A hole stripped by repeated tightening has a pitch diameter above the upper limit for class 2B, and the NO-GO end will enter more than three full turns. The GO end will not reject such a hole because it will screw in without resistance.
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
Hand tap UNF 1/2-20 DIN 2181/2 HSS 2B Fra
Reference: 0000007807 In stock > 10 items
€13.16
€16.18 brutto
Thread ring gauge UNF 1/2-20 2A GO NO-GO
Reference: 15415 In stock < 10 items
€50.00
€61.50 brutto
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