BSP, NPT, UNF/JIC and Metric Threads: How to Identify and Match Them
BSP threads have a 55° profile, while NPT, UNF and metric threads have a 60° profile. BSPT and NPT tapered threads seal on the thread itself; on…
Thread identification tool
Hydraulic Thread Identification Tool
Enter the diameter and thread pitch you measured with a caliper; the tool lists the closest candidates among BSP, NPT, UN/UNF and metric fine threads and shows how to tell commonly confused threads apart.
Quick answer
Identifying a thread takes four pieces of information: the outside diameter (male) or inside diameter (female) measured with a caliper, the pitch or threads per inch (TPI), whether the thread is parallel or tapered, and the seat of the connection. For example, a parallel male thread measuring about 20.9 mm with 14 threads per inch is a G 1/2 (BSPP) candidate; its nominal major diameter is 20.955 mm. If the thread is tapered, the candidate is R 1/2 (BSPT) or 1/2" NPT, and the 55° / 60° profile angle tells these two apart. The tool gives a preliminary identification; confirm the result with a pitch gauge and the mating part.
1. Measure · 2. Enter · 3. Compare
Enter your measurements
Results will appear here
Enter the thread type, the diameter you measured and the thread pitch; the tool lists the three closest of 67 standard threads, with the size differences and any threads they could be confused with.
Examples: male, Ø 20.9 mm, 14 TPI male, Ø 19.0 mm, 16 TPI male, Ø 21.9 mm, pitch 1.5 mm male, Ø 26.9 mm, 12 TPI
Measuring guide
How to measure a thread
Parallel or tapered?
Measure the diameter at the 1st, 4th and last full thread; if it gets smaller towards the end, the thread is tapered (R, Rc, NPT).
Measure diameter
Rest the caliper on the crests of full threads; do not measure damaged or worn threads.
Find the pitch
With a pitch gauge, or by measuring 10 pitches and dividing; TPI for inch threads, mm for metric.
Check the sealing seat
24°, 37° or 45° cone, flat face + O-ring, or port chamfer: the same thread is used on different connections.
Watch for mismatches
Commonly confused threads
Thread pairs that measure almost the same with a caliper, and the features that tell them apart. The risk rises as the OD and pitch differences shrink.
| Thread A | Thread B | Nominal OD differencemm | Pitch difference% | Mix-up risk | How to tell apart |
|---|---|---|---|---|---|
| G 1/2 / R 1/2 | 1/2-14 NPT(F) | 0.381 | 0.0 | Very high | The OD difference is only 0.38 mm and both are 14 TPI; tell them apart by thread angle, crest shape and taper. In 1/2" and 3/4" sizes, a definitive BSPT vs NPT check may require an optical comparator. |
| G 1/2 / R 1/2 | 3/4-16 UNF | 1.905 | 14.3 | Low | JIC/ORB -8 corresponds to 1/2" tube, but its thread is 3/4-16 UNF (Ø19.05 mm), while 1/2" BSP is Ø20.96 mm. A caliper shows the difference. |
| M22×1.5 | 7/8-14 UNF | 0.225 | 21.0 | Medium | OD difference 0.23 mm, pitch difference 21% (1.5 mm / 1.814 mm): a pitch gauge separates them reliably. |
| G 5/8 | 7/8-14 UNF | 0.686 | 0.0 | Medium | Both are 14 TPI with a 0.69 mm OD difference: the thread angle (55° / 60°) and the OD tell them apart. |
| 1/2-14 NPT(F) | 7/8-14 UNF | 0.889 | 0.0 | Low | Both are 14 TPI; they differ in tapered/parallel form and OD (21.3 / 22.2 mm). |
| G 1 / R 1 | 1-5/16-12 UN | 0.089 | 9.1 | Medium | In the 1" group, G 1 (33.25 mm, 11 TPI), 1" NPT (33.40 mm, 11.5 TPI), 1-5/16-12 UN (JIC/ORB -16, 33.34 mm) and M33×2 (33.0 mm) measure almost the same with a caliper; pitches are 2.309 / 2.209 / 2.117 / 2.0 mm. Use a pitch gauge, thread angle and taper check together. |
| 1-1/16-12 UN | M27×2 | 0.013 | 5.8 | High | JIC/ORB -12 (1-1/16-12 UN) and M27×2 (ISO 6149 port) differ by 0.01 mm in OD; pitches are 2.117 / 2.0 mm (5.8%). The risk of mismatching ports and stud ends is high. |
| G 3/4 / R 3/4 | 3/4-14 NPT(F) | 0.229 | 0.0 | Very high | |
| 5/16-24 UNF | M8×1 | 0.063 | 5.8 | High | |
| 1-11.5 NPT(F) | 1-5/16-12 UN | 0.064 | 4.3 | High | |
| G 1 / R 1 | 1-11.5 NPT(F) | 0.152 | 4.5 | High | |
| 1-3/16-12 UN | M30×2 | 0.163 | 5.8 | High | |
| G 1-1/4 / R 1-1/4 | 1-1/4-11.5 NPT(F) | 0.254 | 4.5 | High | |
| 1/4-18 NPT(F) | M14×1.5 | 0.284 | 6.3 | High | |
| 1/8-27 NPT(F) | M10×1 | 0.287 | 6.3 | High | |
| 9/16-18 UNF | M14×1.5 | 0.288 | 6.3 | High | |
| 1-5/16-12 UN | M33×2 | 0.338 | 5.8 | High | |
| 1-7/8-12 UN | M48×2 | 0.375 | 5.8 | High | |
| G 1/4 / R 1/4 | 1/2-20 UNF | 0.457 | 5.3 | High | |
| G 1-1/2 / R 1-1/2 | 1-1/2-11.5 NPT(F) | 0.457 | 4.5 | High | |
| 3/8-24 UNF | M10×1 | 0.475 | 5.8 | High | |
| G 3/8 / R 3/8 | 3/8-18 NPT(F) | 0.483 | 5.6 | High | |
| 1-7/16-12 UN | M36×2 | 0.513 | 5.8 | High | |
| 11/16-16 UN | M18×1.5 | 0.538 | 5.8 | High | |
| G 1/4 / R 1/4 | 1/4-18 NPT(F) | 0.559 | 5.6 | High | |
| G 1/8 / R 1/8 | 1/8-27 NPT(F) | 0.559 | 3.7 | High | |
| 1/4-18 NPT(F) | 9/16-18 UNF | 0.572 | 0.0 | High | |
| 1-1/2-11.5 NPT(F) | 1-7/8-12 UN | 0.635 | 4.3 | Medium | |
| 13/16-16 UN | M20×1.5 | 0.638 | 5.8 | Medium | |
| 1-1/4-11.5 NPT(F) | 1-11/16-12 UN | 0.699 | 4.3 | Medium | |
| G 2 / R 2 | 2-11.5 NPT(F) | 0.711 | 4.5 | Medium | |
| 1-5/8-12 UN | M42×2 | 0.725 | 5.8 | Medium |
How to tell them apart
- thread angle
- Thread angle gauge: 55° (BSP) / 60° (NPT, UN, metric)
- crest shape
- Crest shape: rounded (BSP) / flat (others)
- taper
- Taper check: measure the diameter at the 1st, 4th and last full thread (with a 1:16 taper the diameter changes 1 mm over 16 mm)
- pitch
- Pitch with a gauge: measure at least 10 pitches and divide by the number of pitches
- long-length gauge
- Pitches are almost identical: a pitch gauge alone is not enough; check the gauge fit over a long length
- sealing seat
- Seat: 24° / 37° / 45° cone, flat face + O-ring or port chamfer
- Distinguishing features: thread angle (BSP 55°, NPT/UN/metric 60°), crest shape (BSP rounded, others flat), form (R/Rc and NPT tapered 1:16; G, UN and metric parallel), pitch (TPI for inch threads, mm for metric) and seat (24°, 37°, 45° cone, flat face + O-ring, port chamfer + O-ring).
- Recommended order: measure the diameter at the 1st, 4th and last full thread to tell tapered from parallel; measure the diameter (4th–5th thread if tapered); find the pitch with a gauge; look up the standard in the chart; identify the fitting end by its cone or seat.
- The first seven rows (those with notes) are especially important comparisons that are often confused in the field; their OD or pitch differences may be larger. The other rows are pairs from different thread families with a nominal OD difference under 0.8 mm and a pitch difference under 6.5%.
- Chart values are an identification guide, not inspection criteria. Final verification should be done with thread gauges (GO/NO-GO), a pitch gauge and a cone angle gauge.
Reference standards
- ISO 228-1
- Parallel pipe threads not sealing on the thread (G)
- ISO 7-1
- Pipe threads sealing on the thread (R, Rc, Rp)
- ASME B1.20.1
- General purpose tapered pipe threads (NPT)
- ASME B1.1
- Unified inch screw threads (UN, UNF)
- ISO 261
- ISO metric threads, general plan
Values are nominal; for critical applications, verify against the current standard and the manufacturer's catalog.
Description
Description
The thread identification tool helps you work out which standard a male or female thread belongs to, using measurements taken with a caliper and a pitch gauge. It covers BSP (G and R), NPT, UN/UNF (JIC, ORB, ORFS) and metric fine threads. A correct result starts with a correct measurement.
Step-by-step measuring
Before measuring, clean the thread of oil, sealing tape and old seal residue. You will need a digital or vernier caliper, inch (TPI) and metric pitch gauges and, if possible, a cone angle gauge and a magnifier.
- Male or female? If the thread is on the outside, it is male; if it is on the inside, it is female. This decides which diameter the tool compares.
- Measure the diameter. On a male thread, measure the outside diameter with the caliper jaws square across the crests; on a female thread, measure the inside diameter across the crests with the inside jaws. Repeat two or three times in different directions and take the average.
- Find the pitch. Place the pitch gauge blades on the thread one by one, along the axis, and look against the light; the blade that leaves no gap at the crests and roots is the correct pitch. Try inch and metric gauges separately. Without a gauge, measure a length of at least 10 pitches and divide by the number of pitches: if 10 pitches measure 18.14 mm, the pitch is 1.814 mm, i.e. 14 threads per inch.
- Parallel or tapered? Measure the diameter at the first, fourth and last full thread. On a parallel thread the value does not change; on a tapered thread the diameter gets smaller towards the end. With the 1:16 taper of BSPT and NPT, the diameter changes by 1 mm over a 16 mm length. For a tapered male thread, enter the diameter measured at the 4th–5th full thread.
- Check the profile angle. BSP threads have a 55° profile with rounded crests; NPT, UN/UNF and metric threads have a 60° profile with flat crests. At similar pitches both gauges may seem to fit; slide the gauge along several threads to check.
- Look at the seat. A 24° internal cone in the body or a cutting ring on the tube indicates a metric DIN 2353 / ISO 8434-1 fitting, a 37° flare cone indicates JIC and a 45° flare indicates an SAE 45° connection. An O-ring groove on a flat face points to ORFS, an O-ring under the hex to an ORB or ISO 6149 port, and a washer seal face to a BSPP port.
Reading the results
The tool compares the diameter and pitch you entered with the chart values and lists the candidates in order of closeness. If you entered the thread form (parallel or tapered), candidates that don't match the form are eliminated; you compare the profile angle (55° / 60°) and the seat yourself against the information on the result cards.
- Male threads measure slightly small. Because of tolerances, the actual major diameter of a male thread is below the nominal value; on UN/UNF threads the difference can reach 0.3–0.45 mm. The diameter in the NPT chart is the pipe OD: for 1/2" NPT it is 21.336 mm, while a caliper typically reads 21.14 mm.
- Same thread, different connection. 9/16-18 UNF is the thread of both JIC -6 and ORFS -4; M14×1.5 is the nut thread of both 8L and 6S. Once the thread is found, the seat determines the connection type.
- Close candidates. If the first two candidates are close, the "Commonly confused threads" section shows how to tell that pair apart. The diameter difference between 1/2" BSP (20.955 mm) and 1/2" NPT (21.336 mm) is 0.38 mm and both are 14 TPI; profile angle and taper separate them. Between the JIC -12 thread 1-1/16-12 UN (26.988 mm) and M27×2 the diameter difference is only 0.013 mm; they are told apart by pitch, using a pitch gauge.
Limits of the tool
The result is a preliminary identification, not an inspection decision. On worn, crushed or burred threads the measured diameter comes out smaller than the real one, and the caliper's own error can reach a few hundredths of a millimeter. In 1/2" and 3/4" sizes, a definitive distinction between BSPT and NPT may require an optical comparator. For threads that are not on the list, such as coarse metric, UNC or NPSM, the tool will not give a correct result. SAE 45° flare threads that differ from JIC (5/8-18 UNF for -6, 1-1/16-14 UNS for -12) are not on the list either; if you see a 45° cone, also compare your measurement with these threads. Do the final check with a pitch gauge, thread gauges (go / no-go) and, if possible, the mating part.
Photo support
If you're unsure of the result, photograph the threaded section and the seat of the part with the caliper visible while measuring. Add the diameter and pitch you measured and the equipment the part fits, and send it on WhatsApp. We'll confirm the standard with you and suggest the AISI 316/316L equivalent or, if needed, an adapter that joins the two standards.
FAQ
Frequently asked questions
If your question isn't answered here, message us on WhatsApp with dimensions or a photo, and our technical team will answer directly.
How do I measure the pitch without a pitch gauge?
Measure a length of at least 10 pitches with a ruler or caliper and divide by the number of pitches; for example, if 10 pitches measure 18.14 mm, the pitch is 1.814 mm, i.e. 14 threads per inch. Counting the threads within 25.4 mm can be off by half a thread, so close values such as 27 vs 28 TPI or 11 vs 11.5 TPI are hard to separate by counting. Measuring a longer length reduces the error; use a pitch gauge for a definitive answer.
How do I tell BSP from NPT threads?
BSP has a 55° profile and NPT a 60° profile; BSP crests are rounded and NPT crests are flat. Pitches differ in most sizes (28 vs 27 TPI at 1/8", 19 vs 18 at 1/4" and 3/8", 11 vs 11.5 TPI from 1" to 2"), but at 1/2" and 3/4" both are 14 TPI. In these two sizes, check the profile angle; since BSPP is parallel, a taper check also separates them.
My measured diameter is slightly smaller than the chart value. Is that normal?
Yes. Because of tolerances, the actual major diameter of a male thread is below the nominal value; on UN/UNF threads the difference can reach 0.3–0.45 mm. The diameter in the NPT chart is the pipe OD, and a caliper reads smaller on a tapered thread: for 1/2" NPT the pipe OD is 21.336 mm, while the typical reading at the 4th–5th thread is 21.14 mm. On worn threads the difference is even greater.
Is the same thread used on different connections?
Yes. 9/16-18 UNF is the thread of both JIC -6 and ORFS -4; 1-3/16-12 UN is the thread of both JIC -14 and ORFS -12; M14×1.5 is the nut thread of both 8L and 6S. Once the thread is found, the seat determines the connection type: a 37° cone indicates JIC, a flat face with an O-ring groove indicates ORFS, and a 24° cone with a cutting ring indicates a metric fitting.
What should I send to have my measurement checked?
Send clear photos of the threaded section and the seat taken while measuring with the caliper, together with the diameter and pitch you measured and the equipment the part fits, on WhatsApp. If possible, add a photo of the mating part or port as well; we'll confirm the standard and the right AISI 316/316L part with you.
Not sure?
Send us a photo and we'll confirm the thread together
Send a close-up photo of the part and your caliper reading on WhatsApp; we'll work out the thread standard and the matching AISI 316/316L fitting with you.