Teesing delivers high quality connectivity solutionsLibraryPublicationsBasics of threadFAQ (Basics of thread)
M - ISO thread (metric)
NPT - Pipe thread
G/R/RP - Withworth thread (BSPP/BSPT)
UNC / UNF - unified national thread
Other type of threads
Type of threads - Conversion factor
FAQ (Basics of thread)
Basics of thread
If you can not find your answer in these frequently asked questions, please use the feedback form to submit your question. Do not forget to enter you e-mail address to receive a reply.
How can I identify types of thread?
View our basic information about thread.
- Is the thread male or female (visual)?
- Is the thread conical or cylindrical/parralel (visueel)? Check: Measure the thread with a vernier caliper at the beginning and at the end, if this is equal the thread is straight/parallel.
- Measure the thread diameter (male: outer diameter, female: inner diameter). The diameter obtained in this step is not exactly the same as the specified nominal size of the specified thread. The main reason for this variation is industry or production tolerances.
- Determine the urgency. Easy to measure with a thread gauge.
- Determine the angle of the end connection. Most common angles: 30°, 37° en 45°.
Thread identification example:
According to the method mentioned above:
- The fitting is visibly male.
- The thread is visibly straight/parallel.
- The measure diameter is 26.4 mm (this can easily be converted: inch decimal × 25.4 = mm).
- Threads per inch (TPI) or pitch : 14. If you are not sure, you can search for a suitable diameter in the tables. (For example UNC and UNF have "the same diamter" but the TPI is different: 20 or 28).
- Not applicable. Result: G3/4 male thread.
What does the F in NPTF stand for: Female, fine or fuel?
NPTF stands for National Pipe Taper Fuel. This can be male and female. NPTF is designed to make a more leak-proof seal without the use of teflon tape or other means. A common misconception is that NPTF stands for NPT female or NPT fine. NPT fine does not exist.
Which female threads are conical?
The most common conical internal thread:
- NPT
- BSPT: Rc
BSPT thread is always conical if it is male thread (R), but female thread can both be conical (Rc) and parallel (Rp). BSPP thread is parallel, both male and female (G).
BSPT thread is always conical if it is male thread (R), but female thread can both be conical (Rc) and parallel (Rp). BSPP thread is parallel, both male and female (G).
What is the difference between female parallel Rp-thread and female G-thread?
G-threads have a parallel shape according to the DIN-EN-ISO 228-1 standard.
Rp-threads have a parallel shape according to the ISO 7-1 and DIN EN 10226-1 standard.
In the case of a threadsize 1/8", for example, the threads are designated G1/8 or R1/8.
Male G-thread (parallel) can only be screwed into female G-thread.
Male R-thread (conical) can be screwed into a G or R thread.
The following table shows the internal minimum thread diameter, with upper deviation.
G-Withworth pipe thread
| ∅ | TPI | ∅ | Drill ∅B | Nut thread minor ∅ Min | Nut thread minor ∅ max |
| G 1/16 | 28 | 7.723 | 6.80 | 6.516 | 6.843 |
| G 1/8 | 28 | 9.728 | 8.80 | 8.566 | 8.848 |
| G 1/4 | 19 | 13.157 | 11.80 | 11.445 | 11.890 |
| G 3/8 | 19 | 16.662 | 15.25 | 14.950 | 15.395 |
| G 1/2 | 14 | 20.955 | 19.00 | 18.631 | 19.172 |
| G 5/8 | 14 | 22.911 | 21.00 | 20.587 | 21.128 |
| G 3/4 | 14 | 26.441 | 24.50 | 24.117 | 24.658 |
| G 7/8 | 14 | 30.201 | 28.25 | 27.877 | 28.418 |
| G 1 | 11 | 33.249 | 30.75 | 30.291 | 30.931 |
| G 1.1/8 | 11 | 37.897 | 35.50 | 34.939 | 35.579 |
| G 1.1/4 | 11 | 41.910 | 39.50 | 38.952 | 39.592 |
| G 1.3/8 | 11 | 44.320 | 42.00 | 41.365 | 42.005 |
| G 1.1/2 | 11 | 47.803 | 45.25 | 44.854 | 45.485 |
| G 1.3/4 | 11 | 53.764 | 51.10 | 50.788 | 51.428 |
| G 2 | 11 | 59.614 | 57.00 | 56.656 | 57.296 |
| G 2.1/4 | 11 | 65.710 | 63.10 | 62.752 | 63.392 |
| G 2.1/2 | 11 | 75.184 | 73.00 | 72.226 | 72.866 |
| G 2.3/4 | 11 | 81.534 | 79.00 | 78.576 | 79.216 |
| G 3 | 11 | 87.884 | 85.50 | 84.926 | 85.566 |
| G 3.1/2 | 11 | 100.330 | 98.00 | 97.327 | 98.012 |
| G 4 | 11 | 113.030 | 110.50 | 110.072 | 110.712 |
RP - Withworth pipe thread
| ∅ | TPI | ∅ | Drill ∅B | Nut thread minor ∅ Min | Nut thread minor ∅ max |
| Rp 1/16 | 28 | 7.723 | 6.55 | 6.490 | 6.632 |
| Rp 1/8 | 28 | 9.728 | 8.60 | 8.495 | 8.637 |
| Rp 1/4 | 19 | 13.157 | 11.50 | 11.341 | 11.549 |
| Rp 3/8 | 19 | 16.662 | 15.00 | 14.846 | 15.054 |
| Rp 1/2 | 14 | 20.955 | 18.50 | 18.489 | 18.733 |
| - | - | - | - | - | - |
| Rp 3/4 | 14 | 26.441 | 24.00 | 23.975 | 24.259 |
| - | - | - | - | - | - |
| Rp 1 | 11 | 33.249 | 30.25 | 30.111 | 30.471 |
| - | - | - | - | - | - |
| Rp1.1/4 | 11 | 41.910 | 39.00 | 38.772 | 39.132 |
| - | - | - | - | - | - |
| Rp1.1/2 | 11 | 47.803 | 45 | 44.665 | 45.025 |
| - | - | - | - | - | - |
| Rp 2 | 11 | 59.614 | 56.50 | 56.476 | 56.836 |
| - | - | - | - | - | - |
| Rp 2.1/1 | 11 | 75.184 | 72.25 | 72.083 | 72.443 |
| - | - | - | - | - | - |
| R 3 | 11 | 87.884 | 85.00 | 84.783 | 85.143 |
| - | - | - | - | - | - |
| R 4 | 11 | 113.030 | 110.00 | 109.860 | 110.289 |
The tolerances on the pitch diameter of the internal G-threads correspond to the 'positive' deviation from the diamater tolerances according to ISO 7-1, with the exception of the thread sizes 1/16, 1/8, 1/4 and 3/8, where higher deviations are specified.
Internal g-thread
| Designation of thread | Number of threads in 25.4 mm | Pitch | Tolerance on pitch diameter Internal thread lower deviation | Tolerance on pitch diameter Internal thread Upper deviation |
| 1/16 | 28 | 0.907 | O | +0.107 |
| 1/8 | 28 | 0.907 | O | +0.107 |
| 1/4 | 19 | 1.337 | O | +0.125 |
| 3/8 | 19 | 1.337 | O | +0.125 |
| 1/2 | 14 | 1.814 | O | +0.142 |
| 5/8 | 14 | 1.814 | O | +0.142 |
| 3/4 | 14 | 1.814 | O | +0.142 |
| 7/8 | 14 | 1.814 | O | +0.142 |
| 1 | 11 | 2.309 | O | +0.180 |
| 1 1/8 | 11 | 2.309 | O | +0.180 |
| 1 1/4 | 11 | 2.309 | O | +0.180 |
| 1 1/2 | 11 | 2.309 | O | +0.180 |
| 1 3/4 | 11 | 2.309 | O | +0.180 |
| 2 | 11 | 2.309 | O | +0.180 |
Rp-thread
| Designation of thread | Number of threads in 25.4 mm |
| 1/16 | +/-0.071 |
| 1/8 | +/-0.071 |
| 1/4 | +/-0.104 |
| 3/8 | +/-0.104 |
| 1/2 | +/-0.142 |
| - | - |
| 3/4 | +/-0.142 |
| - | - |
| 1 | +/-0.180 |
| - | - |
| 1 1/4 | +/-0.180 |
| 1 1/2 | +/-0.180 |
| - | - |
| 2 | +/-0.180 |
The combination of an external parallel thread G, tolerance A or B according to DIN-EN-ISO 228-1 with an internal parallel thread Rp according to ISO 7-1, requires special attention.
If this combination is necessary, the positive or negative tolerance of the internal thread according to ISO 7-1 will have to be considered in the relevant product standard, where external parallel thread G is used.
Can I use an O-ring seal with NPT thread?
NPT thread sealing with an O-ring is impossible. NPT thread is conical and can only be partially screwed in. Therefore, the seal itself is placed directly on the thread, with PTFE tape or a liquid sealant.
Are NPT and BSP pipe threads interchangeable?
NPT threadsare common in the United States, whileBSP threads are frequently used in other countries.
NPT/NPS threads have an angle of 60° and have flattened tops and valleys; BSP threads have an angle of 55° and have rounded tops and valleys.
(Sometimes sizes 1/2" and 3/4" NPT and BSP are combined because they are similar in design and have the same pitch).
Never attempt to connect a BSP fitting to an NPT (or NPS) fitting when the perfomance concerning pressure is critical.
| Pipe size | NPT / NPS Pitch | BSP Pitch |
| 1/16 | 27 | - |
| 1/8 | 27 | 28 |
| 1/4 | 18 | 19 |
| 3/8 | 18 | 19 |
| 1/2 | 14 | 14 |
| 5/8 | - | 14 |
| 3/4 | 14 | 14 |
| 1 | 11 1/2 | 11 |
What is thread deprivation and how can it be prevented?
Deprivation arises when two metals rub against each other. Since the metals get hot from friction, the molecules of each bind together, eventually causing problems when the surfaces are "welded" together. The higher the speed and the higher the pressure, the greater the chance that deprivation will happen. This often occurs with fasteners made of stainless steel, aluminum and titanium when they are tightened.
Deprivation can be avoided by good lubrication with PTFE tape and with brass or plastic nuts instead of, for example, stainless steel nuts. Metal-to-metal contacts can be used when mounting slowly or when good lubrication is applied, otherwise stainless steel nuts on stainless steel screws should be avoided.
Which seal is recommended when a PA / PVDF fitting with male male conical thread is screwed into a metal part?
This combination of materials is not ideal and therefore not recommended. However, when this combination is prescribed, our advice is to use PTFE tape. There is a possibility that a leak-tight connection without PTFE tape can be achieved, but there is a greater chance of damaging the thread on the PA / PVDF couplings. Damaged thread increases the risk of leakage and will have to be replaced after disassembly.
What kind of thread does a JIC fitting have?
A JIC fitting -Joint Industry Council- is a "flared" type fitting manufactured with an end connection under a 37 degree angle and a UNF thread. The JIC fitting should be sealed with a metal-to-metal contact between the 37 degree 'nose' of the fitting and the surface below 37 degrees on the inside of the nut.
How can I seal a thread leak-tight?
It is necessary to seal parrallel / cylindrical and conical threads in different ways.
When the female and male threads are both parallel, they are often sealed with an O-ring or metal ring with gasket. O-rings are the most common seals beacuse they are cheap, reliable and easy to assemble.
When the male thread is conical, an O-ring seal can not be used, because the thread is only partially screwed in. At Teesing we use standard PTFE tape or Loctite. Without this additional seal, all conical threads leak! This seal also reduces the chance of deprivation of the threads.
| Fitting size (JIC) | UNF thread |
| D02 | 5/16x24 |
| D03 | 3/8x24 |
| D04 | 7/16x20 |
| D05 | 1/2x20 |
| D06 | 9/16x18 |
| D07 | 5/8x18 |
| D08 | 3/4x16 |
| D10 | 7/8x14 |
| D12 | 1 1/16x12 |
| D16 | 1 5/16x12 |
| D20 | 1 5/8x12 |
| D24 | 1 7/8x12 |
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