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faq (basics of thread)

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Basics of thread

 

MORE INFORMATION?

Do you need more information about thread types or are you looking for a special adapter solution? Please contact our Sales Engineers.

Teesing Sales Engineers

TEESING SALES ENGINEERS
電話.   +886-(0)3-5600560
Mail  teesingtaiwan@teesing.com

FREQUENTLY ASKED QUESTIONS - GENERAL

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.

1. Is the thread male or female (visual)?
2. 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.
3. 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.
4. Determine the urgency.
    • Easy to measure with a thread gauge.
5. Determine the angle of the end connection
    • Most common angles: 30°, 37° en 45°.

Thread identification example:

According to the method mentioned above:
1. The fitting is visibly male.
2. The thread is visibly straight/parallel.
3. The measure diameter is 26.4 mm (this can easily be converted: inch decimal × 25.4 = mm).
4. 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).
5. 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

What is the difference between G-thread (BSPP) and R-thread (BSPT)?

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

RP - Withworth pipe thread

TPI Drill ∅B Nut
thread
minor
 ∅ Min
Nut
thread
minor
 ∅ max
TPI Drill ∅B Nut
thread
minor 
∅ Min 
Nut
thread
minor 
∅ max 
G 1/16 28 7.723 6.80 6.516 6.843 Rp 1/16 28 7.723 6.55 6.490 6.632
G 1/8 28 9.728 8.80 8.566 8.848 Rp 1/8 28 9.728 8.60 8.495 8.637
G 1/4 19 13.157 11.80 11.445 11.890 Rp 1/4 19 13.157 11.50 11.341 11.549
G 3/8 19 16.662 15.25 14.950 15.395 Rp 3/8 19 16.662 15.00 14.846 15.054
G 1/2 14 20.955 19.00 18.631 19.172 Rp 1/2 14 20.955 18.50 18.489 18.733
G 5/8 14 22.911 21.00 20.587 21.128 - - - - - -
G 3/4 14 26.441 24.50 24.117 24.658 Rp 3/4 14 29.441 24.00 23.975 24.259
G 7/8 14 30.201 28.25 27.877 28.418 - - - - - -
G 1 11 33.249 30.75 30.291 30.931 Rp 1 11 33.249 30.25 30.111 30.471
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 Rp1.1/4 11 41.910 39.00 38.772 39.132
G1.3/8 11 44.320 42.00 41.365 42.005 - - - - - -
G1.1/2 11 47.803 45.25 44.854 45.485 Rp1.1/2 11 47.803 45.00 44.665 45.025
G1.3/4 11 53.764 51.10 50.788 51.428 - - - - - -
G 2 11 59.614 57.00 56.656 57.296 Rp 2  11 59.614 56.50 56.476 56.836
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 Rp 2.1/1 11 75.184 72.25 72.083 72.443
G 2.3/4 11 81.534 79.00 78.576 79.216 - - - - - -
G 3 11 87.884 85.50 84.926 85.566 Rp 3 11 87.884 85.00 84.783 85.143
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 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

                                Rp-thread

Designation
of thread
Number of threads in
25.4 mm
Pitch Tolerance on pitch diameter

Thread size Equivalent diametral tolerance on parallel internal threads
(mm)
Internal thread lower deviation Internal thread Upper deviation
1/16 28 0.907 0 +0.107 1/16 +/-0.071
1/8 28 0.907 0 +0.107 1/8 +/-0.071
1/4 19 1.337 0 +0.125 1/4 +-/0.104
3/8 19 1.337 0 +0.125 3/8 +/-0.104
1/2 14 1.814 0 +0.142 1/2 +/-0.142
5/8 14 1.814 0 +0.142 - -
3/4 14 1.814 0 +142 3/4 +/-0.142
7/8 14 1.814 0 +0.142 - -
1 11 2.309 0 +0.180 1 +/-0.180
1 1/8 11 2.309 0 +0.180 - -
1 1/4 11 2.309 0 +0.180 1 1/4  +/-0.180
1 1/2 11 2.309 0 +0.180 1 1/2 +/-0.180
1 34/ 11 2.309  0 +0.180 - -
2 11 2.309 0 +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.

Never attempt to cennect a BSP fitting to an NPT (or NPS) fitting when the perfomance concerning pressure is critical.


 

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

Pitch (threads/inch)
NPT/NPS BSP     
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.

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



Jic fitting

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.

Thread seal types