What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

Yesterday, I did a simple push z-axis, which is used to push a suction device.
This device, after pumping out the waste gas produced by plasma cleaning, rises to the highest point to avoid the higher material on the conveyor belt below it.
The driving stroke is 150 mm, because there is no precision requirement, ± 1 mm is OK.
Therefore, the shaft is driven by double acting cylinder and guided by ordinary single slide linear guide (Use fork sensor to detect whether it has been pushed to the highest position).
Since the cylinder is used, naturally, a simple gas path is involved.
Because we want to keep the suction device at that position for a certain period of time in case of power failure at the highest point, we need a three position five way middle seal solenoid valve ( Plus double side speed control valve (speed control).
Search the system and soon the solenoid valve will be selected.
At first glance, the gas connection is RC1/8. I can’t help thinking of the types and parameters of various gas connections.

For example, in the supplier’s product catalog, there are often NPT1/8, RC3/4, G1/4 and other interfaces.

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What do these pipe thread interfaces NPT, RC, G, etc. mean? What’s the difference between NPT, BSPP and BSPT?

Speaking of these problems, we have to talk about Wyeth thread.
As we know, the profile angle of ordinary connecting thread is 60 degrees.
This Wyeth thread is different from ordinary thread in that it has a 55 degree profile angle.
Why is it 55 degrees? Originally, there is an interesting thing.
Joseph Whitworth, the inventor of Wyeth thread, established his own machine tool factories along with the wave of British Industrial Revolution.
These different factories, because there is no unified thread standard, manufacture thread according to their own experience, resulting in different sizes, which leads to the interchangeability of thread parts become a problem.
For example, the screw hole at the same position of the same part may not be locked after changing a screw.
As the scale of production continues to expand, Whitworth is aware of this serious problem.
Subsequently, he collected thread samples from different factories, measured size and shape and other data. Based on this, he proposed a unified standard for thread processing, which is the origin of Wyeth thread BSW (British Standard Whitworth) and the first thread standard in the world.
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In this standard, the thread profile angle is 55 degrees (BSF = British Standard Whitworth Thread fine).
Although Wyeth thread is not common in nuts and screws nowadays, it has been replaced by unified series and metric series.
However, BSPP (British standard pipe parallel) and BSPT (British standard pipe tape) of 1:16 are developed on this basis, and become international standards (ISO), which have been used up to now.

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BSPP (British Standard Parallel Pipe)

Online some literal translation into parallel pipe thread, bloggers feel a bit inappropriate. Another pipe thread corresponding to it is called “conical pipe thread”, which is not only a conventional name, but also a standard name in textbooks. Translating British standard pipe parallel into “parallel pipe thread” may make people feel a little confused and unprofessional.
For historical reasons, BSPP also has other different names. Such as BSPF (British standard pipe fitting), BSPM (British standard pipe mechanical) and PS (British standard pipe straight), etc.
However, with the development of ISO standardization, all of them were replaced by a simple and clear letter.
This is the G-thread.
Therefore:

  • G = BSPP = BSPF = BSPM = PS = ……

The letter G is the abbreviation of gas (it comes from German Gewinde, meaning “screw thread”, bloggers prefer the former), because it was used in gas pipeline at first. It is a coincidence that we call G pipe thread because the initials of G and “Guan” are the same.
G thread is divided into internal and external threads, which are used for non sealing connection. The executive standard is BS EN ISO 228-1, and the old standard is BS 2779, which is now invalid.
In addition, the meanings of several common versions of relevant standards are as follows:

  • ISO XXX = International Standard = International Standard;
  • EN-ISO-xxx = European version of the international standard;
  • BS-EN-ISO-xxx = British version of the European version of the international standard.
Common BSPP/BSPF thread size table
Sign

Size

Type Major diameter
mm
d=D
Pitch
mm
p
Number of teeth
TPI

Pitch diameter
mm
d2=D2
External thread
Narrow path

d3

Tooth height
mm
H1
Bottom hole
mm
G1/8 1/8″ BSPP 9.728 0.907 28 9.147 8.566 0.581 8.70
G1/4 1/4″ BSPP 13.157 1.337 19 12.301 11.445 0.856 11.60
G3/8 3/8″ BSPP 16.662 1.337 19 15.806 14.950 0.856 15.00
G1/2 1/2″ BSPP 20.955 1.814 14 19.793 18.631 1.162 19.00
G5/8 5/8″ BSPP 22.911 1.814 14 21.749 20.587 1.162 20.75
G3/4 3/4″ BSPP 26.441 1.814 14 25.279 24.117 1.162 24.50
G7/8 7/8″ BSPP 30.201 1.814 14 29.039 27.877 1.162 28.00
G1 1″ BSPP 33.249 2.309 11 31.770 30.291 1.479 30.50
G1-1/8 1-1/8″ BSPP 37.897 2.309 11 36.418 34.939 1.479 35.00
G1-1/4 1-1/4″ BSPP 41.910 2.309 11 40.431 38.952 1.479 39.50
G1-3/8 1-3/8″ BSPP 44.323 2.309 11 42.844 41.365 1.479 41.50
G1-1/2 1-1/2″ BSPP 47.803 2.309 11 46.324 44.845 1.479 45.00
G1-3/4 1-3/4″ BSPP 53.746 2.309 11 52.267 50.788 1.479 51.00
G2 2″ BSPP 59.614 2.309 11 58.135 56.656 1.479 57.00
G2-1/4 2-1/4″ BSPP 65.71 2.309 11 64.231 62.752 1.479 63.00
G2-1/2 2-1/2″ BSPP 75.184 2.309 11 73.705 72.226 1.479 72.50
G2-3/4 2-3/4″ BSPP 81.534 2.309 11 80.055 78.576 1.479 79.00
3 3″ BSPP 87.884 2.309 11 86.405 84.926 1.479 85.50
G3-1/4 3-1/4″ BSPP 93.98 2.309 11 92.501 91.022 1.479 91.00
G3-1/2 3-1/2″ BSPP 100.33 2.309 11 98.351 97.372 1.479 97.75
G3-3/4 3-3/4″ BSPP 106.68 2.309 11 105.201 103.722 1.479 104.00
G4 4″ BSPP 113.03 2.309 11 111.55 110.072 1.479 110.50
G4-1/2 4-1/2″ BSPP 125.73 2.309 11 124.251 122.772 1.479 123.00
G5 5″ BSPP 138.43 2.309 11 136.951 135.472 1.479 136.00
G5-1/2 5-1/2″ BSPP 151.13 2.309 11 149.651 148.172 1.479 148.50
G6 6″ BSPP 163.83 2.309 11 162.351 160.872 1.479 161.50

What’s more interesting is that in Wyeth thread, it is also stipulated that the top and bottom of the tooth should be made into arc.

Why? This is actually related to Whitt’s keen market insight and technology accumulation.
Because in the beginning, the purpose of building huite factory was to sell machine tools, that is, “hens”, rather than selling the parts processed by machine tools. It may be that he is optimistic about the prosperous industrial revolution driven market.
Therefore, when he submitted the thread standard to the British Industrial Standards Association, he implanted the arc. It can be said that he was very wise and made his own thread become the industry standard.
Of course, at that time, only his family could make this kind of circular tooth top and bottom, which was a good technical barrier. This is inseparable from his industry accumulation of more than 10 years.
Even now, if we want to set up standards in an industry, it is not easy to have strong core competitiveness (not necessarily in terms of technology, but in terms of positioning, brand, etc.), because it is easy to be imitated, copied, and lost in the competition.
From the point of view of use, the arc crown and the tooth base have its advantages. It can reduce the wear of the thread appliance on the crown, and its fatigue strength is better than that of the flat bottom thread. Because of the excessive arc, the stress distribution is optimized.
Later, in order to reduce the difficulty of thread processing, the U.S. changed the profile angle to 60 degrees, and the top and bottom of the tooth to be flat according to this standard. Only then did the U.S. standard pipe threads NPT and npsc: National pipe tape thread and national pipe straight coupling thread come into being.
So now, as far as pipe thread is concerned, two pipe thread standard systems with ISO standard as one side and American Standard as the other side have been formed all over the world.

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20210723004245 25600 - What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

BSPT (British standard pipe tape)

BSPT – inch tapered pipe thread, also known as Pt thread, is only used for general sealing connection. The old standard of BSPT is BS 21, but in the New Standard BS EN 10226-1, R thread is used instead.
Therefore:

  • R = BSPT = PT = ……

Having said the two systems of pipe thread, it is easy to understand the classification and difference of various pipe threads, because R series and G thread belong to English pipe thread.

  • RP: Inch sealing straight pipe thread;
  • RC: Imperial sealed internal cone pipe thread;
  • R: British sealing external cone pipe thread, matching with RC, RP, G;
  • R1: British sealed conical external thread, matching with British sealed straight internal pipe thread, RP/R1;
  • R2: English sealing cone external thread, matching with English sealing cone internal thread RC/R2;
  • G: Cylindrical / straight pipe thread.

The design profile of sealing cylindrical pipe thread (RP) is completely consistent with that of non sealing pipe thread g, and the difference lies in the thread precision. The taper pipe thread (R) for sealing is converted into technology based on the taper of 1:16 on the basis of cylindrical pipe thread. The design teeth are as follows:

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Common BSPT/R thread size table
Size Number of teeth
TPI
pitch of screw
mm
P
Tooth height
mm
h
Datum diameter
mm
D=d
Pitch diameter
mm
D2=d2
Narrow path
mm
D1=d1
Reference distance
mm
Limit deviation
T1/2
mm
Effective thread length
mm
Assembly allowance
mm
Limit deviation of internal thread diameterT1/2
mm
1/16 28 0,907 0,581 7,723 7,142 6,561 4 + /- 0,9 6,5 2,5 + /-0,071
1/8 28 0,907 0,581 9,728 9,147 8,566 4 + /- 0,9 6,5 2,5 + /-0,071
1/4 19 1,337 0,856 13,157 12,301 11,445 6 + /- 1,3 9,7 3,7 + /-0,104
3/8 19 1,337 0,856 16,662 15,806 14,95 6,4 + /- 1,3 10,1 3,7 + /-0,104
1/2 14 1,814 1,162 20,955 19,793 18,631 8,2 + /- 1,8 13,2 5,0 + /-0,104
3/4 14 1,814 1,162 26,411 25,279 24,117 9,5 + /- 1,8 14,5 5,0 + /-0,104
1 11 2,309 1,479 33,249 31,77 30,291 10,4 + /- 2,3 16,8 6,4 + /-0,180
1 1/4 11 2,309 1,479 41,91 40,431 38,952 12,7 + /- 2,3 19,1 6,4 + /-0,180
1 1/2 11 2,309 1,479 47,803 46,324 44,845 12,7 + /- 2,3 19,1 6,4 + /-0,180
2 11 2,309 1,479 59,614 58,135 56,656 15,9 + /- 2,3 23,4 7,5 + /-0,180
2 1/2 11 2,309 1,479 75,184 73,705 72,226 17,5 + /- 3,5 26,7 9,2 + /-0,216
3 11 2,309 1,479 87,884 86,405 84,926 20,6 + /- 3,5 29,8 9,2 + /-0,216
4 11 2,309 1,479 113,03 111,551 110,072 25,4 + /- 3,5 35,8 10,4 + /-0,216
5 11 2,309 1,479 138,43 136,951 135,472 28,6 + /- 3,5 40,1 11,5 + /-0,216
6 11 2,309 1,479 163,83 162,351 160,872 28,6 + /- 3,5 40,1 11,5 + /-0,216

It is worth mentioning that many friends think that BSP thread refers to G thread and can be used in common. We have different views on this view.
The sealing effect of general sealing cylindrical pipe thread (RP) is realized by adding sealing filler (e.g. PTFE sealing belt) to the whole thread when necessary, while the sealing effect of non sealing cylindrical pipe thread (g) is realized by using sealing equipment (e.g. O-ring) at both ends of the thread connection or around it.
In addition, there are also differences in the usage and design tolerance between general sealing pipe thread and non sealing pipe thread. For details, please refer to the standard manual of metric, American and English screw threads.
In fact, when machining BSP threads, many machining enterprises usually encounter that the thread ring plug gauge is qualified, but the combination is poor sealing. The main reason is that the RP internal thread for sealing is confused with the G internal thread for non sealing.
Some thread codes and standards related to BSP thread:

  • G – inch cylindrical pipe thread (standard: BS 2779, BS EN ISO 228-1, ISO 7-1);
  • BSPP – Inch cylindrical pipe thread (standard: BS 2779, BS EN ISO 228-1, ISO 7-1);
  • BSPT – Inch tapered pipe thread (standard: BS 21, BS EN 10226-1, ISO 7-1);
  • R (RP, RS, RC, R1, R2) – Inch tapered pipe thread (standard: BS 21, BS EN 10226-1, ISO 7-1, DIN 2999);
  • BSP thread classification diagram.

Use a diagram to show the relationship of inch pipe thread, as follows (from tap notes).

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Above we understand the basic classification of pipe thread.

How to choose the thread?

Here also need to understand the matching way and sealing way of pipe thread.
The first is the matching method: the taper external pipe thread can be matched with the taper internal pipe thread and the straight internal pipe thread.
As shown in the figure above, it is written as RC/R2 and RP/R1. The left represents internal thread and the right represents external thread.
However, straight external pipe thread cannot be matched with tapered internal pipe thread.
In addition, because NPT thread and BSPT thread have different thread profile angle, tooth top, tooth bottom, and thread pitch, they are generally incompatible (except for some 1/2 and 3/4 threads, because their thread pitch is the same). For example, NPT male cannot be used to fit BSP female.
20210723005753 19890 - What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

Intuitively, use the following figure to show the matching method.

20210723005920 98260 - What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

For sealing: cylindrical pipe threads are generally sealed with O-ring or gasket with sealing ring, or with raw material belt, sealant, etc.
The sealing of taper pipe thread depends on the deformation of taper thread, forming extrusion force to seal. But there is always the possibility of air leakage. Therefore, for places with strict requirements, it will be sealed with raw tape or sealant.
Intuitively, the sealing method can be shown in the following figure:

20210723010104 15941 - What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

Therefore, when we choose the connector, we must first see clearly the thread system, BSP with BSP, NPT with NPT, m with m, UNF with UNF, etc.
Then it should be noted that straight pipe external thread can not be matched with tapered pipe internal thread, for example, G1/4 matched with RC1/4 is wrong, and only tapered external pipe thread can be matched with straight pipe internal thread, for example, RP/R1 or RC/R2.
Finally, for sealing, straight pipe thread must be sealed, while taper pipe thread can be sealed by itself. For applications with high sealing requirements, sealant or raw tape should be added. However, suppliers usually bring their own sealing methods when they leave the factory.

What is the global application of the two pipe thread systems?

BSPP and BSPT are Wyeth pipe threads, which are mostly used in Europe, Commonwealth countries, Australia, New Zealand, etc.
In Asia, such as China, South Korea, Japan and so on, BSPT thread is used (Japanese industry standard is called PT). Many solenoid valve products we use, and the gas interface belongs to this system.
The United States, Canada and other regions are using NPT thread.
Here’s a few more words about NPT threads.
NPT thread does not need or cannot be sealed with O-ring, because it is a conical thread, which can only be screwed in, and it can be sealed usually by thread deformation. If you want to ensure the sealing, you can use PTFE tape or loctite to seal.
It should be pointed out that NPTF = National pipe tape fuel refers to American dry sealing tapered pipe thread, including internal thread and external thread.
It differs slightly from NPT only in the apex and the base of the tooth, and the rest is the same. It is easy to mistake NPTF for national pipe thread female.
Finally, a table is used to represent the commonly used British and American threads, and another table is used to compare the commonly used threads, as follows.

20210723010801 11207 - What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?20210723010811 58830 - What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

Source: China Thread Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

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