PT/RC/ZG threads
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Hello, fellow sailors. I’ve read quite a few posts on Haichuan regarding the standards for American, British, and metric threads, and there are various opinions. I still have some misunderstandings regarding PT/RC/ZG threads, so I would like to ask professionals to provide more accurate answers regarding these threads: 1. As far as I know, both PT and RC threads belong to the 55-degree sealed tapered pipe thread category, and they are part of the Whitworth thread family. Are these two standards identical when they have the same specifications? 2. ZG is commonly known as a tube cone, while RC is a cone. I’ve read many posts saying that the ZG national standard is labeled as RC – are the two the same? 3. So what is the relationship among those three? Are PT/RC/ZG threads the same? The question is a bit complicated; please help those who are clear on the concepts, thanks!
This post was last edited by denghl on 2010-7-23 at 10:56. To be more specific: 1. Ordinary metric threads, 60 degrees; these can also be considered as threads according to Chinese standards. There are coarse teeth and fine teeth; to find out the specifics, it’s best to consult the standards. Its diameter and pitch series are specified in detail. Denoted by the symbol M. Here we only discuss the threads that are most closely related to the instruments. For example, a pressure gauge connector thread of M20*1.5 (M) indicates an outer diameter of 20 mm, a pitch of 1.5 mm, and it is a fine-thread external thread. Bolt M12 means an outer diameter of 12 mm, with coarse threads (the pitch for coarse threads is fixed and does not need to be specified). The symbol for external threads is M (Male), abbreviated as “external”; the symbol for internal threads is F (Female), abbreviated as “internal”. Note: The diameter referred to here is the major diameter – it is the outer diameter for external threads, but not necessarily the inner diameter for internal threads. 2. The 55-degree unsealed cylindrical pipe thread is referred to as BSPP thread in imperial units. It is a straight-thread type. Marking method: Size code G, tolerance grade. For external threads, the tolerance grades are A and B; internal threads do not specify a tolerance grade. For example, G1/2A-LH denotes a 1/2'' external cylindrical thread of grade A with a left-hand twist, while G1/2 refers to a 1/2'' internal cylindrical thread. These types of threads are very common, being used extensively in the electrical interfaces of transmitters. The designation G1/2 means exactly this. 3, 55-degree sealed tapered pipe threads (an old Chinese designation, ZG); in imperial units, they are called BSPT threads, which are a type of tapered thread. Thread code: Rc denotes tapered internal threads, while Rp denotes cylindrical internal threads (? , Why is it stuck? , because of R2! ) The R1 tapered external thread mates with Rc, while the R2 tapered external thread mates with Rp. This is a bit confusing. I don’t use it very often; I’ve seen valves from small Japanese manufacturers with this type of thread. Be careful if you encounter it; no need to say more. 4. 60-degree sealed pipe threads (an old Chinese designation, denoted as Z); these are American standard pipe threads, abbreviated as NPT. This type of thread is very common in instruments. Such as NPT1/2-LH (I* usually write it as 1/2NPT, right? ) The British 55-degree Whitworth thread BSW has been phased out, so it is not covered here. For threads expressed in imperial units (the 2, 3, and 4 types mentioned above), the pitch is indicated as the number of threads per inch. Even for a 1/2'' thread, there are differences in the outer diameter and pitch among G, R, and NPT threads. In actual installation, it’s sometimes acceptable to use these threads interchangeably, but it’s best to avoid doing so whenever possible. Especially in explosion-proof areas (where explosion protection requires specific gaps, and mismatched threads can lead to leaks that allow explosions to spread) and in places where high sealing performance is required (such as outdoors). OK, I’ve finished talking again.
This post was last edited by jujiangliu on 2010-7-22 at 13:58. GB/T 7306.1-2000 – 55° sealed pipe threads, Part 1: Circular internal threads and tapered external threads. GB/T 7306.2-2000 – 55° sealed pipe threads, Part 2: Tapered internal threads and tapered external threads. GB/T 193-2003 – Ordinary threads – Diameter and pitch series. GB/T 7307-2001 – 55° unsealed pipe threads. GB/T 12716-2002 – 60° sealed pipe threads. GB/T 1415-2008 – Metric sealed threads. JB/ZQ 4197-2006 – Unified inch threads (introduction to uN and uNR thread profiles). GB/T 20670-2006 – Unified threads – Diameter and number of threads series.
Basic pipe thread specifications: Basic dimensions of pipe threads sealed by threading (R). Size designation, number of threads per inch, pitch, reference dimensions on the reference plane, reference distance, effective thread length, major diameter, minor diameter.
1/16: 28 threads per inch, pitch 0.907; reference dimensions: 7.723, 6.561; major/minor diameters: 4.0, 6.5.
1/8: 28 threads per inch, pitch 0.907; reference dimensions: 9.728, 8.566; major/minor diameters: 4.0, 6.5.
1/4: 19 threads per inch, pitch 1.337; reference dimensions: 13.157, 11.445; major/minor diameters: 6.0, 9.7.
3/8: 19 threads per inch, pitch 1.337; reference dimensions: 16.662, 14.950; major/minor diameters: 6.4, 10.1.
1/2: 14 threads per inch, pitch 1.814; reference dimensions: 20.955, 18.631; major/minor diameters: 8.2, 13.2.
3/4: 14 threads per inch, pitch 1.814; reference dimensions: 26.441, 24.117; major/minor diameters: 9.5, 14.5.
1: 11 threads per inch, pitch 2.309; reference dimensions: 33.249, 30.291; major/minor diameters: 10.4, 16.8.
1¼: 11 threads per inch, pitch 2.309; reference dimensions: 41.910, 38.952; major/minor diameters: 12.7, 19.1.
1½: 11 threads per inch, pitch 2.309; reference dimensions: 47.803, 44.845; major/minor diameters: 12.7, 19.1.
2: 11 threads per inch, pitch 2.309; reference dimensions: 59.614, 56.656; major/minor diameters: 15.9, 23.4.
2½: 11 threads per inch, pitch 2.309; reference dimensions: 75.184, 72.226; major/minor diameters: 17.5, 26.7.
3: 11 threads per inch, pitch 2.309; reference dimensions: 87.884, 84.926; major/minor diameters: 20.6, 29.8.
3½: 11 threads per inch, pitch 2.309; reference dimensions: 100.330, 97.372; major/minor diameters: 22.2, 31.4.
4: 11 threads per inch, pitch 2.309; reference dimensions: 113.030, 110.072; major/minor diameters: 25.4, 35.8.
5: 11 threads per inch, pitch 2.309; reference dimensions: 138.430, 135.472; major/minor diameters: 28.6, 40.1.
6: 11 threads per inch, pitch 2.309; reference dimensions: 163.830, 160.872; major/minor diameters: 28.6, 40.1.
R – Pipe threads for sealing purposes: Rc – tapered internal threads; Rp – straight internal threads; R – tapered external threads.
G – Pipe threads not used for sealing purposes.
Basic dimensions of 60° tapered pipe threads (NPT): Size designation, number of threads per inch, pitch, reference dimensions on the reference plane, reference distance, assembly tolerance (number of threads), major diameter, minor diameter.
1/16: 27 threads per inch, pitch 0.941; reference dimensions: 7.895, 6.389; major/minor diameters: 4.064, 3.
1/8: 27 threads per inch, pitch 0.941; reference dimensions: 10.242, 8.736; major/minor diameters: 4.102, 3.
1/4: 18 threads per inch, pitch 1.411; reference dimensions: 13.616, 11.358; major/minor diameters: 5.786, 3.
3/8: 18 threads per inch, pitch 1.411; reference dimensions: 17.055, 14.797; major/minor diameters: 6.096, 3.
1/2: 14 threads per inch, pitch 1.814; reference dimensions: 21.223, 18.321; major/minor diameters: 8.128, 3.
3/4: 14 threads per inch, pitch 1.814; reference dimensions: 26.568, 23.666; major/minor diameters: 8.611, 3.
1: 11.5 threads per inch, pitch 2.209; reference dimensions: 33.228, 29.694; major/minor diameters: 10.160, 3.
1¼: 11.5 threads per inch, pitch 2.209; reference dimensions: 41.985, 38.451; major/minor diameters: 10.668, 3.
1½: 11.5 threads per inch, pitch 2.209; reference dimensions: 48.054, 44.520; major/minor diameters: 10.668, 3.
2: 11.5 threads per inch, pitch 2.209; reference dimensions: 60.092, 56.558; major/minor diameters: 11.074, 3.
2½: 8 threads per inch, pitch 3.175; reference dimensions: 72.699, 67.619; major/minor diameters: 17.323, 2.
3: 8 threads per inch, pitch 3.175; reference dimensions: 88.608, 83.528; major/minor diameters: 19.456, 2.
3½: 8 threads per inch, pitch 3.175; reference dimensions: 101.316, 96.236; major/minor diameters: 20.853, 2.
4: 8 threads per inch, pitch 3.175; reference dimensions: 113.973, 108.872; major/minor diameters: 21.438, 2.
5: 8 threads per inch, pitch 3.175; reference dimensions: 140.952, 135.872; major/minor diameters: 23.800, 2.
6: 8 threads per inch, pitch 3.175; reference dimensions: 167.792, 162.712; major/minor diameters: 24.333, 2.
Table of names and designations for pipe threads.
Meaning of new national standard designations: G – Straight pipe threads not sealed by threading; R – Pipe threads sealed by threading; RP – Straight internal threads; RC – Tapered internal threads; R – Tapered external threads; NPT – 60° tapered pipe threads.
Meaning of old national standard designations: G – Straight threads; ZG – Tapered threads; Z – Tapered threads.
Meaning of foreign standard designations: BSP – British pipe threads; BSPT – Tapered threads; BSPF/BSPP – Straight threads; NPT – American pipe threads; NPT – Tapered threads
1/16: 28 threads per inch, pitch 0.907; reference dimensions: 7.723, 6.561; major/minor diameters: 4.0, 6.5.
1/8: 28 threads per inch, pitch 0.907; reference dimensions: 9.728, 8.566; major/minor diameters: 4.0, 6.5.
1/4: 19 threads per inch, pitch 1.337; reference dimensions: 13.157, 11.445; major/minor diameters: 6.0, 9.7.
3/8: 19 threads per inch, pitch 1.337; reference dimensions: 16.662, 14.950; major/minor diameters: 6.4, 10.1.
1/2: 14 threads per inch, pitch 1.814; reference dimensions: 20.955, 18.631; major/minor diameters: 8.2, 13.2.
3/4: 14 threads per inch, pitch 1.814; reference dimensions: 26.441, 24.117; major/minor diameters: 9.5, 14.5.
1: 11 threads per inch, pitch 2.309; reference dimensions: 33.249, 30.291; major/minor diameters: 10.4, 16.8.
1¼: 11 threads per inch, pitch 2.309; reference dimensions: 41.910, 38.952; major/minor diameters: 12.7, 19.1.
1½: 11 threads per inch, pitch 2.309; reference dimensions: 47.803, 44.845; major/minor diameters: 12.7, 19.1.
2: 11 threads per inch, pitch 2.309; reference dimensions: 59.614, 56.656; major/minor diameters: 15.9, 23.4.
2½: 11 threads per inch, pitch 2.309; reference dimensions: 75.184, 72.226; major/minor diameters: 17.5, 26.7.
3: 11 threads per inch, pitch 2.309; reference dimensions: 87.884, 84.926; major/minor diameters: 20.6, 29.8.
3½: 11 threads per inch, pitch 2.309; reference dimensions: 100.330, 97.372; major/minor diameters: 22.2, 31.4.
4: 11 threads per inch, pitch 2.309; reference dimensions: 113.030, 110.072; major/minor diameters: 25.4, 35.8.
5: 11 threads per inch, pitch 2.309; reference dimensions: 138.430, 135.472; major/minor diameters: 28.6, 40.1.
6: 11 threads per inch, pitch 2.309; reference dimensions: 163.830, 160.872; major/minor diameters: 28.6, 40.1.
R – Pipe threads for sealing purposes: Rc – tapered internal threads; Rp – straight internal threads; R – tapered external threads.
G – Pipe threads not used for sealing purposes.
Basic dimensions of 60° tapered pipe threads (NPT): Size designation, number of threads per inch, pitch, reference dimensions on the reference plane, reference distance, assembly tolerance (number of threads), major diameter, minor diameter.
1/16: 27 threads per inch, pitch 0.941; reference dimensions: 7.895, 6.389; major/minor diameters: 4.064, 3.
1/8: 27 threads per inch, pitch 0.941; reference dimensions: 10.242, 8.736; major/minor diameters: 4.102, 3.
1/4: 18 threads per inch, pitch 1.411; reference dimensions: 13.616, 11.358; major/minor diameters: 5.786, 3.
3/8: 18 threads per inch, pitch 1.411; reference dimensions: 17.055, 14.797; major/minor diameters: 6.096, 3.
1/2: 14 threads per inch, pitch 1.814; reference dimensions: 21.223, 18.321; major/minor diameters: 8.128, 3.
3/4: 14 threads per inch, pitch 1.814; reference dimensions: 26.568, 23.666; major/minor diameters: 8.611, 3.
1: 11.5 threads per inch, pitch 2.209; reference dimensions: 33.228, 29.694; major/minor diameters: 10.160, 3.
1¼: 11.5 threads per inch, pitch 2.209; reference dimensions: 41.985, 38.451; major/minor diameters: 10.668, 3.
1½: 11.5 threads per inch, pitch 2.209; reference dimensions: 48.054, 44.520; major/minor diameters: 10.668, 3.
2: 11.5 threads per inch, pitch 2.209; reference dimensions: 60.092, 56.558; major/minor diameters: 11.074, 3.
2½: 8 threads per inch, pitch 3.175; reference dimensions: 72.699, 67.619; major/minor diameters: 17.323, 2.
3: 8 threads per inch, pitch 3.175; reference dimensions: 88.608, 83.528; major/minor diameters: 19.456, 2.
3½: 8 threads per inch, pitch 3.175; reference dimensions: 101.316, 96.236; major/minor diameters: 20.853, 2.
4: 8 threads per inch, pitch 3.175; reference dimensions: 113.973, 108.872; major/minor diameters: 21.438, 2.
5: 8 threads per inch, pitch 3.175; reference dimensions: 140.952, 135.872; major/minor diameters: 23.800, 2.
6: 8 threads per inch, pitch 3.175; reference dimensions: 167.792, 162.712; major/minor diameters: 24.333, 2.
Table of names and designations for pipe threads.
Meaning of new national standard designations: G – Straight pipe threads not sealed by threading; R – Pipe threads sealed by threading; RP – Straight internal threads; RC – Tapered internal threads; R – Tapered external threads; NPT – 60° tapered pipe threads.
Meaning of old national standard designations: G – Straight threads; ZG – Tapered threads; Z – Tapered threads.
Meaning of foreign standard designations: BSP – British pipe threads; BSPT – Tapered threads; BSPF/BSPP – Straight threads; NPT – American pipe threads; NPT – Tapered threads
Checking threads is a delicate task; any mistake can lead to delays in work. It would be great if standards could be unified worldwide.
Thank you to the moderator for sharing; I’m currently learning*, and I see these things every time I install it~~~
Thank you, OP! It’s very useful; I’m learning from it! :victory:
Thank you to the experts upstairs for sharing this information. In my work, I often see that the air supply connectors for field devices such as valve positioners and solenoid valves are of 1/2\" NPT standard, while the connectors for some field instruments use standards like RC1/4-8. It’s indeed useful to learn more about these things; thank you.
Hello, is there a way to change the interface of the M20 to 1/2 NPT? I’d appreciate a detailed explanation
The original poster explained in detail; it’s basic yet very useful knowledge. Thank you for sharing!
Thank you to the original poster for sharing; I’ve learned something!