I. Common Thread Symbols
①NPT (National Pipe Thread): A standard American tapered pipe thread with a 60° thread angle, commonly used for general-purpose applications.
②PT (British Standard Tapered Pipe Thread): A 55° thread angle taper thread widely used for sealing. British pipe threads are fine threads to maintain strength, as deeper coarse threads weaken the pipe's outer diameter.
③PF (Parallel Pipe Thread): A straight thread type used in pipe connections.
④G (Whitworth Pipe Thread): A 55° non-threaded sealing pipe thread within the Whitworth family. "G" represents cylindrical pipe threads, where the 55° and 60° variations are based on specific functional applications.
⑤ZG (Tapered Pipe Thread): A conical thread type commonly found in water pipe fittings, formerly denoted as Rc in older Chinese national standards.
⑥Metric vs. Inch Threads: The key difference is representation—metric threads use pitch (distance between threads), while American and British threads are defined by threads per inch (TPI). Metric threads have a 60° equilateral profile, British threads a 55° isosceles profile, and American threads a 60° profile. Metric threads are measured in millimeters, whereas British and American threads follow imperial units.
⑦Pipe Threads: Primarily used in pipeline connections, ensuring a tight fit between internal and external threads. Available in both straight and tapered forms, the nominal diameter refers to the pipeline's size, with the actual thread major diameter exceeding this nominal size.
⑧Nominal Diameters (e.g., 1/4, 1/2, 1/8 inch): These values indicate the thread size in the British standard system, measured in inches.
II. Thread Standards in Different Countries
Unified Inch Thread Standard Widely adopted in inch-based countries, this thread system includes three series: coarse thread (UNC), fine thread (UNF), and extra fine thread (UNFF), with an additional constant pitch series (UN).
Marking Method: Thread diameter – Threads per inch – Series code – Accuracy grade.
Examples:
Coarse Thread: 3/8–16UNC–2A
Fine Thread: 3/8–24UNF–2A
Extra Fine Thread: 3/8–32UNFF–2A
Constant Pitch: 3/8–20UN–2A
The first number (e.g., 3/8) represents the thread's major diameter in inches, which converts to metric by multiplying by 25.4 (e.g., 3/8 × 25.4 = 9.525mm).
The second number represents the number of threads per inch.
The series codes (UNC, UNF, UNFF, UN) define thread types.
The last two characters (e.g., 2A) indicate the thread accuracy grade.
Conversion of 55° Parallel Pipe Threads Originating from the inch-based series, these threads are widely used worldwide for transporting liquids, gases, and electrical wiring connections. However, different countries use different designations, which should be converted according to the international comparison tables.
Conversion of 55° Tapered Pipe Threads These threads have a 55° thread angle with a 1:16 taper ratio. They are widely used globally, but each country has different designations, which can be referenced in the corresponding conversion table.
Conversion of 60° Tapered Pipe Threads Featuring a 60° thread angle and a 1:16 taper ratio, these pipe threads are used in China's machine tool industry, as well as in the U.S. and former Soviet Union. China originally designated them as "K," later changed to "Z," and now conforms to the "NPT" standard. Reference tables provide cross-national designation equivalents.
Conversion of 55° Trapezoidal Threads These threads have a 30° profile and are unified internationally. Their designation remains consistent across different countries.
II. Thread Standards in Different Countries
1.Unified Inch Thread Standard
Widely adopted in inch-based countries, this thread system includes three series: coarse thread (UNC), fine thread (UNF), and extra fine thread (UNFF), with an additional constant pitch series (UN).
Marking Method: Thread diameter – Threads per inch – Series code – Accuracy grade.
Examples:
Coarse Thread: 3/8–16UNC–2A
Fine Thread: 3/8–24UNF–2A
Extra Fine Thread: 3/8–32UNFF–2A
Constant Pitch: 3/8–20UN–2A
The first number (e.g., 3/8) represents the thread's major diameter in inches, which converts to metric by multiplying by 25.4 (e.g., 3/8 × 25.4 = 9.525mm).
The second number represents the number of threads per inch.
The series codes (UNC, UNF, UNFF, UN) define thread types.
The last two characters (e.g., 2A) indicate the thread accuracy grade.
2.Conversion of 55° Parallel Pipe Threads
Originating from the inch-based series, these threads are widely used worldwide for transporting liquids, gases, and electrical wiring connections. However, different countries use different designations, which should be converted according to the international comparison tables.
|
Country |
Code |
|
China |
G |
|
Japan |
G、PF |
|
Britain |
BSP、BSPP |
|
France |
G |
|
Germany |
R（Internal thread）、K（External thread） |
|
Soviet Union |
G、TPУБ |
|
ISO |
Rp |
3.Conversion of 55° Tapered Pipe Threads
These threads have a 55° thread angle with a 1:16 taper ratio. They are widely used globally, but each country has different designations, which can be referenced in the corresponding conversion table.
|
Country |
Code |
|
China |
ZG、R（External thread） |
|
Britain |
BSPT、R（External thread）、Rc（Internal thread） |
|
France |
G（External thread）、R（External thread） |
|
Germany |
R（External thread） |
|
Japan |
PT、R |
|
ISO |
R（External thread）、Rc（Internal thread） |
4.Conversion of 60° Tapered Pipe Threads
Featuring a 60° thread angle and a 1:16 taper ratio, these pipe threads are used in China's machine tool industry, as well as in the U.S. and former Soviet Union. China originally designated them as "K," later changed to "Z," and now conforms to the "NPT" standard. Reference tables provide cross-national designation equivalents.
|
Country |
Code |
|
China |
Z（Old）NPT（New） |
|
USA |
NPT |
|
Soviet Union |
B |
5.Conversion of 55° Trapezoidal Threads
These threads have a 30° profile and are unified internationally. Their designation remains consistent across different countries.
