Overview of Common Thread Symbols and Standards

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.

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.

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.

5.Conversion of 55° Trapezoidal Threads
These threads have a 30° profile and are unified internationally. Their designation remains consistent across different countries.

III. Thread classification
Depending on the use of threads, they can be divided into:
1. International Metric Thread System
The thread adopted by our national standard CNS. The top of the thread is flat, easy to turn, and the bottom of the thread is arc-shaped to increase the strength of the thread. The thread angle is 60 degrees, and the specification is represented by M. Metric threads can be divided into coarse and fine pitch. The representation is such as M8x1.25. (M: code, 8: nominal diameter, 1.25: pitch).
2. American Standard Thread
The top and root of the thread are both flat, and the strength is better. The thread angle is also 60 degrees, and the specification is represented by the number of teeth per inch. This type of thread can be divided into three levels: coarse pitch (NC); fine pitch (NF); extra fine pitch (NEF). The representation is such as 1/2-10NC. (1/2: outer diameter; 10: number of teeth per inch; NC code).
3. Unified Thread

Developed jointly by the United States, the United Kingdom, and Canada, it is the most commonly used British thread.

The thread angle is also 60 degrees, and the specification is expressed in the number of teeth per inch. This type of thread can be divided into coarse thread (UNC), fine thread (UNF), and extra fine thread (UNEF). The representation is 1/2-10UNC. (1/2: outer diameter; 10: number of teeth per inch; UNC code).

4. Sharp V Thread

The top and the root are both pointed, with weak strength, and it is not often used. The thread angle is 60 degrees.

5. Whitworth Thread

The thread adopted by the British national standard. The thread angle is 55 degrees, and the symbol is "W". Suitable for rolling manufacturing. The representation is W1/2-10 (1/2: outer diameter; 10: number of teeth per inch; W code).
6. Knuckle Thread
It is a standard thread set by German DIN. It is suitable for connecting light bulbs and rubber hoses. The symbol is "Rd".
7. Pipe Thread
It is a thread used to prevent leakage and is often used to connect gas or liquid pipe fittings. The thread angle is 55 degrees. It can be divided into straight pipe threads with the code "P.S., N.P.S." and oblique pipe threads with the code "N.P.T.". Its taper is 1:16, that is, 3/4 inches per foot.
8. Square Thread
It has a high transmission efficiency, second only to ball threads, but it cannot be adjusted with nuts after wear, which is its disadvantage. It is generally used for screws in vises and threads of cranes.
9. Trapezoidal Thread
It is also called Acme thread. The transmission efficiency is slightly lower than that of square threads, but it can be adjusted with nuts after wear. The metric thread angle is 30 degrees and the imperial thread angle is 29 degrees. Generally used for the lead screw of lathes. The symbol is "Tr".

10. Buttress Thread
Also known as bevel thread, it is only suitable for single-direction transmission. Such as screw jacks, pressurizers, etc. The symbol is "Bu".
11. Ball thread
It is the thread with the best transmission efficiency. It is difficult to manufacture and has a very high cost. It is used in precision machinery. Such as the lead screw of CNC machine tools.
Indication of British bolts
LH 2N 5/8 × 3 - 13UNC-2A
(1) LH is left-hand thread (RH is right-hand thread, which can be omitted).
(2) 2N double-thread thread.
(3) 5/8 British thread, outer diameter 5/8".
(4) 3 bolt length 3".
(5) 13 thread has 13 teeth per inch.
(6) UNC unified standard thread coarse pitch.
(7) 2nd level fit, external thread (3: tight fit; 2: medium fit; 1: loose fit) A: external thread (can be omitted), B: internal thread.

Imperial thread
The size of an imperial thread is usually expressed as the number of threads per inch of thread length, referred to as "threads per inch", which is exactly the reciprocal of the pitch. For example, a thread with 8 threads per inch has a pitch of 1/8 inch.