Summary of Standard Knowledge on Common Thread Types

I. Common Thread Types

NPT is the American National Pipe Thread, a general-purpose tapered thread with a 60° thread angle.

PT threads are British Standard Tapered Pipe threads with a 55° thread angle, commonly used for sealing applications. British standard pipe threads are fine threads because coarse threads have greater depth, which significantly reduces the strength of the outer diameter of the threaded pipe.

PF threads are parallel pipe threads.

G denotes a 55-degree non-sealing cylindrical pipe thread, belonging to the Whitworth thread family. The designation "G" stands for cylindrical thread and is a general term for pipe threads (from the Chinese word "Guan"). The distinction between 55° and 60° threads is functional.

ZG, commonly known as pipe taper thread, refers to threads machined on a conical surface. Most water pipe fittings use this type, and in old Chinese standards it was marked as Rc.

Metric threads are indicated by pitch, while inch-based threads are specified by the number of threads per inch—this is their main difference. Metric threads feature a 60-degree equilateral profile, whereas imperial threads have an isosceles 55-degree profile, and American threads also have a 60-degree profile. Metric threads use metric units, while inch-based threads use imperial units.

Pipe threads are primarily used for connecting pipes. Their internal and external threads fit tightly and come in two types: straight and tapered. The nominal diameter refers to the diameter of the connected pipe; clearly, the major diameter of the thread is larger than the nominal diameter.

1/4, 1/2, and 1/8 are nominal diameters of imperial threads, measured in inches.

II. Standards from Different Countries

1. Unified Inch Threads  
Widely used in countries adopting the inch system, these threads are divided into three series: UNC (Unified National Coarse), UNF (Unified National Fine), and UNFF (Unified National Extra Fine), plus a fixed-pitch series UN.

Marking method: Thread diameter – threads per inch – series code – tolerance class.

Example:  
UNC series: 3/8–16UNC–2A  
UNF series: 3/8–24UNF–2A  
UNFF series: 3/8–32UNFF–2A  
UN series: 3/8–20UN–2A  

The first number, 3/8, represents the major diameter of the thread in inches. To convert to millimeters, multiply by 25.4: 3/8 × 25.4 = 9.525 mm. The second and third digits (16, 24, 32, 20) indicate the number of threads per inch (number of threads within 25.4 mm). The letters following the numbers—UNC, UNF, UNFF, UN—are series codes, and the last two digits (e.g., 2A) represent the tolerance grade.

2. Conversion of 55° Cylindrical Pipe Threads  
The 55° cylindrical pipe thread originates from the inch system but is widely adopted in both metric and inch-based countries. It is used for connecting pipes carrying liquids, gases, or electrical wiring. However, designations vary by country. Refer to the table below to convert foreign designations to Chinese ones. Below is a list of international designations for 55° cylindrical pipe threads:

Country | Designation  
China | G  
Japan | G, PF  
UK | BSP, BSPP  
France | G  
Germany | R (internal), K (external)  
Soviet Union | G, TPУБ  
ISO | Rp

3. Conversion of 55° Tapered Pipe Threads  
The 55° tapered pipe thread has a 55° thread angle and a 1:16 taper. This thread series is widely used worldwide. Its designation varies by country, as shown in the table below. Convert foreign designations to Chinese equivalents accordingly.

Country | Designation  
China | ZG, R (external)  
UK | BSPT, R (external), Rc (internal)  
France | G (external), R (external)  
Germany | R (external)  
Japan | PT, RISOR (external), Rc (internal)

4. Conversion of 60° Tapered Pipe Threads  
The 60° tapered pipe thread features a 60° thread angle and a 1:16 taper. This thread series is used in China's machine tool industry, as well as in the United States and former Soviet Union. Its designation was previously defined as K in China, later changed to Z, and is now updated to NPT. The thread designation comparison table is shown below.

Country Code  
China: Z (old), NPT (new)  
USA: NPT  
Former Soviet Union: B  

5.55° Trapezoidal Thread Conversion  
Trapezoidal threads refer to metric trapezoidal threads with a 30° thread angle. This series of threads is relatively standardized internationally, and their designations are also largely consistent. See the table below for thread designations.

Country | Designation  
China | T (old), Tr (new)  
ISO | Tr  
Germany | Tr  
Former Soviet Union | Tr  

III. Thread Classification

Depending on application, threads can be classified into the following types:

1. International Metric Thread System  
The thread standard adopted by China's national standard CNS. The thread crest is flat, making it easy to machine, while the root is rounded to increase thread strength. The thread angle is 60 degrees, and specifications are indicated by "M." Metric threads are divided into coarse and fine pitches. For example, M8x1.25 (M: designation; 8: nominal diameter; 1.25: pitch).

2. American Standard Thread  
Both the crest and root are flat, providing higher strength. The thread angle is also 60 degrees, and specifications are expressed as number of threads per inch. This type of thread is categorized into three grades: coarse (NC), fine (NF), and extra-fine (NEF). Example: 1/2-10NC (1/2: outer diameter; 10: threads per inch; NC: designation).

3. Unified Thread  
Developed jointly by the United States, Britain, and Canada, this is currently the most commonly used imperial thread standard.  
The thread angle is 60 degrees, and specifications are given as threads per inch. It includes coarse (UNC), fine (UNF), and extra-fine (UNEF) types. Example: 1/2-10UNC (1/2: outer diameter; 10: threads per inch; UNC: designation).

4. Sharp V Thread  
Both the crest and root are pointed, resulting in lower strength and limited use. The thread angle is 60 degrees.

5. Whitworth Thread  
The British national standard thread. The thread angle is 55 degrees, designated by "W." Suitable for rolling processes. Example: W1/2-10 (1/2: outer diameter; 10: threads per inch; W: designation).

6. Knuckle Thread  
A standard thread defined by German DIN. Used primarily for connecting light bulbs and rubber hoses. Designated by "Rd."

7. Pipe Thread  
Designed to prevent leakage, commonly used for joining pipes carrying gases or liquids. The thread angle is 55 degrees. Divided into straight pipe threads (designated P.S., N.P.S.) and tapered pipe threads (designated N.P.T.), with a taper ratio of 1:16 (i.e., 3/4 inch per foot).

8. Square Thread  
High transmission efficiency, second only to ball screws. However, once worn, adjustment cannot be made using nuts—this is its main drawback. Typically used in vices and cranes.

9. Trapezoidal Thread  
Also known as Acme thread. Slightly lower transmission efficiency than square thread, but wear can be compensated by adjusting the nut. The metric thread angle is 30 degrees, while the imperial version is 29 degrees. Commonly used in lathe lead screws. Designated by "Tr."

10. Buttress Thread  
Also called a single-start square thread, suitable only for unidirectional power transmission. Examples include screw jacks and press machines. The symbol is "Bu". 11. Ball Screw

A screw thread with high transmission efficiency, but difficult to manufacture and costly, used in precision machinery such as lead screws in CNC machine tools. The designation for imperial bolts: LH 2N 5/8 × 3 - 13UNC-2A (1) LH indicates left-hand thread (RH for right-hand thread, which can be omitted). (2) 2N denotes double-start thread. (3) 5/8 refers to the imperial thread size with an outer diameter of 5/8 inch. (4) 3 indicates bolt length of 3 inches. (5) 13 means 13 threads per inch. (6) UNC stands for Unified National Coarse thread. (7) Grade 2 fit, external thread (Grade 3: tight fit; Grade 2: medium fit; Grade 1: loose fit); A: external thread (can be omitted), B: internal thread.

Imperial Thread  
The size of imperial threads is typically indicated by the number of threads per inch, commonly abbreviated 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.