1. NPT is a general-purpose American standard tapered pipe thread with a thread angle of 60°.
PT thread is a British standard tapered thread with a thread angle of 55°, commonly used for sealing. British pipe threads are fine threads; coarse threads have a larger thread depth, which severely reduces the strength of the external diameter of the threaded pipe.
PF thread is a parallel pipe thread.
G is a 55-degree non-threaded sealing pipe thread, belonging to the Whitworth thread family. The designation G represents cylindrical threads; G is a general term for pipe threads (Guan), with the division between 55 and 60 degrees being functional.
ZG is commonly known as pipe taper, meaning the thread is machined from a conical surface. Most water pipe fittings are like this, and the old national standard is marked as Rc.
Metric threads are indicated by pitch, while American and British threads are indicated by the number of threads per inch. This is their biggest difference. Metric threads have a 60-degree equilateral thread profile, British threads have an isosceles 55-degree profile, and American threads have a 60-degree profile. Metric threads use metric units, while American and British threads use imperial units.
Pipe threads are mainly used for connecting pipes, with tight fits between internal and external threads, available in straight and tapered forms. The nominal diameter refers to the diameter of the connected pipe; it is clear that the major diameter of the thread is larger than the nominal diameter.
1/4, 1/2, 1/8 are the nominal diameters of British threads, measured in inches.
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1. Unified Thread Standard
Widely adopted in inch-based countries, this type of thread is divided into three series: coarse thread series UNC, fine thread series UNF, and extra fine thread series UNFF, plus a fixed pitch series UN.
Notation Method: Thread Diameter—Threads Per Inch Series Code—Precision Level
Example: Coarse thread series 3/8—16UNC—2A
Fine thread series 3/8—24UNF—2A
Extra fine thread series 3/8—32UNFF—2A
Fixed pitch series 3/8—20UN—2A
The first number 3/8 indicates the external diameter of the thread, measured in inches. To convert to metric units in mm, multiply by 25.4, i.e., 3/8×25.4=9.525mm; the second and third numbers 16, 24, 32, 20 indicate the number of threads per inch (the number of threads in 25.4mm length); the subsequent letters UNC, UNF, UNFF, UN indicate the series code, and the last two digits 2A indicate the precision level.
2. Conversion of 55° Cylindrical Pipe Threads
55° cylindrical pipe threads come from the inch series but are widely used in both metric and inch-based countries for connecting pipes and fittings for transporting liquids, gases, and electrical installations. However, the codes differ by country; they should be converted according to the table (reference table) below, and the codes for 55° cylindrical pipe threads in various countries are listed in the table below.
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Germany R (internal thread), K (external thread)
Former Soviet Union G, TPУБ
3. Conversion of 55° Tapered Pipe Threads
55° tapered pipe threads refer to threads with a thread angle of 55° and a taper of 1:16. This series of threads is widely used worldwide, and its codes differ by country, as shown in the table below. The foreign codes in the table should be converted to our national codes.
China ZG, R (external thread)
UK BSPT, R (external thread), Rc (internal thread)
France G (external thread), R (external thread)
Germany R (external thread)
ISO R (external thread), Rc (internal thread)
4. Conversion of 60° Tapered Pipe Threads
60° tapered pipe threads refer to threads with a thread angle of 60° and a taper of 1:16. This series of threads is used in our machine tool industry and in the USA and the former Soviet Union. Its code was previously designated as K in our country, later changed to Z, and now it is NPT. The thread code comparison table is shown below.
5. Conversion of 55° Trapezoidal Threads
Trapezoidal threads refer to metric trapezoidal threads with a thread angle of 30°. This series of threads is relatively unified both domestically and internationally, and their codes are also quite consistent, as follows:
Threads can be classified based on their use:
1. International Metric Standard Thread (International Metric Thread System)
This is the thread adopted by our national standard CNS. The thread crest is flat, making it easy to turn, while the thread root is rounded to increase thread strength. The thread angle is 60 degrees, and the specification is represented by M. Metric threads can be divided into coarse and fine threads. The notation is like M8x1.25. (M: code, 8: nominal diameter, 1.25: pitch).
2. American Standard Thread (American Standard Thread)
The thread top and root are both flat, providing better strength. The thread angle is also 60 degrees, and the specification is indicated by the number of threads per inch. This type of thread can be divided into coarse (NC), fine (NF), and extra fine (NEF) levels. The notation is like 1/2-10NC. (1/2: external diameter; 10: number of threads per inch; NC code).
3. Unified Standard Thread (Unified Thread)
Jointly formulated by the USA, UK, and Canada, this is the currently commonly used imperial thread.
The thread angle is also 60 degrees, and the specification is indicated by the number of threads per inch. This type of thread can be divided into coarse (UNC), fine (UNF), and extra fine (UNEF). The notation is like 1/2-10UNC. (1/2: external diameter; 10: number of threads per inch; UNC code).
4. V-Shaped Thread (Sharp V Thread)
The tops and roots are pointed, providing weaker strength and not commonly used. The thread angle is 60 degrees.
5. Whitworth Thread (Whitworth Thread)
This is the thread adopted by the British national standard. The thread angle is 55 degrees, and the symbol is “W”. Suitable for rolling manufacturing. The notation is like W1/2-10. (1/2: external diameter; 10: number of threads per inch; W code).
6. Knuckle Thread (Knuckle Thread)
This is the standard thread set by the German DIN. It is suitable for connecting bulbs and rubber tubes. The symbol is “Rd”.
7. Pipe Thread (Pipe Thread)
This is the thread used to prevent leakage, commonly used for connecting gas or liquid pipe fittings. The thread angle is 55 degrees and can be divided into straight pipe threads marked as “P.S., N.P.S.” and tapered pipe threads marked as “N.P.T.”, with a taper of 1:16, i.e., 3/4 inch per foot.
8. Square Thread (Square Thread)
It has high transmission efficiency, second only to ball screws, but cannot be adjusted with nuts after wear, which is a drawback. It is generally used for the screw of a vise and the thread of a crane.
9. Trapezoidal Thread (Trapezoidal Thread)
Also known as Acme thread. Its transmission efficiency is slightly lower than 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. It is generally used for lead screws in lathes. The symbol is “Tr”.
10. Buttress Thread (Buttress Thread)
Also known as slant square thread, it is only suitable for unidirectional transmission, such as screw jacks and presses. The symbol is “Bu”.
This is the thread with the best transmission efficiency, difficult to manufacture, and very costly, used in precision machinery, such as lead screws of CNC machine tools.
Notation for British bolts:
(1) LH indicates left-hand thread (RH indicates right-hand thread, which can be omitted).
(2) 2N indicates double-line thread.
(3) 5/8 indicates imperial thread with an external diameter of 5/8”.
(4) 3 indicates a bolt length of 3”.
(5) 13 indicates 13 threads per inch.
(6) UNC indicates unified standard coarse thread.
(7) 2 indicates the level of fit, external thread (3: tight fit; 2: medium fit; 1: loose fit) A: external thread (can be omitted) B: internal thread.
The size of imperial threads is usually indicated by the number of threads per inch, referred to as “threads per inch,” which is equal to the reciprocal of the pitch. For example, a thread with 8 threads per inch has a pitch of 1/8 inch.
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