Difference between pipe thread G3/4" and NPT 3/4"

Marking it this way is fine: G3/4" and NPT 3/4"

G3/4" refers to tapered pipe thread without sealing, while NPT 3/4" refers to tapered pipe thread with sealing capability

Common pipe threads mainly include the following types: NPT, PT, G, etc. 　　1) NPT is the abbreviation for National (American) Pipe Thread; it refers to the 60-degree tapered pipe thread standard used in North America. For the national standard, refer to GB/T12716-1991. 　　2) PT (BSPT) is an abbreviation for Pipe Thread; it refers to a 55-degree sealed tapered pipe thread that belongs to the Whitworth thread family. It is widely used in Europe and the Commonwealth**, especially in the water and gas piping industries. The taper ratio is 1:16, and the relevant national standard can be found in GB/T7306-2000. It is called ZG in China. 　　3) G is a 55-degree unthreaded sealing pipe thread, belonging to the Whitworth thread family. Marked as G denotes cylindrical threads. Refer to GB/T7307-2001 for national standards.

　　Pipe threads: Mainly used for connecting pipes, ensuring a tight fit between their internal and external threads; there are straight-thread and tapered-thread types. 　　Common pipe threads mainly include the following types: NPT, PT, G, etc. 　　1) NPT is the abbreviation for National (American) Pipe Thread; it refers to the 60-degree tapered pipe thread standard used in North America. For the national standard, refer to GB/T12716-1991. 　　2) PT (BSPT) is an abbreviation for Pipe Thread; it refers to a 55-degree sealed tapered pipe thread that belongs to the Whitworth thread family. It is widely used in Europe and the Commonwealth**, especially in the water and gas piping industries. The taper ratio is 1:16, and the relevant national standard can be found in GB/T7306-2000. It is called ZG in China. 　　3) G is a 55-degree unthreaded sealing pipe thread, belonging to the Whitworth thread family. Marked as G denotes cylindrical threads. Refer to GB/T7307-2001 for national standards. 　　Difference between metric threads and imperial threads: Metric threads are expressed in terms of pitch, while imperial threads are expressed in terms of the number of threads per inch ； 　　Metric threads have an equilateral profile at 60 degrees, British threads have an isosceles profile at 55 degrees, while American threads have an isosceles profile at 60 degrees ； 　　Metric threads use metric units (such as mm), while imperial and British threads use imperial units (such as inches) ； 　　“Those in the industry usually refer to thread sizes in \"points\"; 1 inch equals 8 points, 1/4 inch equals 2 points, and so on. 　　There is also: ISO—metric thread standard 60 degrees ； UN—Unified thread standard 60 degrees ； API—American Petroleum Pipe Thread Standard 60 degrees ； W—British Whittall thread standard 55 degrees ； 　　NPT BSP thread technology – Differences between NPT, PT, and G threads. The differences among NPT, PT, and G threads: All of these are pipe threads. NPT is the abbreviation for National (American) Pipe Thread; it belongs to the American standard of 60-degree tapered pipe threads and is used in North America. **The relevant standard can be found in GB/T12716-1991.** PT is the abbreviation for Pipe Thread; it represents 55-degree sealed tapered pipe threads and belongs to the Whitworth thread family. It is widely used in Europe and the Commonwealth countries. It is commonly used in the water and gas piping industries, with a taper ratio of 1:16. **The relevant standard can be found in GB/T7306-2000.** G refers to 55-degree non-tapered sealed pipe threads, also belonging to the Whitworth thread family. The designation “G” indicates cylindrical threads. **The relevant standard can be found in GB/T7307-2001.** Additionally, the designations 1/4, 1/2, 1/8 refer to the diameter of the thread, measured in inches. Those in the industry usually refer to thread sizes in fractions, where 1 inch equals 8 fractions; thus, 1/4 inch is equivalent to 2 fractions, and so on. “G” is an umbrella term for pipe threads; the distinctions between 55-degree and 60-degree threads are based on functional aspects, and they are commonly referred to as round pipe threads. That is, the thread is formed by machining a cylindrical surface. 　　ZG is commonly known as pipe taper, meaning that the threads are formed by a conical surface; most water pipe fittings use this type of thread. According to national standards, metric threads are denoted by pitch, while imperial and British threads are indicated by the number of threads per inch – this is their main difference. Metric threads have an equilateral tooth profile at 60 degrees, British threads have an isosceles tooth profile at 55 degrees, and American threads also have a 60-degree tooth profile. Metric threads use metric units, while imperial and American threads use imperial units. Pipe threads are primarily used for connecting pipes; their internal and external threads fit tightly together, and there are two types: straight pipes and tapered pipes. The nominal diameter refers to the diameter of the pipe being connected; obviously, the thread diameter is larger than the nominal diameter. 1/4, 1/2, 1/8 are the nominal diameters of imperial threads, measured in inches. 　　British pipe threads originated from British Whitworth threads; the series of diameters associated with Whitworth threads, together with the thread profiles of Whitworth threads, determined the basic dimensions of British pipe threads. Based on a 1/16 taper ratio, the radial diameter tolerances of Whitworth threads were converted into axial pitch tolerances for British sealed pipe threads (with some rounding and adjustments made). Using the tolerances for British sealed pipe threads as a reference, tolerances for British unsealed pipe threads were established (the tolerance distribution changed from one type to another, with looser tolerances for the major diameter and less strict tolerances for the minor diameter). The three types of threads were developed as follows: In 1841, British Whitworth threads were introduced; in 1905, new standards for Whitworth threads were issued (BS 84). In 1905, standards for British sealed pipe threads were also established (BS 21). From 1905 to 1940, Whitworth threads served as the standard for British unsealed pipe threads. In 1940, a series of unsealed pipe threads based on Whitworth threads was introduced (the BSP series) ； In 1956, a standard for British-style unsealed pipe threads (BS 2779) was issued separately. Europe and the Commonwealth were the first to adopt these British-style pipe thread standards. The ISO/TC5/SC5 Technical Committee for pipe thread standardization and its secretariat are under the control of Europe, and the British-style pipe thread standards were incorporated into ISO standards. In 1955, ISO introduced a standard for British-style sealed pipe threads (ISO R 7) ； In 1961, ISO introduced a standard for British-style unsealed pipe threads (ISO R 228). In 1978, ISO issued official standards for two types of British-style pipe threads (ISO 7-1 and ISO 228-1). Today, British-style pipe threads are widely accepted outside North America and are used extensively in international trade. The British-style pipe thread standards defined by ISO have been converted to the metric system. Converting British-style pipe thread sizes to metric is quite simple: multiply the original inch dimensions by 25.4 to obtain the millimeter values. The use of British-style pipe threads is gradually being phased out; it is unrealistic to expect the use of true pipe thread standards. There is no distinction between genuine metric pipe threads and fake ones. British-style sealed pipe threads come in two types of fits: “straight/tapered” and “tapered/tapered.” These two types use different thread gauges (cylindrical thread gauges and tapered thread gauges) as well as thread plugs (with their reference planes positioned at different distances, half a thread pitch apart). European countries mainly use the “straight/tapered” type of fit for pipe threads ； Outside of Europe, “taper/taper” thread fits are primarily used. For the same type of sealed pipe thread fitting, those that pass the inspection standards in Europe may fail to meet the requirements in other regions. It is essential to be aware of such differences in international trade; otherwise, defective products may result. Before 1994, ISO’s standards for British-style sealed pipe threads and their related gauges were designed based on the “taper/taper” system. Chinese products using British-style sealed pipe threads could enter the international market without any problems, while European products faced disadvantages in this regard. After 2000, ISO’s standards for British-style sealed pipe threads and their gauges were redesigned using the “square/taper” system. As a result, Chinese products using the old British-style sealed pipe thread standards encountered difficulties when entering the international market. To address this issue, China revised its standards for British-style sealed pipe threads in 2000, changing one original standard into two, thereby reminding designers to pay attention to the differences between the two types of thread fits and to choose the appropriate one. When Japan revised its standards for British-style sealed pipe threads in 1999, it continued to use the ISO standards from before 1994. Therefore, the international market for British-style sealed pipe threads became even more complex after 2000, requiring domestic manufacturers to be particularly careful. British-style sealed pipe threads are intended for general-purpose sealing applications, and sealing materials must be added to the thread pair during use. Their advantage lies in their cost-effectiveness, with moderate requirements regarding processing precision. Threads that do not use sealing materials can provide a dry seal, but there are no such dry-sealing threads within the British-style thread system. Sealed pipe threads serve both mechanical connection and sealing functions ； In contrast, unsealed pipe threads have only a mechanical connection function. Therefore, the precision required for sealed pipe threads is higher than that for unsealed pipe threads. Some people, seeing that the tolerance on the pitch diameter of unsealed pipe threads is half that of sealed pipe threads, assume that the precision of unsealed pipe threads is higher; this view is incorrect. Sealed pipe threads require precise tooth profiles, and the tolerances for their outer diameter, pitch diameter, and inner diameter are all the same ； The errors in the tooth side angle and pitch have a significant impact on the sealing performance. In contrast, non-sealing pipe threads generally do not require high precision in terms of tooth shape. The tolerance for the major diameter is greater than that for the mean diameter ； There are no tolerance requirements for its diameter. ； Furthermore, some people believe that a combination of non-sealed circular internal threads and sealed circular external threads can be used. This view is also incorrect; doing so means relaxing the precision requirements for the sealed internal threads, which can lead to problems with sealing. Due to the different applications for sealed pipe threads, as well as differences in machining precision, assembly, and testing techniques, current pipe thread standards cannot guarantee that all threads that meet the standard specifications will provide proper sealing. The British standards for sealed pipe threads do not specify uniform precision requirements for individual thread parameters, yet these parameters have a direct impact on sealing performance. The fundamental solution to this problem is for each industry or company to establish its own internal control measures tailored to its specific products. These internal control criteria are usually kept confidential and are not applicable to companies in other industries; manufacturers must be aware of this. Sealed pipe thread standards are not a panacea, and manufacturers need to pay close attention to sealing issues on their own. Before 1987, China did not have standards for American or British pipe threads. Yet, it was impossible to avoid using these two internationally common types of pipe threads in manufacturing. As a result, the old mechanical drawing standards specified their own codes for American and British pipe threads, based on Chinese pinyin letters, without considering whether they matched the codes used in foreign standards. Since these standards only defined thread codes without specifying thread parameters, the same code could represent different parameter values in different companies or industries. This made it difficult to determine who was at fault when defective products were produced. From 1987 to 1991, China introduced standards for British pipe threads. Since then, thread codes and markings must comply with these standards; the thread codes specified in the old mechanical drawing standards should be abolished immediately.
1. Sealed Pipe Threads (R)
Basic dimensions and tolerances of British sealed pipe threads
Fit methods
There are two fit methods for British sealed pipe threads: a “cylindrical/conical” fit formed by circular internal threads and conical external threads ； Tapered internal threads and tapered external threads form a “taper/taper” fit. In Europe, “shaft/taper” fit threads are primarily used ； Outside of Europe, **“posts/cones” in combination with threads are primarily used; the inspection specifications for these two types of threads differ to some extent** ； The current ISO standard for sealed pipe threads in imperial units (ISO 7-2:2000) is based on a \"column/cone\" fit system.
Designation
The complete designation for sealed pipe threads in imperial units consists of a thread characteristic code, a thread size code, and a rotation direction code.
The characteristic code for sealed cylindrical internal threads in imperial units is: RP ； 　　The characteristic code for imperial sealed tapered internal threads is: RC ； 　　The characteristic code for British imperial sealed tapered external threads is: R1 (used in combination with British imperial sealed cylindrical internal threads) ； 　　R2 (used in conjunction with British standard sealed tapered internal threads) ； 　　The rotation direction code for left-handed threads is LH ； The direction code for right-hand threads is omitted. For sealed pipe threads, RP/R1 and RC/R2 are used to denote \"cylinder/cone\" and \"cone/cone\" thread pairs, respectively. 2. Unsealed pipe threads Basic dimensions and allowable tolerances for British standard unsealed pipes Marking: The complete marking for British standard unsealed pipe threads consists of the thread characteristic code, the thread size code, the pitch diameter tolerance class code, and the direction code. The characteristic code for British standard unsealed cylindrical threads is G. For British standard unsealed cylindrical internal threads, the pitch diameter tolerance class code is omitted ； The metric grade codes for the pitch diameter of British standard unsealed external cylindrical threads are A and B, respectively. The direction code for left-handed threads is LH ； The direction code for right-hand threads is omitted. When indicating a set of threaded connections for imperial non-sealed pipe threads, only the code for the external thread is specified. Examples: Right-hand, non-sealed cylindrical internal thread with size code 2: G2; Right-hand, non-sealed cylindrical external thread of grade A with size code 3: G3A; Left-hand, non-sealed cylindrical external thread of grade B with size code 4: G4 B-LH; A set of threaded connections consisting of a right-hand, non-sealed cylindrical internal thread with size code 2 and a cylindrical external thread of grade A: G2A. Thread taps are used to machine internal threaded holes, while die sets are used to machine external threads. 　　Others: Conversion of 55° tapered pipe threads. 55° tapered pipe threads refer to threads with a thread pitch angle of 55° and a taper ratio of 1:16. This series of threads is widely used around the world, and its designation varies depending on the country. 　　China, UK, France, Japan, ISO. Codes: ZG, R (external), G, PT, R. R (external thread), Rc (internal), R, Rc. Conversion of 60° tapered pipe threads. 60° tapered pipe threads refer to pipe threads with a thread pitch angle of 60° and a thread taper of 1:16; these types of threads are used in China’s machine tool industry as well as in the United States and the former Soviet Union. Its code was designated as K in our country in the past, then as Z, and is now changed to NPT.