Thread Type Guide
Screw thread and other standard parts products have a huge application market in aerospace, automobile, equipment manufacturing and other industries, whose quality directly affects the quality of the target products.
At present, there are many thread styles and standards in the domestic market. Domestic and foreign ones, metric ones, English ones, straight ones, sealed ones, unsealed ones, internal and external ones, 55 ° ones and 60 ° ones are easy to be confused, which directly affects the product conformity rate in product development and trade export. The following small series are common symbols, standards of different countries and common threads for you to comb.
- Common symbols
- Standards in different countries
- Thread classification
- Thread Standards
- How to identify an unknown thread type
- Thread Design
NPT is a general purpose American standard taper pipe thread with a 60 ° profile angle.
PT thread is British taper thread, with a profile angle of 55 °. It is most commonly used in sealing.
British pipe thread is fine thread, because the depth of coarse thread is large, it will seriously reduce the strength of the pipe with the outer diameter of the thread.
PF thread is parallel thread for pipe.
G is 55 degree non thread sealing pipe thread, belonging to the Whitworth Thread family. Mark g for cylindrical thread.
G is the general designation of pipe thread (Guan). The division of 55 and 60 degrees is functional.
ZG is commonly known as pipe cone, that is, the thread is processed from a circular cone. The general water pipe joints are like this. The old national standard is marked RC.
Metric threads are represented by pitch, while American and British threads are represented by the number of threads per inch, which is the biggest difference between them.
Metric thread is 60 degrees of equal profile, English thread is 55 degrees of equal waist profile, American thread is 60 degrees.
Metric screw threads are in metric units, and American and British screw threads are in English units.
The pipe thread is mainly used for the connection of pipes. The internal and external threads are closely matched, including straight pipe and cone pipe. Nominal diameter refers to the diameter of the connected pipe. Obviously, the thread diameter is larger than the nominal diameter.
1 / 4, 1 / 2, 1 / 8 is the nominal diameter of inch thread.
01. Inch unified thread
Widely used in inch system countries, this kind of thread is divided into three series: coarse thread series UNC, fine thread series UNF, super fine thread series UNFF, plus a constant pitch series UN.
Marking method: thread diameter – number of teeth per inch series code – accuracy grade
- Coarse tooth series 3 / 8-16unc-2a
- Fine tooth series 3 / 8-24unf-2a
- Ultra fine tooth series 3 / 8-32unff-2a
- Constant pitch series 3 / 8-20un-2a
The first digit 3 / 8 indicates the outer diameter of the thread, and the unit is inch. When converting to metric unit mm, it is multiplied by 25.4, that is, 3 / 8 × 25.4 = 9.525mm; the second and third digits 16, 24, 32, 20 are the number of teeth per inch (the number of teeth on the length of 25.4mm); the text codes UNC, UNF, UNFF, UN after the third digit are the series codes, and the last two digits 2a are the precision level.
02. Conversion of 55 ° cylindrical pipe thread
55 ° cylindrical pipe thread comes from Inch series, but it is widely used in both metric and inch system countries. It is used for the connection of pipe joints and pipes for conveying liquid, gas and installing wires. However, the codes of different countries are different. The foreign codes in the following table (comparison table) should be changed into Chinese codes. Now, 55 ° cylindrical pipe thread codes of different countries are listed in the following table.
Japan, G, PF
UK, BSP, BSPP
Germany, R (internal thread), K (external thread)
Former Soviet Union, G, TP УБ
03. Conversion of 55 ° taper pipe thread
55 ° tapered pipe thread refers to the thread with a profile angle of 55 ° and a taper of 1:16. This series of thread is widely used in the world. Its code and regulations are different in different countries. See the table below. The foreign codes in the following table are converted into Chinese codes.
China, ZG, R (external thread)
UK, BSPT, R (male), RC (female)
France, G (external thread), R (external thread)
Germany, R (external thread)
Japan, Pt, R
ISO, R (external thread), RC (internal thread)
04. Conversion of 60 ° taper pipe thread
The 60 ° tapered pipe thread refers to the pipe thread with a profile angle of 60 ° and a thread taper of 1:16. This series of thread is applied in China’s machine tool industry, the United States and the former Soviet Union. Its code is k, later Z, now NPT. See the table below for the comparison of thread codes.
China, Z (old) NPT (New)
Soviet Union, B
05. Conversion of 55 ° trapezoidal thread
Trapezoidal thread refers to the metric trapezoidal thread with a profile angle of 30 °. This series of thread is relatively uniform at home and abroad, and its code is also quite consistent. See the following for thread code:
China, t (old) tr (New)
Soviet Union, Tr
It can be divided into:
1. International metric thread system:
The thread used in China’s national standard CNS. The crown is flat and easy to turn, while the bottom is circular to increase the thread strength. The thread angle is 60 degrees, and the specification is in M. Metric thread can be divided into coarse thread and fine thread. For example, m8x1.25. (M: code, 8: nominal diameter, 1.25: pitch).
2. American Standard Thread:
Both the top and the root of the thread are flat, and the strength is better. The thread angle is also 60 degrees, and the specification is indicated by several teeth per inch. This kind of thread can be divided into three grades: coarse thread (NC), fine thread (NF) and extra fine thread (Nef). The representation is as 1 / 2-10nc. (1 / 2: outer diameter; 10: teeth per inch; NC code).
3. Unified thread:
Developed jointly by the United States, the United Kingdom and Canada, it is the British thread commonly used at present. The thread angle is also 60 degrees, and the specification is indicated by several teeth per inch. This kind of thread can be divided into coarse thread (UNC), fine thread (UNF) and extra fine thread (UNEF). The representation is as 1 / 2-10unc. (1 / 2: outer diameter; 10: teeth per inch; UNC code).
4. Sharp V thread:
Both the top and the root are sharp with weak strength, which is not often used. The thread angle is 60 degrees.
5. Whitworth Thread:
Thread used in British national standard. The thread angle is 55 degrees, indicating that the symbol is “W”. Suitable for rolling manufacturing. The representation is as W1 / 2-10. (1 / 2: outer diameter; 10: teeth per inch; W code).
6. Knuckle thread:
Standard screw thread according to DIN. Suitable for the connection of light bulb and rubber tube. The symbol is “Rd”.
7. Pipe thread:
In order to prevent leakage of the thread, often used for gas or liquid pipe connection. The thread angle is 55 degrees, which can be divided into straight pipe thread code “P.S.,” n.p.s., “and inclined pipe thread code” n.p.t., “the taper is 1:16, that is, 3 / 4 inch per foot.
Some industry people also said that: NPT is the American Standard 60 spinal canal thread, the North American Standard, Pt is 55 degree taper pipe thread, belonging to Wyeth thread, used in Europe and the United Kingdom.
8. Square thread:
The transmission efficiency is big, next to ball screw thread, but it can’t be adjusted by nut after wear, which is its disadvantage. Generally used for the screw of vise and the screw of crane.
9. Trapezoid thread:
Also known as the acme thread. The transmission efficiency is slightly smaller than that of square thread, but it can be adjusted by nut after wear. The thread angle of metric system is 30 degrees, and that of British system is 29 degrees. Generally used as the guide screw of lathe. The symbol is tr.
10. Buttress thread:
Also known as beveled thread, it is only suitable for single direction transmission. For example, screw jack, pressurizer, etc., the symbol is “bu”.
11. Inch thread:
The size of inch thread is usually expressed by the number of threads per inch of thread length, referred to as “number of threads per inch”, exactly equal to the reciprocal of pitch. For example, the pitch of 8 threads per inch is 1 / 8 inch.
BSP – British Standard Pipe Thread
The BSP, or Whitworth thread, is a family of thread standards that has been adopted internationally, except in the United States. This thread form is based on a 55° V-thread with rounded roots and crests, as seen in Figure 1. For a thread that conforms to BSP, the major diameter of the pipe thread is slightly smaller than the actual OD of the pipe, and the minor diameter will be very close to (smaller than) the inside diameter of the female thread. There are two types of BSP threads:
- BSPP: Both the male and female threads are parallel. BSPP connections are widely used in UK, Europe, Asia, Australia, New Zealand and South Africa. Sizes can be seen in Table 1.
- BSPT: The male threads are tapered and the female threads are commonly parallel. BSPT connections are especially popular in China and Japan. Sizes can be seen in Table 2.
Figure 1: A BSPP male parallel thread profile (left) and a BSPT taperd male thread profile (right)
BSP threads are identified with letters each of which represents the type of thread and their associated standards1:
- G: external and internal parallel (ISO 228, DIN 259) – BSPP
- R: external taper (ISO 7, EN 10226, BS 21, JIS B 0203) – BSPT
- Rp: Internal parallel (ISO 7-1, EN 10226) – BSPT
- Rc: internal taper (ISO 7) – BSPT
- Rs: external parallel (BS 21) – BSPT- Obsolete
1ISO 7: Pipe threads where pressure-tight joints are made on the threads. ISO 228: Pipe threads where pressure-tight joints are not made on the threads.
Labeling Example: EN 10226 Rp 2 ½
This refers to a British Standard Pipe thread tapered (EN 10226) with an internal parallel form (Rp) and a nominal size of 2 ½.
The actual sizes of the most commonly used BSP threads are listed in Table 1 and Table 2 for BSPP and BSPT threads respectively. Table 3 provides data on pipe sizes associated with these threads.
Note: Each thread size is identified with a number which has little to do with the actual size of the thread. This discrepancy originates from amendments in industrial practices and standardisation throughout the history of standardisation of pipe threads. Therefore, always compare measurements with actual sizes listed in the tables.
|Table 1: BSPP (G) – British Standard Pipe Parallel|
|Nominal Thread Size||Major Diameter (mm)||Minor Diameter (mm)||TPI (in-1)|
|Table 2: BSPT (R/Rp) – British Standard Pipe Tapered|
|Nominal Male Tapered Thread Size (inch)||Nominal Female Parallel Thread Size (inch)||Major Diameter (mm)||Minor Female Diameter (mm)||TPI (in-1)|
|R 1/16||RP 1/16||7.723||6.490||28|
|R 1/8||RP 1/8||9.728||8.495||28|
|R 1/4||Rp 1/4||13.157||11.341||19|
|R 3/8||Rp 3/8||16.662||14.846||19|
|R 1/2||Rp 1/2||20.955||18.489||14|
|R 3/4||Rp 3/4||26.441||23.975||14|
|R 1||Rp 1||33.249||30.111||11|
|R 2||Rp 2||59.614||56.476||11|
|Table 3: British Standard Pipe Dimensions for Standard Thread Sizes|
|Nominal G / R size (in)||Corresponding Pipe|
|DN (mm)||Actual OD (mm)||Wall (mm)|
Figure 2: NPT thread profile mating with the top being a fitting internal thread and the bototm being a pipe external thread.
NP – National (American) Pipe Thread
The American national pipe thread was created based on a 60° V-thread with flattened peaks and valleys (Figure 2) and is widely used in the US and Canada. There are two types of NP threads:
- NPS: Straight threads, meaning the male and female threads are parallel.
- NPT: Tapered threads, meaning the male and female threads are tapered. NPT threads are more commonly used. NPT Thread specifications are based on ANSI B1.20.1 and the threads sizes can be seen in Table 4.
|Table 4: NPT – National (American) Pipe Thread Tapered|
|Nominal Thread Size (inch)||Major Diameter (mm)||TPI (in-1)|
Labeling Example: 3/8 – 18 NPT
National pipe threads are designated with their nominal size (3/8) followed by number of threads per inch (18) and the symbol for the thread series (NPT).
Note: NPS (National Pipe Straight) is not to be confused with NPS (Nominal Pipe Size) which is an American set of standards for pipes. For a given outside diameter, NPS (Nominal Pipe Size) provides multiple pipe schedules (wall thicknesses) while the pipe thread profile remains the same among them.
M – Metric Thread (ISO)
Figure 3: Metric Thread profile
The Metric Thread is one of the first internationally agreed general-purpose thread type. The V-shaped thread form has a 60° flank angle and male and female threads are both parallel (Figure 3). Metric Threads come in different pitch sizes for a given diameter: coarse pitch and fine pitches. Coarse threads have the default pitch size whereas fine threads have smaller pitch sizes and are used less often. As a result, coarse threads are identified by diameter only while fine threads are recognized by diameter as well as pitch size.
Metric threads come in two different pitch sizes for a given diameter:
- Coarse Pitch: These have the default pitch size according to Table 5 and are the most commonly used. They adhere to ISO 724 (DIN 13-1).
- Fine Pitch: These have a smaller pitch size and in the labeling they require the diameter of the pitch size. They adhere to ISO 724 (DIN 13-2 to 11).
Labeling Example: M8
This refers to a metric coarse thread with a diameter of 8 mm (which with reference Table 5, corresponds to a pitch size of 1 mm)
Labeling Example: M4 x 0.5
This indicates a fine thread with a diameter of 4 mm and a pitch size of 0.5 mm.
M 10M 10
|Table 5: Metric Threads (Coarse)|
|Thread Size (mm)||Major Diameter (mm)||Minor Diameter (mm)||Pitch (mm)|
Figure 4: Pitch gauge
To identify an unknown thread, verify the following:
- Gender: Male or female.
- Taper: Parallel or tapered. This can be determined whether by visual inspection or by measuring the diameter of the thread at the beginning and the end via a caliper.
- Thread Diameter: For male threads, measure the major diameter and for female threads measure the minor diameter. Note that this diameter will always be different from the nominal size of the thread.
- Thread Pitch: For this purpose, a pitch gauge can be used (Figure 4). Make sure to measure multiple samples to confirm accurate pitch. If a pitch gauge is not available, count the number of threads in a 1-inch span (TPI) and the reciprocal is your thread pitch in inches. If you are skeptical that the thread is metric, count the number of threads in a 10-mm span. The reciprocal of this number is your pitch size in mm.
Check Tables 1-5 to find a matching actual diameter corresponding to a nominal thread size. Confirm thread pitch or TPI with the values corresponding to that diameter.
Although some BSP and NPT threads might be tightened and engaged together fairly well, pitch and thread angle differences between them will allow spiral leakage. Specifically speaking, at 1/16”, 1/8”, 1/4”and 3/8” sizes, pitch difference results in misalignment of the threads. This misalignment might result in the threads to be fastened only partially. However, at 1/2” and 3/4” sizes, NPT and BSP have the same pitch which will allow them to engage well. Nevertheless, since there are differences in thread forms and thread angles, there will still be issues with this connection. For low end applications, if an appropriate thread sealant is used, these threads might be used together effectively. Table 6 shows the compatibility of threads and the required seal type to be used as a reference for your application.
|Table 6: Thread Compatibility Chart|
|My Fitting||Designation||Matching Fitting||Designation||Seal Type||Notes|
|BSPP Parallel Female||G||BSPP Parallel male||G||O-ring/washer||Rs or NPT male threads are incorrect installations here. Rs threads are slightly smaller than G threads, and NPT threads have a different form.|
|BSPT taper male||R||Tape/pipe dope|
|BSPT Parallel Female||Rp||BSPT taper male||R||Tape/pipe dope||G threads are incorrect installation here as they have different tolerances.|
|BSPT Taper Female||Rc||BSPT parallel male||Rs||Tape/pipe dope||Inserting an NPT male here is an incorrect installation, as NPT threads have a different form.|
|BSPT taper male||R||Tape/pipe dope|
|NP Taper Female||NPT||NP taper male||NPT||Tape/pipe dope||BSP threads are not to be used with NPT threads even if the threads seem to engage fairly well.|
|NP Parallel Female||NPS||NP parallel male||NPS||O-ring/washer||NPT thread is an incorrect installation here as sealing is not possible.|
A thread is a ridge wrapped around a cylindrical or a conical shape, which by mating to another thread can provide a joint. Threads are designed in numerous sizes and shapes to serve a wide range of purposes. Ensuring the compatibility of the threads in a connection and the formation of a proper seal are important in achieving a reliable joint. Threads are identified by their design characteristics and are developed to provide certain degrees of joint reliability. When selecting a product with threads, it is important to understand these important thread specifications:
- Male and female threads
- Right-handed and left-handed threads
- Pitch and threads per Inch
Male and female threads
Threads can be internal (female) or external (male). All matching thread connections consist of a male and a female thread type. In a given connection, it is important for the threads to have the same design characteristics to ensure adequate leak tightness and/or mechanical performance. Generally, threaded port connections of valves are female, as seen in Figure 1 on the right.
Right-handed and left-handed threads
The helix (or spiral) of a thread can be designed to be either right-handed or left-handed. As default, majority of threads are right-handed and follow the right-hand grip rule. The right-hand grip rule means that when a thread is engaged in a connection, it moves upward if rotated in the direction of the right hand. The letters “LH” in a thread spec stand for a left-handed thread and is rotated in the direction of the left hand to move the thread up. Figure 2 shows the differences along with the right-hand rule.
Figure 2: Handedness in Threads
The helix of a thread can be wrapped around a conical or cylindrical shape. A thread on a conical shape, as seen in Figure 3 on the top, creates a tapered thread. A thread on a cylindrical shape, as seen in Figure 3 on the bottom, creates a straight or parallel thread. The ‘taper angle’ is the angle between the conical shape and the centerline of the pipe.
Tapered threads are designed to create leak-tightness through compression and deformation of the male and female threads into each other in a connection. On the other hand, straight threads do not create a leak-tight joint on the threads and are often sealed with O-rings or gaskets. The design differences of these thread types are further discussed here.
Figure 3: Tapered (top) vs. Parallel (bottom) Thread
The major diameter of a thread is the larger of the two extreme dimeters of the thread profile. This is measured from the thread tips or crest for external (male) threads and from the groove or root of the thread for internal (female) threads. Minor diameter is consequently the smaller of the two diameters. The depth of a thread is the difference between the major and minor diameters. The pitch diameter (PD) is measured from a line that intersects the flanks of a thread and is half way between the major and minor diameters. Figure 4 shows these terms on a male thread.
Figure 4: Thread Dimensions
Thread angle or flank angle is the angle between thread flanks (Figure 4). Most commonly used thread angles are 60° and 55°.
Pitch and Threads per Inch
Pitch is the distance between crests of two consecutive threads. Threads per inch or TPI of a thread is essentially the reciprocal of the pitch of that thread. For example, a thread with a TPI of 12 has a pitch size of of an inch. While TPI is used to describe imperial threads, pitch is often used for metric threads and is given in mm.
The number of starts of a thread determines the number of ridges that are wrapped around to create the thread. The number of starts determines the number of threads a screw moves when turned 360°. One-start threads are the most common pipe thread. Figure 5 is color coded to show the difference in ridge numbers per start.
Figure 5: Number of Starts in a Thread
The cross-section shape of a thread is called its form, different examples of which is shown in Figure 6. Threads can be triangular, square, trapezoidal or other shapes. The triangular shaped threads are also called V-threads. Although V-threads are represented as triangular, in practice, the tip and groove of the thread cannot have a perfectly sharp edge and is therefore truncated (cut short) at varying degrees. A Sharp V-thread is a thread where the tips of the triangles are not truncated.
Figure 6: Forms of Threads
Sealing a Threaded Connection
When connecting two threads, it is important to take into consideration the hydraulic seal that they will create. Parallel and tapered threads can be combined to create two types of connections: Jointing threads and longscrew (fastening) threads.
Figure 7: Thread seal tape used for better sealing of jointing threads
Jointing thread connections create pressure-tightness through compressing the threads together. For that purpose, tapered male threads and parallel or tapered female threads are coupled together. In this type of joint, a positive seal is created by deforming the threads into each other through wrench tightening to proper torque. This means that repeated assembly and disassembly can distort the threads. During maintenance, it is important to check this type of joint for any leak paths or damage to the threads.
A sealing coating or a jointing compound on the threads such as a thread seal tape or a pipe dope is often used in this type of connection (Figure 7). When using seal tape, only two turns of sealant is required. It is important to note that an O-ring seal can not be used with a tapered male thread as it will not allow the threads to be tightened completely.
Figure 8: BSPT Connection
Some precise threads are also known as “dry fit” or “dry seal” and do not require any kind of seal material. In these joints, the pressure-tightness is solely created by compression of threads and are especially used to create a gas-tight seal or when the sealant could contaminate or react with the media (e.g. oxygen).
As mentioned, jointing threads may become susceptible to leakage through the course of tightening and untightening during repair or assembly. When a taper male thread is tightened to a parallel female thread, the seal is only made at the base of the female port with 1 or 2 threads due to difference in taper (Figure 8). That means that the area where the crest of male and the root of female thread meet, can form a spiral leak path. This is especially the case with BSPT connections. BSPT threads are commonly used for low-pressure applications and is not recommended for medium or high-pressure systems. This problem is diminished when both male and female threads are tapered (e.g. NPT connections).
Figure 9: O-ring on a male thread
Longscrew or Fastening Threads
When both male and female threads are parallel, pressure-tightness is not achieved on the threads. In this type of joint, a reliable seal is created by compression of a soft material (an o-ring seal or a washer) or a flat gasket between the shoulder of the male pipe and the interior surface of the female thread.
Source: China Thread Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)