Thread Types and International Standards for Fasteners

Understanding Thread Specifications for Fastener Applications

Thread specification is one of the most important factors in custom fastener manufacturing. Correct thread selection ensures compatibility, mechanical performance and reliable assembly.

Thread standards are defined to ensure interchangeability, reliability, and ease of maintenance across mechanical and industrial systems. They are generally based on either the metric or imperial measurement systems, and selecting the correct thread type is critical to achieving a secure and compatible joint in both existing assemblies and new designs.

Among the many thread standards used worldwide, the most common thread form is the 60° profile, which is found in both the Metric ISO thread system and the Unified National (UN) series, including UNC and UNF threads. These standards dominate general fastening applications due to their balance of strength, manufacturability, and global availability.

In addition to these widely used systems, there are several other thread standards designed for specific functions or regional requirements. These may originate from national standards or be developed for specialized industries where performance demands go beyond general fastening. For example, certain thread types are used in precision engineering fields such as medical technology, aerospace engineering, and astronautics, where requirements for sealing, vibration resistance, or extreme operating conditions are significantly higher.

As a result, understanding the differences between thread standards is essential for ensuring correct assembly, preventing compatibility issues, and meeting the technical demands of different industrial applications.

Key Thread Parameters for Fastener Thread Design

1.Gender

Every matched pair of threads, external and internal, can be described as male and female. For example, a screw has male thread, while the matching hole has female thread.

2.Handedness

The helix of a thread twists in one of two possible directions. Most threads are oriented so that the threaded item when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction and moves towards the viewer when it is turned counter-clockwise.

By convention right-handedness is the default handedness for screw threads. Most threaded parts and fasteners therefore have right-handed threads.

Design

The type of thread can be identified by the following characteristics: parallel and tapered design

3.Pitch/TPI

The pitch is the distance from the crest of one thread to the next in milimeters (mm).
TPI (threads per inch) is used by inch thread.

4.Diameter

The major diameter is determined by the thread tips.
The minor diameter is determined by the groove of the thread.
The pitch diameter is the distance between the tips of two opposite flanks or the length of the centreline of the profile.

5.Angle

The flank angle is the angle between the flank of a screw thread and the perpendicular line to the axis of the screw. Tapered threads have a taper angle. This is the angle between the taper and the center axis of the pipe.

6.Crest / root

The outermost part of the thread is called ‘crest’, the innermost part of the thread is called ‘root’.

Common Thread Types and Thread Standards

1.The metric ISO thread (M)

As this type of thread is used very frequently in Europe, you have probably already worked with it yourself. The thread standard for the metric standard thread is a fastening thread that is used for screws and nuts, for example. It has a flank angle of 60° and is characterised by the abbreviation M and the nominal diameter. As this is a metric thread standard, all dimensions are given in mm.

2.The metric ISO fine pitch thread (MF)

This type of thread is a variant of the metric ISO standard thread with a smaller pitch and thread depth. The small pitch of the MF metric fine thread makes it possible to determine the position of a screw very precisely. The thread standard is therefore often used for adjusting screws. In addition, fine threads with this standard have good self-locking properties and can also be used in tight spaces thanks to their shallow depth. The abbreviation MF is used in the thread designation.

3.The trapezoidal thread (Tr)

The trapezoidal thread is also a metric thread standard; accordingly, all Dimensions are specified in mm. The flank angle here is only 30°. A striking feature of this thread is the profile, which is not tapered, but has the shape of an isosceles trapezoid. As a result, the trapezoidal thread is well suited as a movement thread and can be found in lathes or screw presses, for example.

4.Unified National Coarse Thread (UNC)

At this point we enter the area of American thread types. With a flank angle of 60°, the American coarse thread is similar to the metric ISO thread and is also roughly equivalent to this thread standard in the USA in terms of function and distribution. It is important to note that all dimensions are given in inches. In addition, the pitch of American threads is specified by the number of threads in inches. A small but important detail that can be confusing at first glance.

5.Unified National Fine Thread (UNF)

As with metric threads, there is also a fine thread to the coarse thread for American threads. This is the Unified National Fine Thread (UNF). The flank angle of the American fine thread is 60° and here, too, all specifications must be in inches. Like the UNC thread standard, the UNF thread standard is also used worldwide and is also used in the USA, Japan and Australia, among others.

6.The trapezoidal thread (Tr)

The trapezoidal thread is also a metric thread standard; accordingly, all Dimensions are specified in mm. The flank angle here is only 30°. A striking feature of this thread is the profile, which is not tapered, but has the shape of an isosceles trapezoid. As a result, the trapezoidal thread is well suited as a movement thread and can be found in lathes or screw presses, for example.

7.Unified National Special Thread (UNS)

The UNS thread standard is a special standard for threads. This standard also has a flank angle of 60°, but differs from the other American threads in its special diameters, pitches and screw-in lengths. This American thread with a special pitch can be found on microphone stands, for example.

8.National Standard Taper Pipe (NPT)

Although the NPT thread is also one of the American thread standards, it is not part of the Unified Thread Standard (UTS). Rather, it belongs to a group of standards for pipe threads. The American pipe thread is tapered, non-sealing and has a flank angle of 60°.

9.British Standard Whitworth Coarse Thread (BSW)

Let’s continue with the British threads. The British coarse thread is the standardised standard thread in the UK, similar to the metric thread standard on the European mainland. The dimensions are given in inches. Be careful with the flank angle, which is only 55° here. The thread abbreviation for this thread standard is made up of the abbreviation BSW and the nominal diameter in inches.

10.British Standard Fine Thread (BSF)

The British thread standards also recognise the fine thread for the BSW thread. The British Standard Fine Thread (BSF) has a flank angle of 55°. The British fine thread is used to be an important part of the glorious British automotive industry, but has since lost its importance. You can recognise this thread by the thread designation BSF followed by the diameter in inches.

11.British Standard Pipe Parallel (BSP)

While the other British thread standards are mainly used in their country of origin, the BSP pipe thread is also very popular outside the UK. It has established itself as the standard thread for connecting pipes in plumbing and heating technology and can therefore also be found in Germany. Please note that this thread standard also has a flank angle of 55°. In addition, the inches in the thread designation should not be taken literally, as they refer to obsolete pipe geometries. The abbreviation for the BSP thread is G.

12.JIS tapered pipe thread type (PT)

The JIS thread type is identical and interchangeable with the BSPT thread type. However, the male PT thread does not have a 30-degree flare, so it will not mate with the BSPP female swivel with a conical seat. Also, ensure to use a thread sealant with PT threads for a leakproof seal.

13. SAE thread type

SAE (Society of Automotive Engineers) has a set of straight threads having a 90-durometer Buna-N o- ring that aids in sealing. While many thread types require the female and male threads to crush against each other to form the seal, the o-ring in the SAE thread type prevents this and also ensures a leakproof seal.

Thread Specification and Dimensioning Standards for Fasteners

How Thread Requirements Are Defined in Engineering Applications

To specify a thread using the English system, you must provide a minimum of five pieces of information:

• Thread form
• Thread series
• Major diameter
• Class of fit
• Threads per inch

Understanding Different Thread Profiles Used in Fasteners

Thread form is the shape or profile of a screw thread. Many types of thread forms have been developed.

The sharp-V thread was originally developed by William Sellers (September 19, 1824 – January 24, 1905), a mechanical engineer, manufacturer, businessman, and inventor who filed more than 90 patents.

The American National thread replaced the sharp-V thread and is stronger than the sharp-V thread.

The Unified thread is the current standard used in the United States, Canada, and England.

A variation on the Unified thread is the Unified National Round thread, abbreviated UNR.

Diagram: (A) Sharp V; (B) American National; (C) Unified (external) with rounded crest and Unified National Round with flat crest.

• The metric thread is the international standard thread, similar in shape to the American National thread.
• The square, Acme, and buttress threads are used to transmit power in gearing and other types of machines.
• The knuckle thread is usually rolled from sheet metal or cast, and it is used for lightbulb bases, bottle caps, and glass jars.

Diagram: (D) Metric; (E) Square; (F) Whitworth standard; (G) Acme; (H) Knuckle; (I) Buttress.

The thread series refers to the standard number of threads per inch, and there are four classes: coarse (C), fine (F), extra fine (EF), and constant pitch. When used with the Unified thread, they are abbreviated UNC, UNF, and UNEF. The constant pitch series is specified by writing the number before the form designation (4, 6, 8, 12, 16, 20, 28, 32).

• Coarse series fasteners are used for quick assembly or disassembly of cast iron, soft metals, and plastic, and are designated NC or UNC.
• Fine series fasteners are used when a great deal of force is necessary for assembly, and are designated NF or UNF. These fasteners are used extensively in the aerospace and automotive industries.
• Extra fine series fasteners are used when the length of engagement is short and the application calls for high degrees of stress.
• Constant pitch series threads are for special purposes, such as large-diameter or high-pressure environments. It is used when the Coarse, Fine, Extra-Fine Series do not meet the design requirements, and within these series, preference should be given to the 8, 12, and 16 thread series.

Understanding Thread Class 1, Class 2 and Class 3 Fits

• Class 1 – a loose fit where quick assembly is required and looseness or play between parts is acceptable.
• Class 2 – a high-quality, general purpose, commercial class of fit for bolts, nuts, and screws widely used in mass production.
• Class 3 – a very high-quality threaded fastener with a close fit, used for precision tools and for high stress and vibration applications.

How to Read Fastener Thread Callouts on Drawings

Threads are only symbolically represented on drawings; therefore, thread notes are needed to provide the required information. A thread note must be included on all threaded parts, with a leader line to the external thread or to an internal thread in the circular view.

External thread notes are given in the longtitudinal view. Internal thread notes are given on the end view, with a pointer to the solid circle. 

A thread Callouts should contain the following information:

1. Major diameter in fraction or three place decimal form.
2. Number of threads per inch, followed by a space.
3. Thread form designation.
4. Thread series designation.
5. Thread class designation (1,2, or 3).
6. Internal or external symbol (A is for external threads, B is for internal threads), followed by a space.
7. Qualifying information, such as:
   1. LH for left hand threads. If the thread is right-hand, RH is omitted.
   1. DOUBLE or TRIPLE for multiple threads.
   1. Thread length.
   1. Material.

Understanding Metric Thread Designations

Metric thread specifications are based on ISO recommendations and are similar to the Unified standard. The basic designation for a metric thread are:

Here, the note specifies that the thread is metric (M), the diameter of the thread is 16 millimeters, followed by the multiplication sign ‘x’, and the pitch is 1.5 millimeters.

Generally, a complete metric thread callouts should contain the following information:

• Thread form symbol. The letter M is used to designate the metric profile. The J profile class is a modified M profile.
• Nominal size (basic major diameter) in millimeters, followed by an ‘x’.
• Pitch in millimeters, followed by a dash. The pitch can be eliminated for coarse threads, but it is preferred in the American use of the standards.
• General purpose tolerance. The tolerance class designation includes:
• Pitch diameter tolerance: grade, position
• Minor diameter tolerance: grade, position.

Diagram: Internal thread; External thread.

For external threads tolerance lowercase letters are used, for internal threads – uppercase letters.

Screw Thread Representation in Engineering Drawings

Two types of conventions are in general use for screw thread representation, conventional and alternative (pictorial) representation.

Conventional Representation should be used whenever it communicates the required information without confusion, as it requires the least amount of drafting effort.

Diagram: Internal thread; External thread.

This method is independent of the type of screw thread. The type of screw thread and its dimensions have to be indicated.

The alternative representation requires more drafting time but is sometimes necessary to avoid confusion with other parallel lines, or to more clearly portray particular aspects of the thread. This is a close approximation to the actual appearance of the screw thread:

It is simplified, so that crest and roots for full threads are shown sharp, with single straight lines instead of the double curved lines that would be required for the flat crests and roots.

Alternative representation should be used only for enlarged detail and other special applications.

Threaded Assembly Drawing Standards

For general use, the conventional representation for assemblies of threaded parts is recommended.

In sectional views, the externally threaded part is always shown covering the internally threaded part (bolt thread is shown, hole thread is nut shown).

Both methods can be used simultaneously on the same drawing.

Thread Manufacturing Methods for Fasteners

1.Thread Rolling

External threads may be cut using a Die.

A Tap is used to cut smaller diameter internal threads.

However, there is also a type of threads which is not being cut, but rather being rolled.

Rolled threaded products are often made with a reduced body diameter, approximately equal to the pitch diameter. When it is necessary to show this, the feature may be drawn as shown:

Diagram: Blank diameter; Major diameter of thread; Pitch diameter.

Here the thread is depicted bigger than the diameter of the part (both, in conventional and in alternative representations).

2.Thread Cutting

Thread cutting removes material to create the thread profile.

Advantages:

• Suitable for custom threads
• Flexible for small production quantities

Applications:

• Special fasteners
• Prototype components

3.CNC Machined Threads

For custom OEM fasteners:

CNC machining supports:

• Special thread profiles
• Custom dimensions
• Complex components

Standard thread specifications do not always meet every engineering requirement.

Welle Fasteners provides custom thread manufacturing solutions including:

Special thread profiles

Left-hand threads

Custom pitch requirements

Metric and inch thread systems

CNC machined fasteners

Our engineers review drawings, specifications and application requirements to provide suitable manufacturing solutions.

Welle Fasteners Custom Thread Manufacturing Capability

Welle Fasteners supports customized thread solutions for industrial applications.

Our capabilities include:

• Metric threads
• UNC / UNF threads
• Special threads
• Left-hand threads
• Custom studs
• CNC machined fasteners

We manufacture stainless steel fasteners according to:

• Customer drawings
• International standards
• OEM specifications

Applications include:

• Construction
• Marine equipment
• Chemical processing
• Food machinery
• Industrial equipment

Related Products

Stainless Steel Studs

Threaded studs for demanding engineering environments.

Stainless Steel Bolts

Manufactured according to customer specifications and international standards.

Custom Stainless Steel Fasteners

OEM fasteners produced according to drawings and specifications.

Need Custom Thread Fasteners for Your OEM Project?

Welle Fasteners manufactures stainless steel bolts, studs, screws and custom fasteners according to customer drawings, international standards and specific engineering requirements.

Our capabilities include:

✓ Metric thread manufacturing
✓ UNC / UNF thread solutions
✓ Special thread machining
✓ Custom dimensions and tolerances
✓ OEM production support

Send your drawing or specification today for engineering evaluation and quotation.

Technical Disclaimer

Technical information provided in this guide is based on international standards, industry references and manufacturing experience. Final thread specifications should always be confirmed according to application requirements, engineering drawings and customer standards.