Bolts are among the most widely used mechanical fastening components. They are designed to securely connect different parts, structures, and equipment components.

Bolts are commonly used in construction, industrial equipment, machinery, automotive systems, manufacturing, and general assembly applications.

A typical bolt has external threads and is used together with a compatible nut. The bolt passes through aligned holes in the components being joined and is tightened with a nut. This creates clamping force that holds the components securely together.

In this guide, we will explore the main components of bolts, additional fastening elements, important bolt terminology, thread specifications, and metric bolt sizing.

Main Components of a Bolt

A bolt consists of several key parts, including the shaft, threaded section, and head.

Bolt Shaft and Shank

The shank is the smooth, unthreaded section of the bolt located beneath the head.

It helps maintain alignment between the connected components and can resist shear forces. The smooth surface also provides accurate contact within the bolt hole and can contribute to more even load distribution.

For assemblies exposed to vibration, dynamic loads, or repeated impact, selecting the appropriate bolt design is essential for maintaining joint stability.

Bolt Thread

The thread is the continuous spiral profile formed around the outside of the bolt shaft.

It engages with the internal thread of the nut to create the required clamping force. Depending on the application, bolts may be fully or partially threaded.

The thread diameter, pitch, and compatibility with the nut should always be considered when selecting a bolt.

Types of Bolt Heads

The bolt head is designed to receive torque during installation and removal. Different head designs are available depending on installation requirements, available space, and the tools used.

Hex Bolt Head

Hex-head bolts are among the most common bolt types. They can easily be tightened or loosened using a wrench or socket.

They are widely used in construction, machinery, industrial equipment, and general fastening applications.

Countersunk Head

Countersunk bolt heads have a conical shape and can sit nearly flush with the surface after installation.

They are suitable for applications where a smooth surface and minimal protrusion are required.

Dome Head

Dome-head bolts have a smooth, rounded profile. They provide a cleaner appearance and reduce the possibility of snagging on exposed fasteners.

Additional Components of a Bolted Joint

Bolts are generally used together with nuts, washers, and other fastening components.

Nuts

A nut has an internal thread and works with the external thread of the bolt to create clamping force.

When tightened, the nut compresses the connected components and helps secure the joint. In applications subject to vibration, locking nuts, lock washers, and other anti-loosening solutions may be used.

Washers

Washers are placed under the bolt head or nut.

Their main functions include:

• Distributing the load over a larger surface area
• Reducing the risk of surface deformation
• Protecting the connected material
• Improving joint stability

Depending on the application, flat washers, lock washers, and specialized sealing washers may be used.

Important Bolt Terminology

Understanding bolt terminology is essential when selecting the correct fastening component.

Shank Diameter

The shank diameter is the diameter of the smooth, unthreaded section of the bolt. It is an important measurement when determining bolt fit and dimensional compatibility.

Major Diameter

The major diameter is the largest diameter of the bolt thread, measured between opposite thread crests. It generally corresponds to the nominal bolt diameter.

Minor Diameter

The minor diameter, also known as the root diameter, is the smallest diameter of the bolt thread, measured between opposite thread roots.

Pitch Diameter

The pitch diameter is the theoretical diameter located between the major and minor diameters. It is an important parameter for proper thread engagement and compatibility.

Nominal Length

Nominal length refers to the specified length of the bolt and should be selected according to the thickness of the connected materials and the requirements of the joint.

Thread Pitch

Thread pitch is the axial distance between corresponding points on two adjacent threads.

For metric fasteners, thread pitch is normally expressed in millimetres.

How to Read Metric Bolt Sizes

Metric bolt dimensions are commonly written using the following format:

M + nominal diameter + thread pitch + length

For example:

M10-1.0 × 20

This specification means:

• M — metric thread system
• 10 — nominal diameter of 10 mm
• 1.0 — thread pitch of 1.0 mm
• 20 — bolt length of 20 mm

Another example is:

M12 × 50

This indicates a nominal diameter of 12 mm and a bolt length of 50 mm.

If the thread pitch is not separately specified, the applicable standard and product specification should be checked to determine the exact thread type.

Fine Threads vs. Coarse Threads

Thread pitch is an important factor when selecting a bolt.

Fine Threads

Fine threads have a smaller distance between individual threads. They can be suitable for precision fastening, certain high-strength applications, and situations where fine adjustment is required.

Coarse Threads

Coarse threads have a larger pitch and are generally easier to install. In some applications, they are also less susceptible to cross-threading and are commonly used for general-purpose fastening and softer materials.

How to Choose the Right Bolt?

When selecting a bolt, consider the following factors:

• Diameter
• Length
• Thread pitch
• Thread type
• Material
• Strength grade
• Required load capacity
• Corrosion resistance
• Operating environment
• Compatibility with nuts and washers
• Applicable standards

Choosing the correct bolt helps ensure a secure, reliable, and long-lasting mechanical joint.

Conclusion

Bolts are essential fastening components used across construction, industrial, automotive, machinery, and assembly applications.

The bolt head, shaft, shank, and thread each perform important functions, while nuts and washers help create and maintain the required clamping force.

When selecting a bolt, it is important to consider more than just its dimensions. Thread pitch, material, strength grade, load capacity, corrosion resistance, and operating conditions should all be evaluated to achieve a safe, durable, and reliable bolted connection.