How to Read Fastener Specifications: Threads, Grades, Lengths and More

Have you ever looked closely at the markings on the head of a fastener? Those tiny details identify the manufacturer, material, and dimensions. Fastener specifications ensure traceability, help identify parts outside of their original packaging, and allow defects to be linked back to their source.

At first glance, these specifications can seem like a confusing code. This guide will help you understand the basics so you can confidently choose the right fastener for your project.

Note: In Australia, fasteners should comply with AS and AS/NZS Standards. All measurements are given in metric units.

Length and Diameter Measurements

The first part of any fastener specification usually includes length and diameter. Here’s how length is measured:

• Standard bolts: from the underside of the head to the tip of the threaded end.
• Countersunk bolts: from the top of the head to the tip of the threaded end.
• Socket shoulder screws: from the underside of the head to the end of the unthreaded shoulder.

Diameter is measured across the widest point of the thread using a calliper or bolt gauge. Common bolt dimensions are defined in AS1110/1111.

Threads

The thread is the spiral ridge that wraps around a bolt, turning rotation into the clamping force that holds materials together. External threads are on bolts, while internal threads are in nuts or tapped holes.

Coarse threads have fewer threads per length, making them faster to install and resistant to stripping. Fine threads have more threads per length, offering stronger holding power. Thread forms and pitches are defined in AS1275.

Understanding Fastener Grades

Grades indicate the material strength and hardness of a fastener. A higher grade means higher tensile strength, but it must be matched carefully to the project to prevent overtightening or damage.

In the metric system, grades are expressed as two numbers (e.g., 8.8):

• The first digit × 100 = tensile strength in MPa.
• The second digit = ratio of yield strength to tensile strength.

A grade of 8.8 or higher is considered high tensile strength. Relevant standards include AS1110 (precision bolts), AS1111 (commercial bolts), and AS/NZS1252 (structural bolts).

Materials

Fasteners can be made from steel, stainless steel, brass, or engineered polymers. Steel is the most common thanks to its strength and low cost, and it’s often zinc or chrome plated for corrosion resistance or aesthetics. In coastal or outdoor environments, the right coating is especially important. Requirements for coatings are outlined in AS/NZS1214.

Drives and Heads

The drive refers to the tool needed for installation, such as Phillips, slotted, or hex. The head refers to the bolt shape, such as pan, flat, or truss. Some fasteners (e.g., hex bolts) imply their drive and head shape without additional notation. AS/NZS1428.1 defines dimensional requirements for fastener heads.

A Worked Example

Here’s how to read a typical fastener specification:

M10 × 50mm Hexagon Bolt Head, Grade 8.8, Zinc-Plated, AS1275 compliant

• M10 = 10mm thread diameter
• 50mm = bolt length
• Hexagon Bolt Head = type and head shape
• Grade 8.8 = high tensile strength steel
• Zinc-Plated = corrosion-resistant finish
• AS1275 compliant = meets required Australian Standards

Conclusion

Once you understand the basics, fastener specifications are straightforward. Always choose products that comply with Australian Standards to guarantee quality, safety, and performance.

At Bricon Industries, we are one of Australia’s trusted bulk fastener suppliers. With decades of family-owned experience, we provide reliable products and expert advice. Contact our team today for guidance in selecting the right fasteners for your project.