How to Read Bolt Size Markings: A Guide to Identifying Industrial Fasteners

Whether you're replacing a worn fastener, repairing equipment, or organizing shop inventory, correctly identifying a bolt is essential. Bolt size markings provide important information about diameter, thread pitch, length, and strength, helping ensure you select the correct replacement for your application. Understanding how to read these markings can help prevent installation errors, reduce equipment downtime, and improve overall reliability.

Understanding the Basic Bolt Size Format

Bolt sizes are typically identified using either the inch (SAE) or metric measurement system. While the format differs slightly, both provide the information needed to identify the fastener.

Inch (SAE) Example

7/16"-20 x 3"

This marking provides three key measurements:

• 7/16" – Bolt diameter
• 20 – Threads per inch (TPI)
• 3" – Overall bolt length measured from beneath the head to the end of the bolt

In this example, 20 TPI indicates a UNF (Unified National Fine) thread.

A similar bolt marked 7/16"-14 x 3" would have a UNC (Unified National Coarse) thread.

Reading Metric Bolt Sizes

Metric fasteners use a different format.

Metric Example

M12-1.75 x 50

This identifies:

• M12 – Nominal thread diameter of 12 millimeters
• 1.75 – Thread pitch in millimeters between each thread
• 50 – Bolt length in millimeters

Unlike inch fasteners, metric threads are identified by thread pitch rather than threads per inch.

Understanding Strength Markings

The numbers or symbols stamped on a bolt head identify its strength classification.

Metric Fasteners

Common strength classes include:

• Class 8.8 – Medium-strength alloy steel for general industrial applications
• Class 10.9 – High-strength alloy steel for heavy-duty fastening applications

These markings are typically stamped directly onto the bolt head.

Inch Fasteners

SAE fasteners commonly use radial lines to indicate grade.

For example:

• Grade 2 – No radial lines
• Grade 5 – Three radial lines
• Grade 8 – Six radial lines

Always verify the required grade specified by the equipment manufacturer before replacing a fastener.

Don't Forget the Material

Bolt markings identify size and strength, but material and finish are equally important.

Common materials include:

• Alloy Steel
• Yellow Zinc-Plated Alloy Steel
• 18-8 Stainless Steel (AISI 304)

Selecting the appropriate material helps ensure the fastener performs reliably in its intended environment.

Verify Before You Install

Before installing any replacement fastener, confirm:

• System of measurement (SAE or Metric)
• Diameter
• Thread pitch or threads per inch
• Length
• Strength grade or class
• Material
• Finish
• Thread style (Full Thread or Partial Thread)

Taking a few moments to verify these specifications helps prevent damaged threads, improper clamping force, and premature fastener failure.

Choose the Right Fastener Every Time

Correctly identifying bolt size markings is one of the simplest ways to improve equipment reliability and reduce maintenance issues. Whether you're servicing industrial machinery, construction equipment, fleet vehicles, or manufacturing systems, understanding fastener specifications helps ensure safe, dependable assemblies.

Industrial Assortments offers a wide selection of SAE and metric fasteners, organized assortment kits, and industrial storage solutions to help professionals quickly identify, store, and select the right hardware for every job.