Metric vs Imperial Fasteners

What is the difference?

Metric fasteners use millimeters for diameter and thread pitch (for example, M8 x 1.25). Imperial fasteners use inch-based diameters and threads per inch (for example, 5/16-18 UNC). Mixing systems without conversion leads to cross-threading and joint failure.

Metric vs Imperial comparison chart

Common size equivalents for quick reference. Use a screw converter tool for exact pitch matching.

When to use metric fasteners

• Most modern vehicles and imported machinery
• ISO-standardized manufacturing environments
• Builds where metric tools are already standard

When to use imperial fasteners

• Legacy equipment and North American systems
• UNC/UNF thread standards already specified in drawings
• Projects matching existing imperial hardware inventories

Best practices to prevent thread damage

• Always verify diameter and pitch before assembly
• Never force a near-match thread into place
• Use a tap drill chart before cutting internal threads
• Keep conversion references available; use our pitch to TPI converter when needed

FAQ

What is the difference between metric and imperial fasteners?

Metric uses millimeter diameter and pitch (e.g. M8 x 1.25). Imperial uses inch diameter and threads per inch (e.g. 5/16-18 UNC).

When should I use metric vs imperial bolts?

Use metric for modern vehicles and ISO machinery. Use imperial for legacy North American equipment and when specs call for UNC/UNF.

Can I substitute a metric bolt for an imperial one?

Do not substitute without verifying diameter and pitch. Use a conversion chart or tool to find correct equivalents and avoid thread damage.