Metric Bolts, Screws and Nuts: Selection and Specification: Complete Dimensional Table

Complete Dimensional Table — Metric Hex Jam Nuts

ANSI B18.2.4.5M-1979 (R1998) — All dimensions in millimeters

Jam Nut Thickness Comparison

To appreciate how thin jam nuts are compared to standard nuts, consider these ratios:

Jam nuts are consistently about 55–59% the thickness of the corresponding Style 1 hex nut.

Metric Heavy Hex Nuts

Heavy hex nuts feature larger widths across flats and greater thicknesses than standard hex nuts, providing more wrench engagement area and higher load capacity. They are specified primarily in structural steel connections and heavy industrial applications where:

• Higher installation torques are required
• Larger wrenches provide better leverage for field assembly
• Structural bolt standards (ASTM A325M, A490M) mandate heavy hex nut geometry
• Joint designs require maximum bearing surface area without a flange

Governing Standard: ANSI B18.2.4.6M-1979 (R1998)

Complete Dimensional Table — Metric Heavy Hex Nuts

ANSI B18.2.4.6M-1979 (R1998) — All dimensions in millimeters

Heavy Hex vs. Standard Hex — Width Comparison

The heavy hex series extends up to M100 × 6 — sizes far beyond the range of standard hex nuts — serving the largest structural and heavy industrial applications.

Metric Nut Identification Symbols

Being able to read the markings on a metric nut is a forensic skill that separates competent engineers from those who are guessing. These markings tell you the property class, the manufacturer, and by extension whether the nut is appropriate for its application.

Marking Requirements in the supplied reference

Reading the Property Class

The metric property class number is stamped or embossed directly on the nut face or flat. Unlike inch-series nuts (which use dot-and-line marking systems), metric nuts carry a numeric designation that directly indicates their proof load capability:

• Class 5 — Nonheat-treated, standard duty
• Class 9 — Nonheat-treated, high strength
• Class 10 — Heat-treated, high strength
• Class 12 — Heat-treated, maximum strength

If you cannot read the marking on a nut, do not assume its property class. Counterfeit fasteners are a documented problem in the industry, and the only reliable way to verify a nut's properties is through laboratory testing of hardness, elongation, and ultimate loading.

Metric Nut Designation System

The designation system for metric nuts follows a standardized sequence that ensures unambiguous specification on drawings, purchase orders, and bills of materials. Every engineer and procurement professional must master this format.

Designation Sequence

Metric nuts are designated by the following data, in this preferred sequence:

1. Product name (nut type and style)
2. Nominal diameter and thread pitch
3. Steel property class or material identification
4. Protective coating (if required)

Designation Examples

Hex nut, style 1, M10 × 1.5, ASTM A563M class 10, zinc plated

Heavy hex nut, M20 × 2.5, silicon bronze, ASTM F467, grade 651

Slotted hex nut, M20, ASTM A563M class 10

The Thread Pitch Convention

It is common practice in ISO Standards to omit the thread pitch from the product designation when the nut threads are the metric coarse thread series. For example:

• M10 stands for M10 × 1.5 (coarse series)
• M20 stands for M20 × 2.5 (coarse series)

When a nut has fine-series threads, the pitch must be explicitly stated. If you see a designation without a pitch value, assume coarse thread series.

Building Your Designation — A Decision Framework

Quick-Reference Decision Matrix

Use this table when you need to select a metric nut type quickly:

Your Next Step

You now hold the most comprehensive metric nut reference available in a single document. Every dimensional table, every property class assignment, every design rule you need to specify metric nuts with absolute confidence is here.

But knowledge without action is just data. Here is what you should do right now:

1. Audit your current fastener specifications. Pull the last three assemblies you specified or approved. Check every nut against the property class matching rules in this guide. Are your Style 1 and Style 2 assignments correct? Are you using prevailing-torque nuts where vibration is present?

2. Create your own M10 policy. Decide right now whether your organization defaults to 15 mm or 16 mm width across flats for M10 metric nuts, and document that decision so procurement never has to guess.

3. Build your designation template. Create a fill-in-the-blank template that forces complete metric nut designations on every drawing and purchase order. Incomplete designations cause specification drift that accumulates into failures.

4. Bookmark this guide. You will return to these tables. When you do, they will be exactly as complete as they are today — every tolerance, every exception, every rule waiting for you.

The difference between an engineer who specifies fasteners and an engineer who specifies them correctly is one catastrophic failure. Make sure your first time reading this guide is the last time you need to learn these lessons the hard way.

This guide references the following ANSI/ASME, ASTM, and ISO standards: ANSI/ASME B18.2.4.1M, B18.2.4.2M, B18.2.4.3M, B18.2.4.4M, B18.2.4.5M, B18.2.4.6M, B18.16.3M, ASTM A563M, ASTM F467M, ANSI B1.13M, ISO 4032, ISO 4033, ISO 4035, ISO 4161. Always verify current revision status before use in critical applications.