Metric Bolts, Screws and Nuts: Selection and Specification: Recommended Bolt and Nut Strength Grade Combinations

Recommended Bolt and Nut Strength Grade Combinations

Rule: Nuts of a higher strength grade may always be substituted for nuts of a lower strength grade. Never the reverse.

Finish Specifications

• General metric screws and bolts (except socket head cap screws): Furnished with natural (as processed) finish, unplated or uncoated unless otherwise specified
• Alloy steel socket head cap screws: Furnished with an oiled black oxide coating (thermal or chemical) unless a protective plating or coating is specified by the purchaser

Metric Screw and Bolt Designation: How to Order Correctly

Getting the designation right eliminates ambiguity in procurement. Here is the exact format:

Standard Metric Screws and Bolts

Format: Product name, nominal diameter × thread pitch × nominal length, property class or material, protective coating (if required)

Examples:

• Hex cap screw, M10 × 1.5 × 50, class 9.8, zinc plated
• Heavy hex structural bolt, M24 × 3 × 80, ASTM A490M
• Hex lag screw, 6 × 35, silicon bronze

Socket Head Cap Screws (Metric Series)

Format: ANSI Standard number — nominal size × thread pitch × nominal length, product name, material and property class, protective finish (if required)

Examples:

• B18.3.1M—6 × 1 × 20 Hexagon Socket Head Cap Screw, Alloy Steel
• B18.3.1M—10 × 1.5 × 40 SHCS, Alloy Steel Zinc Plated

ISO convention note: It is common ISO practice to omit the thread pitch from the product size designation when screws use the metric coarse thread series. For example, "M10" stands for "M10 × 1.5."

Socket Head Cap Screws (Metric Series) — ANSI/ASME B18.3.1M

Socket head cap screws are the precision fastener of choice for machine tool assemblies, jigs, fixtures, and any application where high strength, compact head profile, and precise clamping are required. They use an internal hexagon (Allen) drive.

Dimensional Reference Table

All dimensions in millimeters.

ᵃ The M14 × 2 size is not recommended for use in new designs.

Thread Tolerance for Socket Head Cap Screws

Socket head cap screws use tolerance class 4g6g (tighter than the 6g class used for general hex screws and bolts). For plated screws, the allowance "g" may be consumed by plating thickness, so the maximum limit after plating becomes tolerance class 4h6h.

Clearance Holes for Metric Hex Screws and Bolts

Proper clearance hole sizing is essential for assembly, alignment, and joint integrity. Three categories exist:

• Close Clearance: For critical alignment, thin walls, or other tight-tolerance applications. Countersinking may be required at the entry side to ensure proper head seating.
• Normal Clearance (Preferred): For general-purpose applications. Specify this unless design conditions dictate otherwise.
• Loose Clearance: For maximum adjustment capability during assembly.

Master Clearance Hole Table

All dimensions in millimeters (minimum hole diameters).

ᵃ Applies only to heavy hex structural bolts.

Recommended Hole Tolerances

Does not apply to: hex lag screws, hex socket head cap screws, or round head square neck bolts. These have separate clearance hole specifications.

Drill and Counterbore Sizes for Metric Socket Head Cap Screws

When installing socket head cap screws, you need precisely sized clearance holes, counterbores for head recessing, and countersinks for fillet clearance.

Best practice: It is considered good practice to countersink or break the edges of holes which are smaller than the maximum body diameter in parts having a hardness which approaches, equals, or exceeds the screw hardness. If such holes are not countersunk, the heads of screws may not seat properly or the sharp edges on holes may deform the fillets on screws, thereby making them susceptible to fatigue in applications involving dynamic loading.

Thread Series and Tolerance Classes

Standard Thread Series

Metric screws and bolts (except hex lag screws) use the ISO metric coarse thread series as specified in ANSI B1.13M.

• General hex screws and bolts: Tolerance class 6g (applies to plain finish fasteners and to plated/coated fasteners before treatment)
• Socket head cap screws: Tolerance class 4g6g (tighter tolerances for precision applications)
• Plated fasteners with additive finish: The 6g diameters may be exceeded by the amount of the allowance—i.e., the basic diameters apply after plating or coating
• Plated socket head cap screws: The allowance "g" may be consumed by plating thickness, resulting in maximum tolerance class 4h6h after plating

Metric Hex Lag Screws

Hex lag screws use a special thread (not the standard ISO metric coarse pitch). This thread is covered specifically in the ANSI B18.2.3.8M standard and is designed for self-tapping engagement in wood.

Identification Symbols: Reading the Head Markings

Metric screws and bolts are identified on the top of the head by two marks:

1. Property class symbol (the numerical grade, such as 8.8 or 10.9)
2. Manufacturer's identification symbol (a registered trademark identifying the producer)

This marking system is mandatory for steel bolts, screws, and nuts of 6 mm diameter and larger manufactured to strength grade designations 8.8 (for bolts/screws) and 8 (for nuts) or higher.

Bolts and screws are identified as ISO metric by either of the symbols "ISO M" or "M", embossed or indented on the top of the head.

Why Head Markings Matter

Remember the practitioner's bridge? The substitute bolts he found had no clear property class markings—or markings that were illegible from wear. This is a red flag.

Counterfeit fasteners are a documented and dangerous problem in the global supply chain. Fasteners that bear markings identifying them as belonging to a specific grade or property class are counterfeit if they do not meet the standards established for that class. Detection is difficult because counterfeits look genuine. The only certain verification method is laboratory testing of hardness, elongation, ultimate loading, and chemical composition.

Protect yourself: Always purchase from reputable distributors who can provide mill certifications and test reports. For critical applications, independent testing is prudent.

Drilled Head Dimensions for Socket Head Cap Screws

Drilled head socket cap screws are used in applications requiring safety wiring (lock wiring) to prevent fastener loosening under vibration. The drilled holes accept lock wire routed between multiple fasteners.

Key Specifications

• Available range: M3 through M36
• M3 and M4: Two drilled holes spaced 180° apart
• M5 and larger: Six drilled holes spaced 60° apart (unless purchaser specifies two holes)
• Hole alignment: The alignment check plug must pass completely through the head without deflection
• Optional chamfer: Edges of holes on the outside surface may be chamfered 45° to a depth of 0.30 to 0.50 mm when specified

Drilled Head Dimensional Reference

All dimensions in millimeters.

Diameter-Length Combinations: What Is Actually Available

Not every combination of diameter and length exists as a standard product. The ANSI standards define recommended diameter-length combinations for each fastener type.

General Rules

• Hex bolts in sizes M5 through M24 and heavy hex bolts in sizes M12 through M24 are standard only in lengths longer than 150 mm or 10D, whichever is shorter
• When shorter lengths of these sizes are ordered, hex cap screws are normally supplied in place of hex bolts, and heavy hex screws in place of heavy hex bolts
• Hex bolts M30 and larger and heavy hex bolts M30 and M36 are standard in all lengths
• Formed hex screws, hex flange screws, and heavy hex flange screws: Recommended lengths do not extend above 150 mm
• Heavy hex screws: Recommended lengths do not extend below 20 mm

Thread Length Rule for Round Head Square Neck Bolts

Basic thread lengths for metric round head square neck bolts:

Metric Nuts: Complete Specification Guide

Hex Nuts, Styles 1 and 2

The two styles of metric hex nuts differ in height and mechanical properties:

• Style 1 (ANSI B18.2.4.1M): Standard height, matched with ISO 4032
• Style 2 (ANSI B18.2.4.2M): Slightly higher, matched with ISO 4033, provides greater resistance to stripping

Metric Heavy Hex Nuts (ANSI B18.2.4.6M)

Heavy hex nuts are wider across flats than standard hex nuts and are available from M12 through M100.

Metric Hex Jam Nuts (ANSI B18.2.4.5M)

Jam nuts are thinner than standard hex nuts and are used as lock nuts in combination with a standard nut, or alone in low-load applications.

Nut Bearing Surfaces and Tops

• Hex nuts (Styles 1 and 2), slotted hex nuts, and hex jam nuts: Double chamfered in sizes M16 and smaller. In M20 and larger, either double chamfered or washer-faced bearing surface with chamfered top, at manufacturer's option
• Heavy hex nuts: Optional either way in all sizes
• Hex flange nuts: Flange bearing surface with chamfered top
• Prevailing-torque hex nuts: Chamfered bearing surface
• All metric nuts: Tapped hole countersunk on bearing face

Metric Nut Materials and Property Classes

Metric Plain Washers — ANSI B18.22M

Three Series Available

Metric plain washers come in three series based on outside diameter:

• Narrow series: Smallest outside diameter relative to nominal size
• Regular series: Standard outside diameter for general-purpose use
• Wide series: Largest outside diameter for maximum bearing area

Types and Applications

• Soft (as fabricated) washers: Available in sizes 1.6 mm through 36 mm. Used in low-strength applications to distribute bearing load, provide a uniform bearing surface, and prevent marring
• Hardened steel washers: Available in sizes 6 mm through 36 mm (narrow and regular series only). Hardened to 38–45 Rockwell C. Intended for high-strength joints to minimize embedment, provide uniform bearing surface, and bridge large clearance holes or slots

Metric Plain Washer Designation

Format: Description, nominal size, series, material type, finish (if required)

Examples:

• Plain washer, 6 mm, narrow, soft, steel, zinc plated
• Plain washer, 10 mm, regular, hardened steel

Representative Washer Dimensions

All dimensions in millimeters.

ISO note: The washers covered by this ANSI Standard are nominally similar to those in ISO documents. Outside diameters were selected from ISO/TC2/WG6/N47 where possible. Thicknesses are similar to nominal ISO thicknesses, but tolerances differ. Inside diameters also differ.

Quick-Reference Decision Framework

Choosing the Right Metric Fastener Type

Your Next Step

You have just read what amounts to a portable reference library for metric fasteners. But knowledge without application is just information.

Here is what to do now:

1. Audit your fastener inventory. Pull samples from your bins and verify the head markings match what your drawings specify. If you find unmarked fasteners in bins labeled for Class 8.8 or higher, investigate immediately.

2. Review your clearance hole specifications. Are you defaulting to "loose" when "normal" would suffice? Are you using the correct table for your fastener type? Socket head cap screws have different clearance requirements than hex bolts.

3. Verify your bolt-nut grade matching. A Class 8.8 bolt paired with a Class 4 nut is a joint waiting to strip. Use the combination table in this guide to confirm every match.

4. Bookmark this guide. The next time you are writing a purchase order, specifying a joint, or inspecting incoming material, the answers are here.

What is the most common metric fastener mistake you have encountered on the shop floor or job site?

The answer to that question—your answer—might save someone's bridge, machine, or career.

All specifications referenced in this guide are based on ANSI/ASME B18 series standards coordinated with ISO international standards. For complete manufacturing and acceptance specifications, reference should be made to the respective governing standards, available from the American National Standards Institute. Manufacturers should be consulted concerning items and sizes currently in stock production.

Context and scope

A complete engineering reference covering every metric nut type — from hex nuts and slotted hex nuts to prevailing-torque designs and hex flange nuts — with full ANSI/ISO dimensional tables, property class breakdowns, and real-world selection strategies.

The Assembly That Nearly Ended a Career

the practitioner stared at the shipping manifest on her desk and felt her stomach drop.

Forty-eight industrial gearbox housings — each one assembled with metric hex nuts she had personally approved — were being recalled from three different countries. The customer reports all described the same failure: nuts backing off under vibration, clamping force lost, bolted joints opening up in service.

The root cause wasn't dramatic. No exotic metallurgical failure. No manufacturing defect in the castings. The problem was painfully simple: the practitioner's team had specified Style 1 metric hex nuts where Style 2 nuts were required, used property class 5 nuts on property class 10.9 bolts, and installed standard hex nuts in a vibration-critical application where prevailing-torque nuts were mandatory.

Three specification errors. Forty-eight assemblies. A recall cost that exceeded her department's entire quarterly budget.

the practitioner wasn't incompetent. She held a mechanical engineering degree and had five years of fastener experience. But metric nut standards are a labyrinth of overlapping ANSI designations, ISO cross-references, property class hierarchies, and dimensional subtleties that punish even experienced engineers who don't know exactly where to look.

This guide exists so you never become the practitioner. What follows is the most comprehensive metric nut reference you will find anywhere — every nut type, every dimension, every property class, every selection rule — organized so you can find exactly what you need in seconds and apply it with confidence for decades.

What You Will Master in This Guide

• Metric Nut Fundamentals — How ANSI and ISO standards align (and where they diverge)
• Tops, Bearing Surfaces, and Chamfer Rules — The geometry details that determine joint performance
• Mechanical Properties and Property Classes — How to match nuts to bolts without guessing
• Hex Nuts, Styles 1 and 2 — Complete dimensional tables with every tolerance
• Slotted Hex Nuts — When and why you need castellation
• Thread Series and Tolerance Rules — The 6H standard and what happens with plated fasteners
• Hex Flange Nuts — Built-in bearing surfaces and their advantages
• Prevailing-Torque Type Nuts — The three designs that resist vibration loosening
• Hex Jam Nuts and Heavy Hex Nuts — Locking companions and structural workhorses
• Metric Nut Identification Symbols — Reading property class markings like a forensic engineer
• Metric Nut Designation System — How to specify any metric nut unambiguously on a drawing or purchase order

Metric Nuts and the Standards That Govern Them

The American National Standards covering metric nuts were developed in cooperation with the Department of Defense, ensuring they could serve both commercial and military procurement. These standards are your primary dimensional and material authority for metric nuts used in North American manufacturing — and they are carefully coordinated with international ISO standards to ensure global interchangeability.

The governing standards and their ISO counterparts are:

Comparison with ISO Standards

American National Standards for metric nuts have been coordinated to the maximum extent possible with comparable ISO Standards. The dimensional differences between each ANSI Standard and its comparable ISO Standard are very few, relatively minor, and none affect the interchangeability of nuts manufactured to the requirements of either system.

This is an engineering achievement worth appreciating. In May 1977, ISO Technical Committee TC2 convened in Varna to study technical reports analyzing the optimal widths across flats for hex bolts, screws, and nuts. The primary technical objective was to achieve a logical ratio between the nut bearing surface area (which governs compressive stress on bolted members) and the tensile stress area of the screw thread (which governs the clamping force developed by tightening).

The widths across flats in ANSI Standards agree with those selected by ISO/TC2 to become ISO Standards — with one exception.

The M10 Exception You Must Know

For metric hex nuts (Styles 1 and 2), metric slotted hex nuts, metric hex jam nuts, and prevailing-torque metric hex nuts, the M10 size presents a critical ordering consideration:

M10 nuts are currently produced in the United States with a width across flats of 15 mm. This width is NOT an ISO Standard. Unless M10 nuts with 15 mm width across flats are specifically ordered, the M10 size with 16 mm width across flats will be furnished.

This means if you are maintaining equipment originally assembled with 15 mm M10 nuts and you reorder without specifying, you will receive 16 mm nuts — which require a different wrench size and have different bearing surface characteristics. Always specify the width across flats when ordering M10 metric nuts.