Washers: Selection, Standards and Joint Function: BS 4320 Black Metal Washers

BS 4320 Black Metal Washers — Extra Large Diameter (Form G)

All dimensions in millimeters.

Comparison insight: Notice how the Form G (extra large) M24 washer has an OD of 72 mm — compared to just 44 mm for the Form E (normal) at the same bolt size. That is nearly 2.7 times the bearing area, which can be the difference between a stable joint and one that sinks into the base material.

BS 4320 Washer Designation System

How to Order British Standard Washers

The BS 4320 standard specifies that ordering designations should include:

1. General description — Bright or Black washers
2. Nominal bolt/screw size — e.g., M5, M12
3. Form designation — Form A through Form G
4. Chamfer specification — if required (bright washers only)
5. Standard number — BS 4320:1968 (1998)
6. Coating information — if required, with the appropriate British Standard number and coating thickness

Designation Examples

Bright washer, normal diameter, normal thickness, chamfered, for M12 bolt:

Bright washers M12 (Form A) chamfered to B.S. 4320

Bright washer, large diameter, light thickness, for M16 bolt:

Bright washers M16 (Form D) to B.S. 4320

Black washer, extra large diameter, for M20 bolt, zinc plated:

Black washers M20 (Form G) to B.S. 4320, zinc plated to B.S. 1706

BS 4320 Form Quick-Reference Matrix

British Standard Metric Spring Washers — BS 4464:1969 (1998)

When the practitioner Needed More Than a Flat Washer

The hydraulic press was fixed. The European contract was on track. But the practitioner's next challenge involved something flat washers couldn't solve: vibration-induced loosening.

A conveyor drive assembly with M12 bolts was losing preload over weeks of continuous operation. The vibration was well within design limits, but the bolted joints were slowly walking loose. the practitioner needed spring washers — and she needed to understand the different types available in the metric series.

BS 4464:1969 (1998) covers several types of metric spring washers, each with distinct characteristics:

Spring Washer Types at a Glance

BS 4464 Single Coil Rectangular Section Spring Washers — Types B and BP

All dimensions in millimeters. Sizes in parentheses are non-preferred.

BS 4464 Double Coil Rectangular Section Spring Washers — Type D

The double coil design provides approximately twice the spring force of a single coil washer at the same nominal size. The free height before compression is normally approximately five times the thickness, though washers with other free heights may be obtained by arrangement with the manufacturer.

All dimensions in millimeters. Sizes in parentheses are non-preferred.

Metric Washer Selection Decision Framework

The 5-Question Washer Selection Process

After her year of hard lessons, the practitioner developed a systematic approach that she now teaches to every junior engineer on her team. Before specifying any metric washer, answer these five questions in order:

Question 1: What is the bolt property class?

Question 2: What is the base material?

Question 3: Is vibration present?

Question 4: Which standard governs the project?

Question 5: Are there special clearance requirements?

If the bolt hole is oversized or slotted, always use a washer with sufficient OD to fully bridge the hole with at least 2 mm of bearing surface beyond the hole edge on all sides.

Cross-Standard Comparison: ANSI vs. BS 4320 vs. ISO

Dimensional Comparison at Common Sizes

To illustrate the differences that can trip up engineers working across standards, here is a comparison of M12 washers across the three major systems:

Key observation: The ANSI "Regular" M12 washer has an OD of 34 mm — 10 mm larger than the BS 4320 Form A at 24 mm. These are not interchangeable in applications where bearing area is critical. Always verify dimensional compatibility when substituting across standards.

Improvement method and result

Eighteen months after the hydraulic press incident, the practitioner stood before her engineering team with a new company standard: WSP-001 — Metric Washer Selection and Specification Protocol.

The document codified everything she had learned through expensive failures and late-night research. It required:

• Hardened washers on every high-strength joint — no exceptions, no substitutions
• Explicit washer callouts on every drawing — not just the bolt and nut, but the washer type, series, material, hardness, and finish
• Incoming inspection of washer hardness — random Rockwell testing of every lot of hardened washers
• Cross-standard verification on international projects — formal compatibility checks documented in the joint design file

The protocol eliminated washer-related joint failures entirely. Not reduced — eliminated.

That's what happens when you stop treating washers as commodity items and start treating them as precision-engineered components of the bolted joint system.

Master Quick-Reference Card

ANSI B18.22M Series at a Glance

BS 4320 Forms at a Glance

Key Standards Reference

Your Next Step

You now have the most complete metric washer reference available in a single document. Every dimension, every tolerance, every form designation, every material specification — it's all here.

But knowledge without action is just trivia.

Here is what to do right now:

1. Audit your current washer specifications. Pull up your most recent assembly drawing. Is the washer fully specified — type, series, material, hardness, and finish? Or does it just say "washer" or "flat washer M12"? If it's the latter, you have a procurement substitution risk waiting to happen.

2. Verify your high-strength joints. Every bolt at property class 8.8 or above needs a hardened washer. Check your bills of materials. If you find soft washers paired with high-strength bolts, flag them immediately.

3. Bookmark this guide. The dimensional tables alone will save you hours of cross-referencing standards documents the next time you need to specify a washer for an international project.

The question is not whether you can afford to get washer selection right. The question is whether you can afford to get it wrong.

the practitioner learned that lesson at the cost of eleven days of production. You just learned it in the time it took to read this guide.

Use it.

What washer specification challenge are you dealing with right now? What joint design has you reaching for the standards book? The tables are here. The decision framework is here. The only thing left is your next assembly.

British Washers: The Definitive Engineering Guide to BS 4464 and BS 4320 Standards

The Bolt That Shattered a Reputation

the practitioner had built his fabrication shop over seventeen years. From a single lathe in a rented garage to a 400-square-metre facility supplying precision assemblies to marine and industrial clients across three continents, his name meant one thing: reliability.

That reputation nearly died on a Tuesday afternoon in May.

A client's offshore pump housing—assembled with M16 bolts and what the practitioner later discovered were incorrect washers—developed catastrophic joint loosening within ninety days of installation. The bolts hadn't failed. The threads were perfect. The torque values had been applied correctly.

The washers were wrong.

the practitioner had specified rectangular-section spring washers. His purchasing team, unfamiliar with British Standard designations, had substituted generic flat washers from an overseas supplier. No spring tension. No vibration resistance. No lock.

The repair bill exceeded what he'd earned on the entire contract. But the real cost was the six months he spent rebuilding trust with a client who had every right to walk away.

What the practitioner learned in the aftermath—and what this guide will teach you in the next thirty minutes—is that British Standard washers aren't interchangeable commodities. They're precision-engineered components governed by exact specifications, and choosing the wrong type, form, or dimension can turn a perfect assembly into a liability.

What You'll Master in This Guide

This is not a summary. This is a complete technical reference covering every British Standard washer type you'll encounter in professional engineering practice.

You'll walk away with:

• Complete dimension tables for every spring washer type under BS 4464:1969
• Full specifications for bright and black metal washers under BS 4320:1968
• Form selection logic (A through G) so you never specify the wrong washer again
• Material and finish requirements for both bright and black categories
• Designation protocols that ensure your purchase orders are unambiguous
• Decision frameworks for matching washer type to application

British Standard Spring Washers — BS 4464:1969 (1998)

Spring washers are the frontline defence against vibration-induced loosening. Unlike flat washers that only distribute load, spring washers actively resist rotation of the nut or bolt head by maintaining elastic tension in the joint.

BS 4464:1969 (confirmed in 1998) defines four distinct spring washer configurations, each engineered for specific load and vibration profiles. the practitioner's mistake was treating them as interchangeable. They are not.

The Four Types at a Glance

Understanding which type you need starts with understanding the section geometry.

Single Coil Rectangular Section Spring Washers — Types B and BP

BS 4464:1969 (1998)

These are the workhorses of the British spring washer family. The rectangular cross-section provides a high width-to-thickness ratio, which translates to greater contact area with the bearing surface and more predictable spring behaviour under compression.

Type B is the standard configuration. Type BP adds a pip (a small raised projection, dimension k) on the inner edge that bites into the bolt shank or bearing surface for additional anti-rotation resistance.

Key Engineering Characteristics

• Section shape: Rectangular (width b is greater than thickness s)
• Spring action: Elastic deflection resists bolt rotation under vibration
• Size range: M1.6 through M68
• Tolerance control: Width and thickness held to ±0.1 mm for sizes up to M5, scaling to ±0.25 mm for sizes above M30
• Pip feature (Type BP only): Available from M3.5 upward, dimension k ranges from 0.15 mm to 1.0 mm

How to Read This Table

The width (b) is the radial dimension of the washer's cross-section—how far it extends from the inner diameter to the outer diameter. The thickness (s) is the axial dimension—how tall the wire section stands. For rectangular sections, the width is always greater than the thickness, giving the washer a broad, stable footprint.

The pip dimension (k) on Type BP washers represents the maximum protrusion height of the locking pip. This feature is absent on sizes below M3.5 because the wire section is too small to form a reliable pip.

Double Coil Rectangular Section Spring Washers — Type D

BS 4464:1969 (1998)

When the practitioner rebuilt the failed pump housing assembly, his metallurgist recommended double coil washers for every joint exposed to continuous cyclic loading. The reason is straightforward: two coils provide approximately twice the elastic deflection range of a single coil, which means the washer can absorb greater axial movement without going flat.

The Critical Dimension: Free Height

The free height of double coil washers before compression is normally approximately five times the thickness. If your application requires a different free height, washers with alternative free heights can be obtained by arrangement with the manufacturer.

This is a specification detail that many engineers overlook. If you're designing for a joint with unusual deflection requirements, the standard explicitly permits custom free heights—but you must communicate this to your supplier.

Comparing Single Coil vs. Double Coil — Same Nominal Size

Notice how the dimensions differ between Type B (single coil) and Type D (double coil) for the same nominal bolt size. Taking M16 as an example:

The critical difference: The double coil washer has a thinner individual section (2 mm vs. 3.5 mm) but compensates with two coils, providing greater total deflection capacity. The smaller thickness tolerance (±0.1 mm vs. ±0.2 mm) on the Type D reflects the tighter manufacturing control needed when two coils must work in concert.

Single Coil Square Section Spring Washers — Types A-1 and A-2

BS 4464:1969 (1998)

Square-section spring washers represent the third major category under BS 4464. Where rectangular sections offer a broad contact footprint, square sections provide a more uniform spring rate because the width and thickness are equal. This makes them particularly suitable for applications where predictable, symmetrical deflection behaviour matters more than maximum bearing area.

The standard defines two sub-types:

• Type A-1: Standard square-section configuration
• Type A-2: Alternative square-section configuration with slightly different dimensional parameters

Both types share the same nominal size range and are specified in the metric series. Sizes shown in parentheses are non-preferred and are not usually stock sizes.

When to Choose Square Over Rectangular

The decision between square and rectangular section depends on your joint's primary failure mode:

British Standard for Metric Series Metal Washers — BS 4320:1968 (1998)

Now we shift from spring washers to flat metal washers—the load-distribution workhorses that sit between the fastener head (or nut) and the bearing surface. BS 4320:1968 (confirmed in 1998) divides these into two fundamental categories based on their manufacturing process and surface finish:

• Bright Metal Washers — precision machined, close tolerances
• Black Metal Washers — hot-formed, wider tolerances, heavier duty

the practitioner learned this distinction the hard way. After the pump housing failure, he instituted a purchasing protocol that required every washer order to specify not just the size, but the form designation, finish category, and applicable British Standard number. No exceptions.

Bright Metal Washers — BS 4320:1968 (1998)

Materials and Manufacturing

Bright metal washers are manufactured from one of two materials in the hard condition:

• CS4 cold-rolled strip steel to BS 1449: Part 3B
• CZ 108 brass strip to BS 2870:1980

By mutual agreement between purchaser and supplier, washers may be made available with the material in any other condition, or they may be made from another material, or may be coated with a protective or decorative finish to some appropriate British Standard.

Surface Quality Requirements

• Washers are reasonably flat and free from burrs
• Normally supplied unchamfered
• May have a 30-degree chamfer on one edge of the external diameter (if specified)

Form Designations — The Classification System

Bright washers are organised into four forms across two size categories and two thickness ranges:

The thickness relationship: A light-range washer (Form B or D) is from ½ to ⅔ the thickness of a normal-range washer (Form A or C).

Complete Dimension Table — Bright Metal Washers, Normal Diameter Sizes

All dimensions in millimeters. Nominal bolt/screw sizes in parentheses are non-preferred.

Note: Form B (light range) washers are not available below M6. For bolt sizes M1.0 through M5, only Form A (normal thickness) is specified.