British and Legacy Thread Systems: ISO Metric Hexagon Slotted Nuts and Castle Nuts

ISO Metric Hexagon Slotted Nuts and Castle Nuts

Slotted nuts: Available in sizes M4 to M39 only (six slots)

Castle nuts: Available in sizes M12 to M39 (six slots) and M42 to M68 (eight slots)

Table: ISO Metric Precision Hexagon Slotted and Castle Nuts — BS 3692:1967

Nominal Lengths and Tolerances

Getting the length right sounds simple. It isn't. The nominal length of a bolt or screw is the distance from the underside of the head to the extreme end of the shank including any chamfer or radius. This is where errors accumulate if you don't know the tolerance bands.

ISO Metric Bolt and Screw Nominal Lengths — BS 3692:1967

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

Extended lengths (continued):

Bolt and Screw Ends

Ends may be finished with either:

• A 45-degree chamfer to a depth slightly exceeding the depth of thread, or
• A radius approximately equal to 1-1/4 times the nominal diameter of the shank

With rolled threads, the lead formed at the end by the thread rolling operation is considered sufficient chamfer.

Thread Length Formulas

Standard thread lengths for bolts (BS 3692):

Where $d$ = nominal diameter and $L_t$ = thread length.

Tolerance on bolt thread length: Plus two pitches for all diameters.

Screws: Threaded to permit a screw ring gage being screwed by hand to within a distance from the underside of the head not exceeding:

• 2.5 × pitch for diameters up to and including 52 mm
• 3.5 × pitch for diameters over 52 mm

Critical rule: Bolts that are too short for minimum thread lengths are threaded as screws and designated as screws.

Screw Thread Form and Tolerances

The screw threads conform to BS 3643:Part 1:1981 (1998) — ISO Metric Screw Threads, Principles and Basic Data. Tolerances are for the medium class of fit (6H/6g) as specified in BS 3643:Part 2:1981 (1998).

Chamfering, Washer Facing, and Countersinking

• Bolt and screw heads: Chamfer approximately 30° on upper faces; washer face or full bearing face on underside (manufacturer's option)
• Nuts: Countersunk at an included angle of 120° ± 10° at both ends of thread
• Countersink diameter: Shall not exceed the nominal major diameter of the thread plus 0.13 mm (up to 12 mm diameter) or plus 0.25 mm (above 12 mm diameter)
• This stipulation does not apply to slotted, castle, or thin nuts

Strength of Steel Bolts, Screws, and Nuts

This section cost the practitioner a second sleepless night. After solving his thread compatibility issue, he discovered that his replacement bolts — while dimensionally correct — were the wrong strength grade for the operating pressure. The two-figure grade designation system for British fasteners is elegant once you understand it, but cryptic if you don't.

Strength Grade Designation System for Steel Bolts and Screws

The system consists of two figures:

First figure = 1/10 of the minimum tensile strength in kgf/mm²

Second figure = 1/10 of the ratio between minimum yield stress (or stress at permanent set limit, $R_{0.2}$) and minimum tensile strength, expressed as a percentage

$$\text{Second figure} = \frac{1}{10} \times \frac{R_{0.2}}{\text{Minimum Tensile Strength}} \times 100$$

Example — Grade 8.8:

• First figure "8" → minimum tensile strength = 80 kgf/mm²
• Second figure "8" → yield-to-tensile ratio = 80%
• Therefore: minimum yield stress = 80 × 0.80 = 64 kgf/mm²

Complete Strength Grade Table for Bolts and Screws

Note: For grades 8.8 and above, the yield stress is replaced by the stress at permanent set limit ($R_{0.2}$), which is the stress at which the material exhibits 0.2% permanent deformation.

Strength Grade Designation for Steel Nuts

The nut designation is simpler — a single number equal to 1/10 of the specified proof load stress in kgf/mm².

The proof load stress corresponds to the minimum tensile strength of the highest grade of bolt or screw with which the nut can be used.

Recommended Bolt and Nut Combinations

This is the table that prevents joint failures. Always match or exceed the nut grade for the bolt grade in use.

Critical rule: Nuts of a higher strength grade may always be substituted for nuts of a lower strength grade. Never substitute downward.

Marking Requirements

Marking and identification are mandatory for:

• Steel bolts, screws, and nuts of 6 mm diameter and larger
• Manufactured to strength grade designations 8.8 (bolts/screws) and 8 (nuts) or higher

Bolt and screw identification:

• Marked with either "ISO M" or "M" embossed or indented on top of the head

Designation format:

"Bolts M10 × 50 to BS 3692 — 8.8"

This specifies: Bolts, 10 mm diameter, 50 mm long, steel strength grade 8.8.

"Brass screws M8 × 20 to BS 3692"

For non-steel fasteners, no strength grade is required in the designation.

"Nuts M12 to BS 3692 — 6, plated to BS 3382: Part 1"

For plated nuts, the coating standard and part are included.

The Clearance Hole System — BS 4186:1967

No fastener specification is complete without the clearance holes that receive them. British Standard BS 4186:1967 provides three series of clearance holes for ISO metric bolts and screws.

Three Fit Series

Clearance Hole Sizes (Selected Sizes)

These sizes comply with ISO Recommendation R273 and are designed to require the minimum number of drills across the full range.

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

Washer Categories

Light-range thickness is from 1/2 to 2/3 the thickness of a normal-range washer.

Washer characteristics:

• Reasonably flat and free from burrs
• Normally supplied unchamfered
• May have a 30° chamfer on one edge of the external diameter (optional)

Selected Bright Washer Dimensions (Form A — Normal)

Designation example: "Bright washers M12 (Form A) chamfered to B.S. 4320"

Quick-Reference Decision Framework

When to Use Which Standard

Master Bolt-Nut Pairing Card

Never install a bolt without confirming the nut grade matches or exceeds these pairings:

Grade 4.6 / 4.8 → Nut Grade 4 | Grade 5.6 / 5.8 → Nut Grade 5 | Grade 6.6 / 6.8 → Nut Grade 6 | Grade 8.8 → Nut Grade 8 | Grade 10.9 → Nut Grade 12 | Grade 12.9 → Nut Grade 12 | Grade 14.9 → Nut Grade 14

Your Next Step

You now have the most comprehensive reference on British fasteners available in a single guide. Every dimension, every tolerance, every strength grade pairing — it's all here.

But information without application is just data.

Here's what to do right now:

1. Identify the thread system on your current or next project. Are you working with BSW, BSF, Unified, or ISO metric? Verify — don't assume.

2. Cross-reference the strength grade of every bolt and nut combination against the pairing table above. A single mismatch can cause joint failure under load.

3. Confirm your clearance holes match the fit series appropriate for your application — close, medium, or free.

4. For studs, determine whether general-purpose (BS 2693:Part 1) or high-grade (BS 2693:Part 2) studs are required based on your interference fit needs.

5. Bookmark this guide. Print the strength grade table and the bolt-nut combination chart. Post them in your workshop or office.

The engineers who never have fastener failures aren't luckier than the ones who do. They're better prepared.

Which British fastener specification are you working with right now? What's the application that brought you to this guide?

This guide references British Standards BS 84:1956, BS 1083:1965, BS 1580, BS 1768:1963, BS 1769:1951, BS 2693:Part 1:1956, BS 2693:Part 2:1964, BS 3382, BS 3643:Part 1:1981, BS 3643:Part 2:1981, BS 3692:1967, BS 4183:1967, BS 4186:1967, BS 4190:1967, and BS 4320:1968. All dimensional data is sourced from authoritative engineering handbooks. Always verify current standard status and revision through the British Standards Institution before use in safety-critical applications.

British Threads: The Complete Engineer's Guide to Tolerances, Engagement, and Diameter/Pitch Selection (BS 3643 · BSW · BSF · BA)

"The difference between a thread that holds and a thread that fails is not the material — it is the millimeter you ignored."

What You Will Find in This Guide

• The Architecture of British Thread Standards — how the BS/ISO system is structured
• Tolerance Classes for Nuts (Internal Threads) — every class, every position, every scenario
• Tolerance Classes for Bolts (External Threads) — grades, positions, and the fit philosophy
• Lengths of Thread Engagement — short, normal, and long categories with complete tables
• Limits and Tolerances (BS 3643: Part 2) — dimensional data from M1 to M39+
• Diameter/Pitch Combinations (BS 3643: Part 1) — first, second, and third-choice selections
• British Standard Whitworth (BSW) and Fine (BSF) — the legacy 55° thread family
• British Association (BA) Threads — the miniature thread system still found in instrument work
• The Decision Matrix — how to choose the right tolerance class for your application

The Architecture — How British Thread Standards Are Organised

Before you can read a tolerance table, you need to understand the conceptual structure that generates it.

British threads sit within a layered standards hierarchy. At the top is the international ISO framework. Below it sits the British Standards Institution (BSI), which has adopted ISO metric threads as the primary standard through BS 3643, while also maintaining legacy standards for Whitworth (BS 84) and British Association (BS 93) threads that remain relevant for maintenance, repair, and heritage engineering.

The Three Families

The BSI formally declared in 1965 — at a conference representing major British industry sectors — that Whitworth, BA, and BSF threads should be treated as obsolescent, and that the ISO metric thread should become the first choice for all future designs, with ISO Unified (inch) as the second choice. That policy has been consistent ever since.

Nevertheless, any engineer working on legacy British machinery, heritage industrial equipment, or British-made instruments from the mid-twentieth century will encounter all three families. This guide covers them comprehensively.

The ISO Metric Thread — Basic Profile

The ISO metric thread is defined by a 60° included angle. Its geometry is shared with the Unified (inch) thread family but is expressed entirely in millimetres.

Profile Geometry

        30° 30°
       /|    |\
      / |    | \
     /  |    |  \
    /   P/8  |   \
   /    |    |    \
--+-----+----+-----+--  Pitch Line
   \         |   /
    \    5/8 H  /
     \        /
      \      /
       \    /
        \  /
         \/
       H/4 (Root — rounded in practice)

Symbol Definitions

The fundamental relationship:

$$H = \frac{\sqrt{3}}{2} P \approx 0.866025P$$

The basic profile is the theoretical sharp-V profile from which all tolerances are calculated. In practice, crests and roots are truncated and/or rounded to clear the mating thread. Thread contact is confined to the flanks only — never on crests or roots of the basic profile.

The Tolerance System — Understanding What the Numbers and Letters Mean

This is where most engineers lose their footing, and it is the source of the practitioner Hartley's midnight crisis.

The BS 3643 tolerance system assigns every thread a tolerance class — a two-part designation consisting of:

1. A tolerance grade (a number) — defines the size of the tolerance zone
2. A tolerance position (a letter) — defines the location of the tolerance zone relative to the basic size

Tolerance Grades

A higher grade number = larger tolerance zone (looser). A lower grade number = tighter tolerance. Grade 6 is the middle reference grade from which all others are scaled.

Tolerance Positions

For nuts (internal threads): positions G and H

For bolts (external threads): positions e, f, g, and h

The tolerance position defines the fundamental deviation — the distance between the basic size (the zero line) and the nearest end of the tolerance zone:

• Position H (nuts): zero deviation — tolerance zone starts at basic size
• Position G (nuts): positive deviation — tolerance zone is shifted away from basic size
• Position h (bolts): zero deviation — tolerance zone starts at basic size
• Positions e, f, g (bolts): negative deviations — tolerance zones are shifted below basic size (providing clearance / allowance)

Fundamental Deviations Table

All values in micrometres (µm). EI = lower deviation for internal (nut) threads; es = upper deviation for external (bolt) threads.

The Formulas Behind the Deviations

$$EI_G = +(15 + 11P) \quad \text{(Position G, nut)}$$

$$EI_H = 0 \quad \text{(Position H, nut)}$$

$$es_e = -(50 + 11P) \quad \text{except for } P \leq 0.45\,\text{mm}$$

$$es_f = -(30 + 11P)$$

$$es_g = -(15 + 11P)$$

$$es_h = 0$$

In all formulas, $EI$ and $es$ are in micrometres; $P$ is in millimetres.

Crest Diameter Tolerance Formula

The tolerance for the major diameter of bolt threads (grade 6) is:

$$T_d(6) = 180\sqrt[3]{P^2} - 3.15 \quad \text{(µm; } P \text{ in mm)}$$

Scaling between grades:

$$T_d(4) = 0.63 \cdot T_d(6) \qquad T_d(8) = 1.6 \cdot T_d(6)$$

Minor Diameter Tolerance Formula (Nut Threads, Grade 6)

For pitches 0.2 to 0.8 mm:

$$T_{D1}(6) = 433P - 190P^{1.22} \quad \text{(µm)}$$

For pitches 1 mm and coarser:

$$T_{D1}(6) = 230P^{0.7} \quad \text{(µm)}$$

Scaling:

Tolerance Classes for Nuts (Internal Threads)

The Standard specifies that tolerance classes shall be selected from a reduced set — this deliberately limits the number of gages and tools required.

Table 3 — Tolerance Classes for Nuts (BS 3643)

The commercial standard is 6H. For the overwhelming majority of general-purpose nuts, 6H is correct.

When to Use Each Nut Tolerance Class

Key principle: The tolerance position letter comes after the grade number when describing a tolerance class. 6H means grade 6, position H. Do not confuse this with a material hardness designation or any other engineering notation.

Tolerance Classes for Bolts (External Threads)

Table 4 — Tolerance Classes for Bolts (BS 3643)

The commercial standard is 6g. This provides a small built-in allowance (clearance before tolerance begins) that accommodates plating, coating, and manufacturing variation.

The 6H/6g Fit — Why It Is the Default

The combination 6H/6g — nut at 6H, bolt at 6g — is the standard commercial thread fit for ISO metric fasteners worldwide. It provides:

• A clearance fit under all conditions (no interference assembly)
• Sufficient allowance to accommodate thin electroplated coatings
• A tolerance zone that is achievable with standard commercial machining and rolling processes

The designation on a drawing reads as the nut class first, then the bolt class, separated by an oblique stroke:

M10-6H/6g
M20 × 2-6H/5g6g

When both pitch diameter and crest diameter share the same tolerance class, the symbol is written once. When they differ (as in 5g6g), both are stated — pitch diameter first.

The Fine Tolerance Bolt Classes

For precision work where close control of fit is essential — instrument mechanisms, adjustment screws, precision gages — the 4h and 5h classes are applied.

Critical note for coated threads: Unless otherwise specified, size limits for standard tolerance classes 6g and 4g6g apply before coating. The allowance (fundamental deviation) can accommodate the coating thickness, provided the maximum coating thickness does not exceed one-quarter of the allowance. After coating, the thread must not exceed maximum material limits for tolerance position h or g respectively.

Lengths of Thread Engagement — Short, Normal, and Long

Thread engagement length is the axial distance over which the external and internal threads are in contact. It is not the same as bolt length, grip length, or thread run-out.

Why it matters: Longer engagement amplifies the effect of pitch and flank angle errors. A tolerance class calibrated for normal engagement may produce a tighter-than-expected fit in a long engagement, or a looser-than-expected fit at short engagement. The Standard therefore categorises engagement into three bands and adjusts tolerance class selection accordingly.

Engagement Length Categories

Table 5 — Lengths of Thread Engagement (BS 3643 / ISO 965-1)

All dimensions in millimetres.