British and Legacy Thread Systems: Applying the Table

Applying the Table — The Rules

When your engagement length falls in the short category:

Reduce the pitch diameter tolerance of the external thread (bolt) by one tolerance grade number. Example: Normal class would be 6g → use 5g for short engagement.

When your engagement length falls in the long category:

Increase the allowance (fundamental deviation) at the pitch diameter of the external thread (bolt). Example: Normal class 6g → use 7g for long engagement.

When the actual engagement length is unknown (as in standard bolt manufacture):

Use the normal category.

Limits and Tolerances — BS 3643: Part 2 (Complete Dimensional Data)

Part 2 of BS 3643 specifies fundamental deviations, tolerances, and limits of size for:

• Tolerance classes 4H, 5H, 6H, 7H for internal threads (nuts)
• Tolerance classes 4h, 6g, 8g for external threads (bolts)
• Coarse-pitch series: M1 to M68
• Fine-pitch series: M1 to M33
• Constant-pitch series: M8 to M300 (refer to the Standard for full data)

How to Read the Limits Table

For external threads (bolts), the table provides:

• Fundamental deviation — the offset from basic (position g = negative; h = zero)
• Major diameter: Maximum value and tolerance (subtracted)
• Pitch diameter: Maximum value and tolerance (subtracted)
• Minor diameter: Minimum value

For internal threads (nuts), the table provides:

• Major diameter: Maximum (this is the basic diameter — not exceeded)
• Pitch diameter: Maximum and tolerance
• Minor diameter: Maximum and tolerance

Selected Limits and Tolerances — BS 3643: Part 2: 1981 (Normal Engagement)

All dimensions in millimetres.

M1 (Nominal Diameter: 1 mm)

M1.4

M2

M3

M6

M8

M10

M12

M16

M20

M24 through M39 (Selected)

Note: For constant-pitch series and diameters larger than M39, consult BS 3643 directly. The data above covers the first-, second-, and third-choice coarse and fine pitch combinations from BS 3643: Part 2: 1981.

Designation Examples

Diameter/Pitch Combinations — BS 3643: Part 1

Part 1 of BS 3643 provides a structured choice of diameter/pitch combinations, organised into first, second, and third choices.

Always prefer first-choice combinations. Second choice is acceptable when design requirements demand it. Third choice should be avoided unless no alternative exists.

Table 7 — Diameter/Pitch Combinations (Partial, BS 3643: Part 1: 1981)

Maximum Diameter for Fine Constant-Pitch Threads

When a constant pitch finer than the coarse pitch is required, the following maximum diameter limits apply:

Note: In the diameter range 150–300 mm where a pitch larger than 6 mm is required, use the 8 mm pitch.

British Standard Whitworth (BSW) and Fine (BSF) Threads

The 55° Thread That Built British Industry

Joseph Whitworth introduced his thread standard in 1841, and for over a century it was the foundation of British manufacturing. The 55° included angle, rounded crests and roots, and inch-based pitch table gave British engineering its interchangeable fastener standard long before the ISO system existed.

Today, BSW and BSF are formally obsolescent (BS 84:1956). You will still encounter them when:

• Servicing or repairing pre-metric British machinery
• Sourcing spare parts for legacy equipment
• Working on classic vehicles (pre-metric British motorcycles, aircraft, engines)
• Interpreting old engineering drawings

Thread Form

         55°
        / | \
       /  |  \
      / H |   \
     /  = |    \
    / 0.640327p \
---+----------+---  Pitch Line
    \         /
     \   r   /  r = 0.137329p
      \     /
       \   /
        \_/

Where $n$ = threads per inch and $p = 1/n$:

$$d = 0.640327 \times p = \frac{0.640327}{n} \quad \text{(depth of thread)}$$

$$r = 0.137329 \times p = \frac{0.137329}{n} \quad \text{(radius at crest and root)}$$

Tolerance Classes for BSW / BSF

The tolerance formula variable is:

$$T = 0.002\sqrt{D} + 0.003\sqrt{L} + 0.005\sqrt{p}$$

Where $D$ = major diameter (inches), $L$ = length of engagement (inches), $p$ = pitch (inches).

Table 1 — Tolerance Formulas for BSW and BSF Threads (BS 84)

All tolerances are positive for nuts and negative for bolts.

Allowances (BSW / BSF)

Only Free Class and Medium Class bolts carry an allowance:

• For nominal sizes from ¼ inch to ¾ inch: allowance = 30% of Medium Class bolt effective-diameter tolerance (0.3T)
• For sizes below ¼ inch: use the ¼-inch allowance
• Allowances are applied minus from basic bolt dimensions

Stainless steel advisory: For stainless steel bolts ¾ inch and below, do not use Close Class limits. Use Medium or Free Class instead to avoid galling and seizing. For sizes above ¾ inch, apply maximum and minimum limits 0.001 inch smaller than the table values.

Table — BSW Coarse Thread Series: Basic Dimensions (BS 84:1956)

All dimensions in inches.

Tap drill diameters (BS 1157:1975) provide 77–87% of full thread.

Whitworth Form Constants

British Association (BA) Threads

The Miniature Thread for Instrument Work

The British Association thread (BS 93:1951, obsolescent) was developed for small instrument, electrical, and precision mechanisms. Sizes run from 0 BA (6.0 mm diameter) down to 16 BA (0.79 mm diameter).

The BA thread is a 47.5° symmetrical V-thread with rounded crests and roots, with a basic depth of 0.6 times the pitch.

BA geometry:

$$H = 1.13634 \times p$$

$$h = 0.60000 \times p \quad \text{(depth of BA thread)}$$

$$r = 0.18083 \times p \quad \text{(radius at root and crest)}$$

$$s = 0.26817 \times p \quad \text{(root and crest truncation)}$$

Table 4 — BA Standard Thread Basic Dimensions (BS 93:1951)

Preference: In selecting BA sizes, preference should be given to even-numbered BA sizes.

Table 5 — BA Tolerance Formulas (BS 93, in mm)

All tolerances: positive for nuts, negative for bolts.

Allowances:

• Close Class bolts: No allowance — maximum bolt diameter equals minimum nut diameter
• Normal Class bolts (0–10 BA): Allowance of 0.025 mm

The Decision Matrix — Choosing the Right Tolerance Class

Use this matrix when specifying British ISO metric threads. The three axes are: quality requirement, engagement length, and manufacturing context.

Step 1 — Determine Your Quality Requirement

Step 2 — Determine Engagement Category

From the thread engagement table (Part VI): is your engagement Short (S), Normal (N), or Long (L)?

Step 3 — Apply the Selection Matrix

The one rule that covers 80% of all applications: Unless you have a specific reason not to, specify 6H/6g. It is the universal commercial standard, it accommodates standard coatings, it is what gage calibration and commercial inspection assume, and it is what the bolt and nut manufacturers have optimised their processes around.

Step 4 — Check the Coating/Plating Situation

The Resolution — What the practitioner Hartley Found

Back in that factory at 06:00.

the practitioner pulled the drawing for the housings. They had been tapped to 7H — a tolerance the subcontractor used for all their cast aluminium work, because their blind holes were notoriously difficult to tap to closer limits.

The bolts supplied were standard commercial 6g.

The fit: 6g bolt in a 7H nut. On paper, this should assemble — there is clearance. But the practitioner noticed that the actual engagement length was 28 mm on an M16 × 2 thread. That put the engagement firmly in the long category (anything over 24 mm at this diameter/pitch combination falls into long).

At long engagement, the 7H tolerance on the nut, combined with the 6g allowance on the bolt and cumulative pitch and flank angle errors over a long helix, had produced a functional interference condition on one end of the tolerance band. Not all bolts failed. Only those at the worst-case tolerance boundary of both parts.

The fix: specify the subcontractor taps to 6H for these housings. Standard 6g bolts would assemble without issue. The engagement table would confirm that 6H/6g at normal to long engagement is well within clearance.

The lesson: it was never about the individual tolerance class. It was about the combination of nut class, bolt class, and engagement length — operating simultaneously at their worst-case limits.

Quick-Reference Summary Card

The British ISO Metric Thread Essentials

The Legacy Thread Essentials

Common Fit Combinations — Quick Check