BIZOMAT MAKEOS · Threads
British Association (BA) Thread Basic Sizes and Limits (BS 93)
For a British Association (BA) screw thread of my pitch and major diameter, what are the basic thread form dimensions, and what are the bolt and nut limits of size in the close or normal class of BS 93?
British Association threads by the handbook’s formulas: a 47½° thread with H = 1.13634p, depth h = 0.6p, crest and root radius r = 0.18083p and truncation s = 0.26817p, effective diameter D − h and minor diameter D − 2h, and the BS 93 tolerances: close class bolts (0 to 10 BA) major 0.15p, effective 0.08p + 0.02 and minor 0.16p + 0.04 mm; normal class bolts major 0.20p (0 to 10 BA) or 0.25p (11 to 16 BA), effective 0.10p + 0.025 and minor 0.20p + 0.05 mm with a 0.025 mm allowance for 0 to 10 BA; and nuts of all classes effective 0.12p + 0.03 and minor 0.375p mm.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| BA size number | BA | BA number of the thread, 0 being the largest size. |
| Pitch p | mm | Pitch of the thread from the BA table, in millimetres. |
| Major diameter D | mm | Major diameter of the thread from the BA table, in millimetres. |
| Bolt class | choice | BS 93 class of the bolt; nuts have one class. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: BA size number
- What it is
- Size number of the British Association thread, 0 being the largest.
- Units and format
- Enter a number in BA: BA. The form accepts 0 BA to 25 BA. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 BA as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets which tolerance rules and allowance apply.
- Get it right
- Read it from the screw designation, e.g. 4 BA.
What this means: Pitch p
- What it is
- Axial distance between neighbouring threads, from the BA table.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the thread depth, radii and every tolerance.
- Get it right
- Take it from the BA table, e.g. 0.66 mm for 4 BA; a pitch gauge confirms it.
What this means: Major diameter D
- What it is
- Outside diameter of the BA screw, from the BA table.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The effective and minor diameters follow from it.
- Get it right
- Take it from the BA table, e.g. 3.60 mm for 4 BA.
What this means: Bolt class
- What it is
- BS 93 class of the bolt or screw being made or checked.
- Units and format
- Choose one of 2 options from the list: Normal class (general use); Close class (precision parts, 0 to 10 BA).
- Example
- The form opens with “Normal class (general use)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Sets the bolt tolerances and whether an allowance is taken.
- Get it right
- Close class is for precision parts under stress; normal class for general use.
Method and formulas
- H = 1.13634p; h = 0.6p; r = 0.18083p; s = 0.26817p; effective diameter E = D − h; minor diameter K = D − 2h.
- Bolts, close class (0 to 10 BA): tolerances major 0.15p, effective 0.08p + 0.02, minor 0.16p + 0.04 mm, no allowance.
- Bolts, normal class: major 0.20p (0 to 10 BA) or 0.25p (11 to 16 BA), effective 0.10p + 0.025, minor 0.20p + 0.05 mm, below an allowance of 0.025 mm for 0 to 10 BA.
- Nuts: from the basic sizes, effective + (0.12p + 0.03) and minor + 0.375p mm.
- Example: the bolt effective diameter is 5.375 to 5.25 mm.
Limitations
- Enter the pitch and major diameter of the BA size from the BA table; BS 93 is obsolescent and BA threads are no longer recommended for new designs.
- The close class is defined for 0 to 10 BA only.
- The nut major diameter has no maximum in the tolerance formulas.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Make or check BA screws for instruments and older British equipment
- Find the tapping and minor diameters of a BA thread
- Set the limits for close or normal class BA bolts and nuts
Who it is for
- Instrument makers
- Machinists
- Maintenance fitters
What you get
- Thread depth, radius and truncation
- Basic effective and minor diameters
- Bolt major, effective and minor limits
- Nut effective and minor limits
Interpreting the result
- BA threads have rounded crests and roots like Whitworth threads but a smaller 47½° angle.
- Close class bolts have no allowance on the nut, for precision parts under stress.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1981–1982. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Useful alongside · Library collection
Choosing fasteners and threads: thread systems, grades, screws for sheet and plastics, inserts and pins
In Machine Elements and Mechanical Design
What it covers: Fasteners are cheap, numerous and easy to get wrong. How to identify thread systems, read grades, choose screws, inserts and pins, and cut fastener variety without cutting reliability.
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