BIZOMAT MAKEOS · Threads
Whitworth Thread Basic Dimensions
What are the basic pitch and minor diameters of this BSW, BSF or BSP (parallel) Whitworth thread?
Basic depth, pitch and minor diameters and crest/root radius of a 55° Whitworth form thread (BSW, BSF, BSP parallel) from its major diameter and threads per inch.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Major diameter | in | Basic major diameter in inches (BSP sizes use the pipe thread outside diameter, e.g. 0.825 in for ½ BSP). |
| Threads per inch | TPI | Threads per inch (e.g. 12 for ½ BSW, 14 for ½ BSP). |
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: Major diameter
- What it is
- The basic major diameter of the Whitworth thread in inches. For BSW and BSF it is the nominal size; for BSP parallel pipe threads it is the outside diameter of the thread, which is larger than the pipe size name.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 20 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A BSP size name is not its diameter: ½ BSP is not 0.5 in. Take the thread major diameter from ISO 228 or a pipe thread table.
What this means: Threads per inch
- What it is
- The number of thread crests per inch of length for this Whitworth size, from the BSW, BSF or BSP series table.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 100 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fewer threads per inch means a larger pitch and depth, so smaller pitch and minor diameters.
- Get it right
- BSW, BSF and BSP of the same nominal size have different threads per inch; check the series.
Method and formulas
- Fundamental triangle H = 0.960491p; depth h = 0.640327p; radius r = 0.137329p (p = 1/n).
- Pitch diameter = D − h; minor diameter = D − 2h.
- Example: ½ BSW (12 TPI) has a pitch diameter of 0.44664 in and a minor diameter of 0.39328 in.
Limitations
- Basic dimensions; tolerances come from BS 84 (BSW/BSF) or ISO 228 (BSP parallel). BSPT is tapered and not covered.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Repair and replace parts on older British equipment
- Inspect Whitworth form threads
- Convert Whitworth thread sizes to metric dimensions
Who it is for
- Maintenance fitters
- Machinists
- Quality inspectors
What you get
- Thread depth
- Basic pitch and minor diameters
- Crest and root radius
Interpreting the result
- Do not mix Whitworth (55°) and unified or metric (60°) threads; they look similar but do not fit properly.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1980–1989, 1995–1998. Topic reference; no source tables or recommended settings are reproduced.
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