KEVOS® MAKEOS · Threads
Whitworth Thread Limits to BS 84 (BSW, BSF)
What are the major, effective and minor diameter limits of a Whitworth form (BSW or BSF) bolt and nut in the BS 84 close, medium, free or normal classes for my length of engagement?
Limits of size of parallel screw threads of Whitworth form by the BS 84 tolerance formulas the handbook gives: T = 0.002 ∛D + 0.003 √L + 0.005 √p; bolt effective diameter tolerances 2/3 T, T and 3/2 T for close, medium and free classes, bolt major tolerances of the same plus 0.01 √p, bolt minor tolerances 2/3 T + 0.013 √p, T + 0.02 √p and 3/2 T + 0.02 √p; nut effective tolerances 2/3 T, T and 3/2 T for close, medium and normal classes and nut minor tolerance 0.2p plus 0.004, 0.005 or 0.007 in, with the basic Whitworth dimensions and the stainless steel adjustment.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Major diameter D | in | Major diameter of the Whitworth thread in inches. |
| Threads per inch | TPI | Threads per inch of the thread. |
| Length of engagement L | in | Length of engagement of the bolt in the nut, in inches. |
| Bolt class | choice | BS 84 bolt class. |
| Nut class | choice | BS 84 nut class; normal nuts suit medium and free bolts. |
| Bolt material | choice | Stainless steel bolts have the BS 84 adjustment applied. |
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 D
- What it is
- Nominal outside diameter of the Whitworth bolt, in inches.
- Units and format
- Enter a number in in: inches. The form accepts 0.0625 in to 6 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.
- Effect on the result
- Sets the basic diameters and the ∛D term of the tolerance unit.
- Get it right
- Use the nominal size, e.g. 0.5 for a 1/2 in BSW thread.
What this means: Threads per inch
- What it is
- Number of thread pitches in one inch along the axis.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 2 TPI to 72 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
- Sets the pitch, thread depth and the √p terms of the tolerances.
- Get it right
- BSW and BSF sizes have different pitches; check with a Whitworth pitch gauge.
What this means: Length of engagement L
- What it is
- Axial length over which the bolt and nut threads are engaged.
- 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.
- Effect on the result
- A longer engagement raises the tolerance unit T.
- Get it right
- Use the nut thickness or the length of thread in a tapped hole.
What this means: Bolt class
- What it is
- BS 84 class of the bolt, from a fine snug fit to ordinary commercial quality.
- Units and format
- Choose one of 3 options from the list: Close; Medium; Loose fit (BS 84 free class).
- Example
- The form opens with “Medium” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Close, medium and free take 2/3, 1 and 3/2 of T.
- Get it right
- Medium suits better interchangeable work, free most commercial bolts.
What this means: Nut class
- What it is
- BS 84 class of the nut that mates with the bolt.
- Units and format
- Choose one of 3 options from the list: Close; Medium; Normal.
- Example
- The form opens with “Normal” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Close, medium and normal take 2/3, 1 and 3/2 of T on the effective diameter.
- Get it right
- Normal class nuts are meant for medium or free class bolts.
What this means: Bolt material
- What it is
- Whether the bolt is stainless steel, which BS 84 treats differently.
- Units and format
- Choose one of 2 options from the list: Carbon or alloy steel; Stainless steel.
- Example
- The form opens with “Carbon or alloy steel” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Above 3/4 in the bolt limits move 0.001 in smaller.
- Get it right
- Choose stainless steel for austenitic or other stainless bolts.
Method and formulas
- p = 1 ÷ n; h = 0.640327p; effective = D − h; minor = D − 2h; r = 0.137329p.
- T = 0.002 ∛D + 0.003 √L + 0.005 √p (inches).
- Bolts (minus from basic): effective 2/3 T, T, 3/2 T; major 2/3 T, T, 3/2 T + 0.01 √p; minor 2/3 T + 0.013 √p, T + 0.02 √p, 3/2 T + 0.02 √p for close, medium, free.
- Nuts (plus from basic): effective 2/3 T, T, 3/2 T for close, medium, normal; minor 0.2p + 0.004 (26 TPI and finer), + 0.005 (22 and 24 TPI) or + 0.007 (20 TPI and coarser).
- Stainless steel bolts above 3/4 in: both limits 0.001 in smaller; at 3/4 in and below, medium or free class rather than close.
- Example: the medium class bolt has an effective diameter of 0.44664 to 0.44149 in.
Limitations
- The formulas are for parallel threads of Whitworth form (BSW and BSF); pipe threads and BSPT are not covered.
- Use the length of engagement of the actual joint; the tolerance grows with it.
- The handbook gives the formulas only; check limits for acceptance against BS 84.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Make replacement bolts and nuts for older British machinery
- Inspect Whitworth threads to BS 84 limits
- Choose a bolt class for a Whitworth thread
Who it is for
- Maintenance fitters
- Machinists
- Quality inspectors
What you get
- Basic depth, effective and minor diameters
- Tolerance unit T
- Bolt major, effective and minor diameter limits
- Nut effective and minor diameter limits and the minimum major diameter
Interpreting the result
- Bolt limits are the basic sizes minus the tolerances; nut limits are the basic sizes plus the tolerances.
- Close class bolts are for special work where refined accuracy of pitch and form is needed.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1953–1954. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® 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.
Useful alongside · Library collection
Inch and metric conversion on legacy drawings: tolerances, threads, pipe sizes and other traps
In Practical Mathematics for Engineering and Manufacturing
What it covers: How to convert imperial drawings and data to metric without errors: exact factors, converting tolerance limits, why threads and pipe sizes must not be converted, and other common traps.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Whitworth Thread Limits to BS 84 (BSW, BSF) calculator