BSW and BSF Whitworth Thread Gauges: A Buyer’s Guide

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What Makes a Whitworth Thread Different

The 55° angle and rounded profile were the original British standard for screw threads. Unified threads replaced them in most new designs, but the installed base of Whitworth threads is still large.

Common BSW and BSF Sizes

Where You Still Find Whitworth Threads

• Maintenance and spare parts for older British machinery and machine tools
• Classic and older vehicles and agricultural equipment
• Repair shops rebuilding legacy equipment
• BSP pipe threads, which share the 55° Whitworth form (but use different sizes)

How to Confirm a Thread Is Whitworth

1. Measure the major diameter — it will be close to an inch fraction.
2. Count threads per inch with a pitch gauge.
3. Check the flank angle with a 55° pitch gauge leaf or thread profile gauge; a 60° leaf will not seat cleanly.
4. Compare with the BSW and BSF columns above.
5. If the diameter matches a pipe size (e.g. 1/2″ pipe ≈ 21 mm OD), it is probably BSP, not BSW.

A Short History of the Whitworth Thread

Joseph Whitworth proposed his thread in 1841, making it one of the first national thread standards. For more than a century BSW and BSF were used across British and Commonwealth engineering — in machine tools, steam and motor vehicles, railways and shipbuilding.

After the Second World War, the need for interchangeability with American equipment led to the Unified thread (UNC/UNF), which uses a 60° angle. The UK later moved to ISO metric threads. Whitworth threads were no longer used for new designs, but the huge installed base means they are still met every day in maintenance and restoration work.

Whitworth Fit Classes

BS 84 defines tolerance classes for Whitworth threads, traditionally described as close, medium and free (or normal) fits. The class sets how much variation is allowed in the pitch diameter, and therefore which gauge limits apply.

If the drawing or old part does not state a fit, medium fit is the usual choice. For replacement gauges, measuring an existing good gauge or part helps confirm which class was used originally.

Whitworth Form in BSP Pipe Threads

BSP pipe threads use the same 55° Whitworth form, but the sizes refer to nominal bore, not outside diameter, and the pitches are different. For example, 1/2″ BSP has an outside diameter of about 20.96 mm and 14 TPI, while 1/2″ BSW is about 12.7 mm with 12 TPI.

Replacing Whitworth Gauges Without a Drawing

Many Whitworth gauges in use today are decades old, and their original drawings are long gone. A replacement can still be made accurately:

1. Send the old gauge (even if worn) or a known-good part.
2. We measure diameter, pitch, angle and pitch diameter.
3. We identify the size, series (BSW or BSF) and most likely fit class.
4. We confirm the specification with you before manufacture.
5. The new gauge is made and supplied with a measured-value certificate on request.

If the old gauge is badly worn, measuring a good mating part as well helps confirm the original limits.

Workshops with Mixed Thread Systems

Restoration and maintenance workshops often handle Whitworth, Unified and metric threads side by side. Clear organisation prevents costly mistakes:

• Store Whitworth gauges in a separate, clearly labelled drawer or case.
• Mark each gauge with “BSW” or “BSF” as well as the size.
• Keep a wall chart of Whitworth sizes and TPI near the inspection bench.
• Identify any unknown thread fully before choosing a gauge.

More BSF Sizes

BSF was widely used where finer adjustment or thinner walls were needed — for example on motorcycles, cars and machine tools. Additional common sizes:

Some of these sizes, such as 7/32″ and 9/32″, have no BSW equivalent and are strong clues that a thread is BSF.

BA Threads: Another British Small Thread

Small British instruments and electrical equipment often use BA (British Association) threads rather than Whitworth. BA threads have a 47.5° angle and are numbered from 0BA (6.0 mm) downwards to very small sizes. They are metric-based, not inch-based, and need their own gauges.

Caring for Legacy-Thread Gauges

Whitworth gauges are often used only occasionally, so they spend long periods in storage. Protect them from corrosion with a light oil film and closed cases, and inspect them before each use. Because replacements take longer to obtain than common metric gauges, keep a record of each gauge’s size, class and calibration so a replacement can be ordered quickly if one is lost or damaged.

Gauging Rounded-Crest Threads

Whitworth threads have rounded crests and roots. Whitworth gauges are made with matching rounded profiles so they contact the flanks correctly. A gauge with the wrong crest or root shape may touch the part in the wrong place and give false results — another reason not to substitute Unified gauges, even where the pitch matches.

When worn Whitworth parts are being assessed for reuse, remember that rounded crests wear and flatten over time. A thread that looks “sharp” in places may already be well worn.

Summary

BSW and BSF threads are legacy British standards that are still widely met in maintenance and restoration work. Their 55° angle and rounded form make them incompatible with Unified and metric gauges. Identify them carefully, order Whitworth gauges by series, size and fit, and keep them clearly separated from other thread systems in the workshop.

Key Terms

Quick definitions of the terms used in this guide:

Practical Tip: Label Legacy Parts

When a Whitworth thread is identified on a machine, label the part or note it in the maintenance record. The next person will not need to repeat the identification, and spare parts and gauges can be ordered correctly the first time.

Next Steps

Send the Whitworth sizes you need, stating BSW or BSF, plug or ring, and the fit class if known. If you only have an old gauge or a part, send it or photos with measurements and we will identify the thread first. We confirm the specification with you before making the gauge.

How to Order Whitworth Gauges

• BSW or BSF, size and TPI, e.g. 1/2" BSW
• Plug (internal) or ring (external), GO/NO-GO, quantity
• Fit class if the drawing gives one
• Certificate requirement

Frequently Asked Questions