British Standard Whitworth (BSW) is one of a number of imperial unit based screw thread standards which use the same bolt heads and nut hexagonal sizes, the others being British Standard Fine thread (BSF) and British Standard Cycle. These three are collectively called Whitworth threads.
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The Whitworth thread was the world's first national screw thread standard [1], devised and specified by Joseph Whitworth in 1841. Until then, the only standardization was what little had been done by individual people and companies, with some companies' in-house standards spreading a bit within their industries. Whitworth's new standard specified a 55° thread angle and a thread depth of 0.640327p and a radius of 0.137329p, where p is the pitch. The thread pitch increases with diameter in steps specified on a chart. The Whitworth thread system was later to be adopted as a British Standard to become British Standard Whitworth. An example of the use of the Whitworth thread is the Royal Navy's Crimean War gunboats. These are the first instance of 'mass-production' techniques being applied to marine engineering as the following quotation from the obituary from The Times of 24 January 1887 to Sir Joseph Whitworth (1803-1887) shows:
The Crimean War began, and Sir Charles Napier demanded of the Admiralty 120 gunboats, each with engines of 60 horsepower, for the campaign of 1855 in the Baltic. There were just ninety days in which to meet this requisition, and, short as the time was, the building of the gunboats presented no difficulty. It was otherwise however with the engines, and the Admiralty were in despair. Suddenly, by a flash of the mechanical genius which was inherent in him, the late Mr John Penn solved the difficulty, and solved it quite easily. He had a pair of engines on hand of the exact size. He took them to pieces and he distributed the parts among the best machine shops in the country, telling each to make ninety sets exactly in all respects to the sample. The orders were executed with unfailing regularity, and he actually completed ninety sets of engines of 60 horsepower in ninety days – a feat which made the great Continental Powers stare with wonder, and which was possible only because the Whitworth standards of measurement and of accuracy and finish were by that time thoroughly recognised and established throughout the country.
An original example of the gunboat type engine was raised from the wreck of the SS Xantho by the Western Australian Museum. On disassembly, all its threads were shown to be of the Whitworth type.[2]
With the adoption of BSW by British railway lines, many of which had previously used their own standard both for threads and for bolt head and nut profiles, and improving manufacturing techniques, it came to dominate British manufacturing.
In the USA, BSW was replaced when steel bolts replaced iron, but was still being used for some aluminium parts as late as the 1960s and 1970s when metric based standards replaced the Imperial ones.
American Unified Coarse was originally based on almost the same imperial fractions. The Unified thread angle is 60° and has flattened crests (Whitworth crests are rounded). Thread pitch is the same in both systems except that the thread pitch for the 0.5 in bolt is 12 threads per inch (tpi) in BSW vs 13 tpi in the UNC.
The British Standard Fine (BSF) standard has the same thread angle as the BSW, but has a finer thread pitch and smaller thread depth. This is more like the modern "mechanical" screw and was used for fine machinery and for steel bolts.
The British Standard Cycle (BSC) standard which replaced the Cycle Engineers' Institute (CEI) standard was used on British bicycles and motorcycles. It uses a thread angle of 60° compared to the Whitworth 55° and very fine thread pitches.
(To simplify matters the term hexagon will be used in this paragraph to denote either bolt head or nut). Whitworth spanner (wrench) markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion also arises because BSF hexagon sizes can be one size smaller than the corresponding Whitworth hexagon. This leads to instances where a spanner marked 7/16BSF is the same size as one marked 3/8W. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same. However, in World War II the size of the Whitworth hexagon was reduced to the same size as the equivalent BSF hexagon purely to save metal during the war, and they never went back to the old sizes afterwards. Thus it is today uncommon to encounter a Whitworth hexagon which takes the nominally correct spanner. Spanners in this case may be marked 7/16BS to indicate that they have a jaw size of 0.710 in and are designed to take either the (later) 7/16 BSW or 7/16 BSF hexagon. The table here [1] illustrates the differences between the old and new hexagon standards.
The British Association screw thread (BA) standard is sometimes classed with the Whitworth standard fasteners because it is often found in the same machinery as the Whitworth standard. However it is actually a metric based standard that uses a 47.5° thread angle and has its own set of head sizes. BA threads have diameters of 6 mm (0BA) and smaller, and were and still are particularly used in precision machinery.
The Whitworth 55° angle remains commonly used today worldwide in form of the 15 British standard pipe threads defined in ISO 7, which are commonly used in water supply, cooling, pneumatics, and hydraulic systems. These threads are designated by a number between 1/16 and 6 that originates from the nominal internal diameter (i/d) in inches of a steel pipe for which these threads were designed. These pipe thread designations do not refer to any thread diameter.
Other threads that used the Whitworth 55° angle include Brass Threads, British Standard Conduit (BSCon), Model Engineers (ME), and British Standard Copper (BSCopper).
| Whitworth size (in) | Core diameter (in) | Threads per inch | Pitch (in) | Tapping drill size |
|---|---|---|---|---|
| 1/16 | 0.0411 | 60 | 0.0167 | Number Drill 56 (1.2 mm) |
| 3/32 | 0.0672 | 48 | 0.0208 | Number Drill 49 (1.85 mm) |
| 1/8 | 0.0930 | 40 | 0.025 | Number Drill 39 (2.55 mm) |
| 5/32 | 0.1162 | 32 | 0.0313 | Number Drill 30 (3.2 mm) |
| 3/16 | 0.1341 | 24 | 0.0417 | Number Drill 26 (3.7 mm) |
| 7/32 | 0.1654 | 24 | 0.0417 | Number Drill 16 (4.5 mm) |
| 1/4 | 0.1860 | 20 | 0.05 | Number Drill 9 (5.1 mm) |
| 5/16 | 0.2414 | 18 | 0.0556 | Letter Drill F (6.5 mm) |
| 3/8 | 0.2950 | 16 | 0.0625 | 5/16 in (7.94 mm) |
| 7/16 | 0.3460 | 14 | 0.0714 | Letter Drill U (9.3 mm) |
| 1/2 | 0.3933 | 12 | 0.0833 | Letter Drill Z (10.5 mm) |
| 9/16 | 0.4558 | 12 | 0.0833 | 12.1 mm (0.4764 in) |
| 5/8 | 0.5086 | 11 | 0.0909 | 13.5 mm (0.5315 in) |
| 11/16 | 0.5711 | 11 | 0.0909 | 15 mm (0.5906 in) |
| 3/4 | 0.6219 | 10 | 0.1 | 16.27 mm (0.6406 in) |
| 13/16 | 0.6845 | 10 | 0.1 | 18 mm (0.7087 in) |
| 7/8 | 0.7327 | 9 | 0.1111 | 19.25 mm (0.7579 in) |
| 15/16 | 0.7953 | 9 | 0.1111 | 20.75 mm (0.8169 in) |
| 1 | 0.8399 | 8 | 0.125 | 22 mm (0.8661 in) |
| 1 1/8 | 0.9420 | 7 | 0.1429 | |
| 1 1/4 | 1.0670 | 7 | 0.1429 | |
| 1 1/2 | 1.2866 | 6 | 0.1667 | |
| 1 3/4 | 1.4939 | 5 | 0.2 | |
| 2 | 1.7154 | 4.5 | 0.2222 | |
| 2 1/2 |
Nearly all current cameras accept a 1/4 in Whitworth tripod thread in their baseplate, while a 5/8 in thread is the accepted standard for tripod mounted land surveying equipment.
The Leica Thread-Mount used on rangefinder cameras and on many enlarging lenses is 39 mm by 26 turns-per-inch Whitworth, an artifact of its having been developed by a German company specializing in microscopes and thus equipped with tooling capable of handling threads in inches and in Whitworth.
British Morris and MG engines from 1919 to 1955 were built in a factory that used metric threads but with bolts and nuts for Whitworth spanners (wrenches) and sockets.[4]
In the 2011 movie 'Cars 2' by Disney / Pixar, the vital clue to the discovery of the villain, Sir Miles Axlerod, is that he uses Whitworth bolts. The hero, Mater, a tow-truck, knows his stuff! Although Axlerod doesn't precisely resemble any real car (whereas numerous other characters are closely modelled on real cars), he seems most closely to match the original Range Rover Classic. In reality, early model Range Rovers used parts with imperial dimensions, although the photograph of the villain's engine is virtually identical to the later 3.9 litre single plenum Rover V8.
Other thread standards:
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