british standard whitworth thread

British Standard Whitworth

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, some others being British Standard Fine thread (BSF) and British Standard Cycle. These three are collectively called Whitworth threads.


The original Whitworth thread form was proposed by Joseph Whitworth in 1841 to replace various proprietary fasteners. This standard specifies a 55° thread angle and a thread depth of 0.640327 p and a radius of 0.137329 p, where p is the pitch. The thread pitch is specified by a chart. The Whitworth thread system was later to be adopted as a British Standard to become British Standard Whitworth. Contrast this with the American Unified Coarse another standard based on imperial fractions used in the United States. These are similar except that the Unified thread angle is 60° and has flattened crests (Whitworth crests are rounded). There is one important exception in this comparison; the thread pitch for the 1/2" bolt is 12 threads per inch (tpi) in BSW vs 13 tpi in the AUC.

With the adoption of BSW by all the British railway lines, many of which had previously used their own standard both for threads and for bolt head and nut profiles, it came to dominate mechanised British manufacturing.

In the USA, BSW mostly died out as steel bolts replaced iron, but was still used for aluminium parts into the 1960s and 1970s when metric based standards replaced the Imperial ones. In some other countries, such as Australia, BSW is still heavily used.

Comparison of standards

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 .710" - the width across the hexagon flat - is the same. However a later standardisation in World War II reduced the size of the Whitworth hexagon to the same size as the equivalent BSF hexagon. Spanners in this case may be marked "7/16BS" to indicate that they have a jaw size of .710" and are designed to take either the (later) 7/16 BSW or 7/16 BSF hexagon. The table here illustrates the differences between the old and new hexagon standards.

The British Association (BA) fastener 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. See British Association screw threads.

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 inner inch-diameter 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).

Thread sizes

Whitworth Size Core Diameter Threads
per inch
Pitch inches 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 inch (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 inch)
5/8 " 0.5086" 11 0.0909" 13.5 mm (0.5315 inch)
11/16" 0.5711" 11 0.0909" 15 mm (0.5906 inch)
3/4 " 0.6219" 10 0.1" 41/64 inch (16.27 mm)
13/16" 0.6845" 10 0.1" 18 mm (0.7087 inch)
7/8 " 0.7327" 9 0.1111" 19.25 mm (0.7579 inch)
15/16" 0.7953" 9 0.1111" 20.75 mm (0.8169 inch)
1 " 0.8399" 8 0.125" 22 mm (0.8661 inch)
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 1/2 0.2222"
2 1/2"

See also

Other thread standards:


Further reading

  • Erik Oberg et al. "Whitworth threads." (2008). Machinery's Handbook, 28th ed. New York: Industrial Press. p.1858-1860.
  • British Standard 84:1956, "Parallel Screw Threads of Whitworth Form."

External links

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