RLH Bus Information Centre

BSW Whitworth Thread System

In Britain, road vehicles, aeroplanes and machinery until the 1970s used British Standard thread forms. The following families of threads existed;

The U.S. equivalents are the Unified Thread System formed by ANSI/NATO in 1948. These standards were agreed between Britain, Canada and the US and were based on the previous American National standard, except with rounded roots and optionally rounded crests. This composed the following:

Whitworth threads are used for most mechanical equipment in the RLH.

With the UNF/UNC system the number stamped on the wrench or socket is the dimension of the hexagonal bolt head measured across the flats (AF = Across Flats); so the wrench required for a 1/4 UNC bolt would have be stamped 7/16. Whitworth wrench sizes, however, indicate the size of the bolt, i.e., a wrench required for the equivalent 1/4 BSW bolt will have 1/4W stamped on it.

Whitworth heads are NOT the same size as the equivalent UNC/UNF - they are larger. For this reason, it is imperative that a set of Whitworth wrenches are purchased before working on any antique British machinery. Otherwise, expect rounded-off heads and busted knuckles, trademarks of the careless craftsman.

Whitworth wrenches and sockets are available through clubs that cater to British cars. It's also a good idea to get a set of taps and dies to fix the attempts of those who have gone before you in the vain attempt to convert, intentionally or otherwise, to SAE format.

Another area that causes confusion is the fact that some BSW/BSF nuts can be screwed on UNF/UNC bolts and vice-versa. Under NO circumstances should this be practised. Most of the coarse threads share the same threads per inch, which means BSW nuts can be screwed onto UNC bolts and vice-versa (the exception to this being ½ inch, of which the BSW size has 12 threads per inch whereas the UNC size has 13). Whitworth and UNC/UNF thread forms differ greatly, the primary difference being the thread angle. Consequently, if these fasteners are interchanged, considerable loss of holding force, fatigue resistance and strength will result.

Some outfits in the US have been known to sell so-called Whitworth hardware, which has SAE sized hex heads mixed with Whitworth thread forms. This really only serves to cause confusion and increase the risk of the wrong equipment being used in future. Fully correct Whitworth fasteners are available in the US.
Abbreviation Name Introduced Diameter/ Range Thread angle, degrees Notes
BA British Association 1884 16BA to 0BA (0.03 to 0.24 inch) 47.5 Mainly electrical
BSW British Standard Whitworth 1841 1/16 to 2.5 inches 55 Coarse
BSF British Standard Fine 1908 3/16 to 1 inch 55
UNC Unified National Coarse 1918 0.05 to 1 inch 60
UNF Unified National Fine 1918 0.05 to 1 inch 60
ISO Metric Coarse International Standards Organization Metric Coarse 1 to 27mm 60
ISO Metric Fine International Standards Organization Metric Fine 3 to 27mm 60

SOCKETS and WRENCHES (SPANNERS)
Key to Nut & Bolt Sizes
Nut size Across Flats British sizes
decimal in. mm in. BA BSF BSW
0.250 1/4
0.276 7
0.282 3
0.312 5/16
0.316 8
0.324 2
0.340 3/16 1/8
0.343 11/32
0.354 9
0.365 1
0.375 3/8
0.394 10
0.412 0
0.433 11
0.437 7/16
0.445 1/4 3/16
0.472 12
0.500 1/2
0.512 13
0.525 5/16 1/4
0.551 14
0.562 9/16
0.590 15
0.600 3/8 5/16
0.625 5/8
0.629 16
0.669 17
0.687 11/16
0.708 18
0.710 7/16 3/8
0.748 19
0.750 3/4
0.787 20
0.812 13/16
0.820 1/2 7/16
0.827 21
0.866 22
0.875 7/8
0.906 23
0.920 9/16 1/2
0.938 15/16
0.945 24
0.984 25
1.000 1
1.010 5/8 9/16
1.024 26
1.063 27 1 1/16
1.100 11/16 5/8
1.102 28
1.125 1 1/8
1.181 30
1.188 1 3/16
1.200 3/4 11/16
1.250 1 1/4
1.260 32
1.300 7/8 3/4
1.313 1 5/16
1.339 34
1.375 1 3/8
1.390 13/16
1.417 36
1.438 1 7/16
1.480 1 7/8
1.496 38
1.500 1 1/2
1.563 1 9/16
1.575 40 15/16
1.625 1 5/8
1.670 1 1/8 1
1.688 1 11/16
1.750 1 3/4
1.811 46
1.813 1 13/16
1.850 47
1.860 1 1/8
1.875 1 7/8
1.890 48
1.938 1 15/16
1.969 50
2.000 2
2.050 1 1/4

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