Spanner Sizes Explained

 

Spanners, it would have been good if a size meant what it said but no, sadly that is not the case.

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What size spanner to use?

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The correct spanner for the size of nut or bolt is the right answer. 'W', 'BSF', 'BA' for Imperial, 'AF' for UNC or UNF and 'mm' for metric. BSP sizes are the same as 'BSF'. As you can see from the table below, there are no jaw sizes, the distance across the flats, that are the same between all the different thread types and their associated hexagons.

 

Using the wrong size spanner or socket is likely to round off the nut or bolt as the torque applied increases.

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Why it is not simple?

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The easy answer is that sizes of spanners and the nuts and bolts they fit have evolved over time.

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In my biased view, the British system started off well. It did not follow the emerging metric systems. If you picked up a 1/4" spanner it was for a 1/4" diameter bolt whereas if you picked up a 10mm metric spanner it would fit a 6mm bolt. In the world of Withworth nuts and bolts, the humble spanner marked with '1/4' originally used to fit a 1/4" diameter bolt be it BSW or BSF.

 

Several British standards between 1911 and 1940 changed the width across the flats of the hexagon of a nut or at the head of a bolt, and that 1/4" spanner now fitted two different sizes, 1/4" and 5/16". The first change for the automobile industry before WWI provided a reduced size BSF small hexagon which sold in parallel with the original large sized hexagon.

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Another later British standard removed the large sized BSF hexagons as they were no longer being purchased. In recognition of this a British standard was created just for spanners. This standard provided the double size markings seen on spanners. That simple 1/4" spanner end was now marked 1/4W to show that it was for use on 1/4" BSW hexagons and also marked with 5/16BS to show that it could also be used on 5/16" BSF hexagons. 

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Then another standard in 1924 provided a BSW with a small hexagon just as previously implimented for BSF. Three sizes of hexagons were now being produced, large and small BSW and small BSF. To end the trio of hexagons, an amendment in WWII removed the large sized BSW to save metal. 

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CEI, later BSC, has always followed the BSW and BSF hexagon standards with both large and small sizes.

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