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The British Association series of fixings may seem a strange and eclectic set of screw thread series, but they are actually really well worked out. Based on Swiss practice, but defined in the UK, the basic thread size is 0BA which is the same diameter and pitch (6mm and 1mm) as M6 screws. The different thread angle (47.5 degrees) means that although M6 and 0BA may fit together, the two threads don't engage properly.

As you move to smaller BA sizes, each increase in number is a reduction in size of 0.9 times. This means that the BA series offers ideally spaced fixings throughout the range with no awkward gaps. In fact just using even-numbered sizes is sufficient for most practical purposes and many would argue it gives a better range than comparable metric sizes. The odd-numbered sizes can be useful to get the biggest possible fixing in a small space.

A boxed set of LAL BA taps and dies

My Grandfather's set of LAL BA Taps and Dies, still in use today.

BA fixings are a real boon for scale modellers, as the 0.9 ratio means you can always find a screw thread within about 5% of scale size, a difference unlikely to be noticed. For modelling scale fixings, especially post 1940s Whitworth ones it is customary to use screws and nuts made with 'one size smaller' BA hex, although standard BA hex sizes do match proportions well with many older (e.g. Victorian) examples.

A final note, as raw hex stock in BA sizes becomes more difficult to obtain, BA fixings may be supplied with hex from a near metric equivalent. The difference is usually so small that it goes unnoticed, unless two sizes of hex are used side by side.

BA Fixings Data Chart

BA Fixings Data Chart