SPANNER SIZES & THREAD TYPES - WHAT IT ALL MEANS


There are four basic bolt specifications still commonly found today, these are:

Metric
In Europe the most common size is the metric 'M' standard, the spanners to fit these are identified by an across flats measurement of the bolt head or nut. This is distinct from the thread size or form.

AF
The Unified threads standards, UNF (fine) & UNC (coarse). Originally standardised in 1948 between the US, Canada and the UK as the standard for countries using the inch unit of measurement. The spanners to fit these are again identified by an across flats measurement of the bolt head or nut in fractional inch measurements, 1/2" AF, 3/4" AF etc. These are sometimes incorrectly referred to as American Fine.

Whitworth
A somewhat older standard but still quite common in the UK and its areas of former influence is Whitworth. Sir Joseph Whitworth proposed this thread in 1841. This was the first standardised thread form. The relevant standard for this thread form is BS 84: 1956. The thread form is now redundant and has been replaced by Unified and Metric threads. The British Standard Fine (BSF) thread has the same profile as the BSW thread form but was used when a finer pitch was required for a given diameter. This form of thread was officially declared obsolete in 1965.

BA
The fourth standard is that of BA, British Association. This thread was used for small diameter threads (below 0.25 inches diameter). The size varies from BA number 23 (0.33 mm diameter with a pitch of 0.09 mm) to BA number 0 (6mm diameter with a pitch of 1 mm). Relative to the Whitworth thread the depth of the BA thread is smaller size for size. The thread form is now obsolete and has been replaced by Unified and Metric threads since 1965.

For more detailed information on thread forms and standards we would recommend visiting this site:
the Boltscience web pages

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