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Metric Bolt System
In November 1948, the "Unified thread" was agreed upon by the UK, the US and Canada to be used as the single standard for all countries using inch units.
In 1965 the British Standards Institution (BSI) issued a policy statement requesting that organisations should regard the previous BSW, BSF and BA standard threads as obsolescent. The first choice replacement for future designs was to be the ISO metric thread with the ISO inch (Unified) thread being the second choice.
Metric threads are designated by the letter M followed by the nominal major diameter of the thread and the pitch in millimeters. For example M10 x 1.0 indicates that the major diameter of the thread is 10mm and the pitch is 1.0mm. The absence of a pitch value indicates that a coarse thread is specified. For example, stating that a thread is M10 indicates a coarse thread series of diameter 10mm (giving the thread a pitch of 1.5mm).
The thread form for Unified and Metric threads are identical. If:
- p = pitch of the thread
- d = depth of the thread
- r = radius at the top and bottom of the threads
Then:
- d = 0.54127 p
- r = 0.14434 p
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