British Standard Cycle

British Standard Cycle (BSC) or (BSCy) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth and British Standard Fine) the thread runs at a 60 degrees rather than a 55 degrees angle. It generally comes as 26 threads per inch (tpi) and is similar to 1 mm ISO threads which are equivalent of 25.4 threads per inch and also run at a 60 degrees angle. It was originally used with both bicycles and motorcycles.[1] However it is now believed to be obsolete in motorcycle manufacture. In the bicycle industry it is still found on virtually all bottom bracket threads and the wheel axles of low-end models manufactured in China.

Traditionally the parts it would be found on are:

Note that M8 × 1.00 metric threaded parts can interchange with 5/16th × 26 tpi parts with a "Class B fit", meaning you should not go back and forth between threadings. British Bicycle standards for other parts which are not strictly BSC but come in similar fine fines include:


The thread was defined to meet a requirement for a thread form for bicycles and motorcycles, originally specified by the Cycle Engineering Institute. Before being adopted as a British Standard it was known as the CEI thread.

See also


  1. Morgan, Brian. "BSC (British Standard Cycle) Thread Data". Retrieved 2009-06-05.
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