British Standard Cycle

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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. All sizes ​14 inch and larger use 26 threads per inch (tpi), making them similar to 1 mm ISO threads, which are 25.4 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[by whom?] 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.[citation needed]

Cycle thread in 7/16" and 1/2" sizes also come in 20tpi and 24tpi options.

Traditionally the parts it would be found on are:

  • Front axle and seat pin 5/16th inch diameter (7.94 mm), 24 or 26 tpi threading
  • Rear axle 3/8th inch diameter (9.53 mm), 26 tpi
  • GB Stems using 5/16th inch diameter (7.94 mm), 26 tpi for the handlebar clamp

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:

  • Bottom Bracket cups diameter with a 24 tpi ISO standard or a 26 tpi Raleigh (now obsolete) standard


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[edit]


  1. ^ Morgan, Brian. "BSC (British Standard Cycle) Thread Data". Retrieved 2017-04-26.