British Standard Cycle

British Standard Cycle (BSC or BSCy or CEI) 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 1⁄4 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, which are derived from pre-WWII British roadsters.[citation needed]

BS 811: 1950 provides specifications for British standard cycle threads. [2]

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

1/4" diameter cycle thread nuts and bolts have the same 26tpi as 1/4" BSF, which means that they will fit each other in this diameter only.

Traditionally the parts it would be found on are:

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

Note that M8 × 1.00 metric threaded parts can interchange with 5⁄16 inch (7.94 mm) diameter × 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 threads include:

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

History

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.[citation needed]

See also

References

  1. ^ Morgan, Brian. "BSC (British Standard Cycle) Thread Data". Archived from the original on 2017-04-27. Retrieved 2021-12-29.
  2. ^ "BS 811:1950 | 24 May 1950 | BSI Knowledge".


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