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 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]
Cycle thread in 7⁄16 and 1⁄2 inch sizes also come in 20 tpi and 24 tpi options.
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
^Morgan, Brian. "BSC (British Standard Cycle) Thread Data". Archived from the original on 2017-04-27. Retrieved 2021-12-29.
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