What Is BS Cycle Thread?
British Standard Cycle thread is an imperial thread that was originally engineered for use on bikes and motorcycles. It is no longer used in motorbike production but it is still commonly used on the bottom brackets of most bikes. This thread type is often abbreviated to BSC or BSCy.
| STANDARD BOTTOM BRACKET WITH BSC THREAD 24 TPI |
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The technical bit...
Most imperial threads like British Standard Whitworth (BSW) and British Standard Fine (BSF) have a thread that runs at a 55 degree angle, whereas BS Cycle threads run at 60 degrees. All sizes over ¼” use a standard 26 threads per inch (TPI) regardless of the diameter of the fastener, although larger sizes are also made in 20 and 24 TPI variants. Bottom brackets on bikes generally use 24 TPI BSC threads.
Common Uses
On bikes, the most common uses for BSC are as follows;
- Front Axle (5/16”)
- Seat Clamp (5/16”)
- Rear Axle (3/8”)
- Stem (5/16”)
- Bottom Bracket (various sizes, usually 24 TPI)
Alternatives to BS Cycle Thread
British Standard Brass (BSB), which is sometimes called brass thread, also has 26 TPI for every diameter. BSB has a 55 degree thread angle, but you can interchange BSB with BS Cycle for non-critical applications.
