Cycle Thread Bolts & Screws

Standardised by the Cycle Engineers Institute in 1902 and used for threaded parts of cycles and motorcycles.

Known as CEI or Cycle up to 1938 and latterly as BSCy, BSc, Cycle or BSC.

Applying to most British motorcycles is 26 threads per inch (tpi) up to 7/16 inch bolt diameter and 20 tpi for sizes 1/2 inch and above.

A progressive phasing out of this thread form began in the later 1960's for BSA, Norton, and Triumph motorcycles.