Threaded bottom brackets use a shell with internal threads to hold threaded parts. The frame shell may be threaded directly at the shell, or there may be an insert installed that has internal threads. 

Threaded bottom brackets shells have been made over time in various thread standards that are not interchangeable. The common threaded bottom bracket uses cups or adaptors with the thread specification of 1.37″ x 24 threads per inch (approximately 34.8mm diameter). The most commonly used term for this standard is “English” threading, sometimes abbreviated as ENG. The non-drive side (left side) on the English standard is a right hand threading, tightening clockwise. The drive side (right side of the bike) uses a left hand thread, tightening counterclockwise. The shell inside diameter will be approximately 33.7mm.

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