BSA: the classic threaded bottom bracket
BSA is the most common threaded bottom bracket standard. It uses a 1.37″ × 24 TPI thread and the frame shell is 68 mm (road) or 73 mm (MTB). The drive side is left-hand threaded (tightens the opposite way) so it won't loosen itself. It's the easiest to maintain and almost never creaks; it fits Shimano 24 mm and SRAM DUB spindles.
If your bike is alloy or mid-range, it's almost surely BSA. It's the oldest, most universal standard and every mechanic's favorite because it's easy to service.
What is it?
BSA (British Standard) is a bottom bracket that threads straight into the frame, pressing nothing. It seats firmly, doesn't depend on tight frame tolerances and rarely makes noise. It dominates alloy and mid-range bikes.
Specification
What fits
It natively accepts 24 mm spindles (Shimano Hollowtech II) and SRAM DUB (28.99 mm), with external cups. It can also take 30 mm spindles, but only with thin-wall bearings (less durable). Almost every crank on the market has a BSA bottom bracket version.
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Frequently asked questions
Are BSA and English threaded the same?
Yes. BSA and English refer to the same standard: a 1.37 inch × 24 TPI thread.
Why does the drive-side cup tighten the opposite way?
Because it's left-hand threaded: pedaling forces tend to tighten it rather than loosen it (precession).
Is BSA compatible with SRAM DUB?
Yes, there are BSA DUB cups for 68/73 mm that house the 28.99 mm spindle.