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Adjustable Cup
From WikiPedalia
In a conventional threaded bottom brackets, the left cup is adjustable, and its position is secured by a lockring. The right cup is not adjustable, its position is fixed, usually by a shoulder which presses against the side of the bottom-bracket shell. The fixed cup is screwed all the way into the bottom bracket, until the shoulder runs up against the end of the bottom-bracket shell.
The bearings of a cup-and-cone bottom bracket are adjusted by screwing the adjustable cup in or out of the bottom bracket shell, and the adjustment is secured by the lockring.
See also the Bottom Bracket Glossary Entry and my Tool Tips article on Bottom Bracket Adjustment.
