Cranks are measured in millimeters from the center of the bottom bracket to the center of the pedal spindle. It's usually written on the inside of the crank near the pedal. Bottom brackets can have a lot more variables. If it's a threaded shell, it's mostly a matter of measuring the width of the bottom bracket shell to get the right bottom bracket. If it's a press fit then you need to know both the width and the inner diameter of the BB shell itself.
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