They can be fixed if they're loose, but I dont think they can be replaced in a sealed unit. I know before I turned mine into powder, it would creak sometimes, All I would do is pull my cranks/BB and put grease on EVERYTHING. That always solved my problem.
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"the bus came by and I got on, that's when it all began...there was Cowboy Neal at the wheel of a bus to never-ever land."