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...the better loose bearing BB spindles and cups are case hardened. This allows the use of a low carbon steel that is easily machined, and remains somewhat resistant to surface wear, while still not failing in use due to brittleness. If you wear them down past this exterior layer of case hardening (for example by trying to resurface a pitted bearing race on the spindle or in the cups), they wear out again pretty quickly. So it's a bad idea to try to resurface them by grinding and polishing.