3. Grease that's white or milky has been contaminated with water. If it's gritty it has dirt or sand in it. If it's black it has probably been hot. All these mean that it's time for a cleaning and repack. Once you have the old bearings out, give them an inspection for wear, pitting, rust, or scarring. Check for cracked or broken bearing cages. The cage is the piece that holds all the rollers to the bearing. Slight discoloration isn't bad, but if it's dark like burnt metal, it's time to replace. If you're reusing the bearings, take the time to clean them with solvent and compressed air, but do not let the air spin the bearings because this can damage the rollers.