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The holes in the jewels and the pinions must be cylindrical. Any geometry other than this will cause problems. Any chips or cracks in the jewels are a problem. The cylindrical portion of the pinion must be the only thing that rides in the cylindrical hole in the jewel. A cracked or chipped jewel can cut into the pinion and cause it to wear in an uneven fashion. The best way to spot these issues is with a microscope.