Product Features Product Name: Steel Ball Material: GCR15 bearing steel Tolerance: 0.01mm Hardness: HRC60 Size (diameter): 15.081mm,15.5mm,15.875mm,16mm,16.5mm,16.669mm,17mm,17.463mm,17.5mm,18mm,18.256mm,18.5mm,19mm,19.05mm,19.5mm,19.844mm,20mm,20.5mm,20.638mm,21mm,21.431mm,22mm,22.225mm,22.5mm,22.8mm,23mm,23.5mm,23.813mm,24mm,24.5mm,24.606mm,25mm,25.2mm,25.3mm,25.4mm,25.5mm,26mm,26.2mm,26.5mm,27mm,27.5mm,28mm,28.57mm28.575mm,29mm,29.2mm,29.3mm,29.5mm,29.6mm,30mm1.high hardness, wear resistance, smooth surface, good roundness, not easy to rust