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螺栓和螺钉用通孔尺寸表攻螺纹前底孔直径的确定（1）普通螺纹攻螺纹前底孔的钻头直径（2）非螺纹密封的管螺纹攻螺纹前钻底孔的钻头直径螺纹公称直径（）1/81/43/81/23/4111/413/811/2每25.4mm内牙数281919141411111111钻头直径（mm）8.811.715.218.924.430.639.241.345.1（3）英制锥管螺纹和英制锥螺纹攻丝前钻底孔的钻头直径英制锥管螺纹英制锥螺纹公称直径（）每25.4mm内牙数钻头直径（mm）公称直径（）每25.4mm内牙数钻头直径（mm）1/8288.41/8278.61/41911.21/41811.13/81914.73/81814.51/21418.31/21417.93/41423.63/41423.211129.71111/229.211/41138.311/4111/237.911/21144.111/2111/243.921155.82111/256