This standard of the GB carriage bolt stipulates the thread size from M5-M20. As the aquare neck,Website:http://www.chinajinrui.net,the depth of square shall be measured,parallel to th axis of bolt at the midpoint of the flats   of square,from the bottom of square to the bearing surface for bolts having flat bearing surface type heads,and to the intersectio of flats of square with the conical bearing surface for bolts having countersunk head.The corners of square neck may be rounded to the extent specified in the respective dimensional tables and need not be filled out for the total depth of square.StandardMaterialFinishThreadGB/T14carbon steelPlainUNCZinc platedAlloy steelYellow zinc platedPTFEStainless steelBlackUNFDacrometOtherHDGOther