Hexagon head bolts are classified into grades A, B, and C. Among them, Class A is the most accurate and Class C is the least accurate. Class A is used for important, high-precision assembly and places subject to greater impact or variable load. GB5780 is a grade C bolt with a specified thread length, and GB5781 is a grade C bolt with a full thread. The thread tolerance of the two is 8g, and the mechanical performance level is 3.6, 4.6, and 4.8 when d is less than M39, and when d is greater than M39, it is in accordance with the order agreement. GB5782 is grade A and B bolts with specified thread length, and GB5783 is grade A and B bolts with full thread. The thread tolerance of the two is 6g, and the mechanical performance levels are 5.6, 8.8, 10.9, etc. respectively.