The outer hexagon screw is a matching nut used to fasten and connect two connected parts with through holes and components. Hex head screws are commonly used bolts. It is more important to use Class A and Class B outer hexagon. In addition, it is often used in the occasion of high assembly accuracy, large impact, vibration or cross rate load. Grade C outer 66 screws are used in situations where the surface is rough and the assembly accuracy is not required.