It's not the grade of the bolt that creates the problem, it's the design. What I learned about bolts and threads in the aircraft industry is that bolt threads are supposed to be designed so that when the nut is tightened there should be at least 3-4 threads still left untouched by the nut. The extra threads act as a spring that stretches as you tighten the nut. If you leave too few threads left on the bolt there is nothing to "stretch" so all the torque is quickly transferred to the last thread without any "spring" to absorb the shock of the torque. The shackle bolt threads end very abruptly at the shoulder and there is where they always break. Too few threads to stretch in the short distance between where the nut ends up and the shoulder. If fact, the number of threads that are left (that can be stretched) is basically the number of threads in the thickness of the shackle...not much. Of course it could also be the cheap Chinese cr@P!!!!
