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Just about everything blykins has to say is correct.
Grade 8 bolts will handle more stress than grade 5 in every loading case (assuming identical bolts). It doesn't matter if the load is tensile, shear, or bending moment. That is assuming they are installed correctly. Grade 8 bolts are much more succeptable to failure if they are not pretensioned to their minimum approved value, or if washers are not used when required. ESPECIALLY in cases of cyclic or redundant loading...which I would imagine is the case in most vehicle chassis.
However, grade 8 bolts' reduced ductility means that microscopic crack propogation due to manufacturing defect or otherwise will occur at a much faster rate. Grade 8 bolts will typically fail catasrophically, whereas grade 5 may give you some warning.
Using grade 8 in every application would not only be irresponsible, but wasteful. Even a properly installed grade 8 bolt becomes useless when the connection will fail in block shear or tearout before the bolt itself fails.
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