[Scoober]

bolts are an energy storage device like a capacitor or bungee cord. Normally bolts are used as really stiff springs to hold the parts together. Bolts will stretch a few thousands to creat clamp load. The part being clamped needs to be stiffer than the spring(bolt) to hold and not have a loss of preload wihich will cause self loosening. Tighten it too much and you will arrive in yield or permanent deformation. If you tighten to proof (about 80% of yield) and there is alot of vibration or thermal expansion will yield the fastener. Some belting type applications that I have experienced are a shoulder type bolt that will allow movement of the belt mounting bracket and the shoulder is contacting the secure surface. Maybe a tube over the bolt and inside the belt bracket is the way to go for movement type and a large hardened flat washer for non movement.

Grade 5 are more compliant, they will be more tollerant to extreme vibration than grade 8 will. Grade 8 is a bit more brittle but are stronger. Metric fasteners are typically stronger than SAE in the same sizes.