[kitcarbp]

DV, To begin, the bolt grades describe the working stress, yield stress and tensile (axial direction) properties of the bolts. see attached chart from a bolt vendor. I am not a seatbelt expert and I have wondered the same question as I am planning my seat belt mounts. The answer to your question will depend on the design of bolted joint, ie. how the seat belt is being bolted to the frame, I have seen many different solutions on these cars. But in general bolted joints need to have strength in both bending (shear) and axially. For seatbelts, if you take look at modern car designs, the typical 3-point system is set-up whereby the bolted joints are set-up to provide most but not all of the strength in SHEAR or in other words, in a crash the 3-point seatbelt loads when transferred to the bolts are trying to rip the head of the mounted bolt off in shear. (many seat belt buckles are bolted to or part of the seat base frame). The bolted joint needs to withstand alot of "G" loading in a crash beyond 10 G's in many cases (depends on the design scenario since, Force =mass x accel). I have noticed most aftermarket and some OEM seatbelt hardware is Grade 5 for street applications but I dont know what the professional class racers are using as far as bolt grades. Others who do know should add to this thread.