What grade of hardware to mount seats?

[mocwon]

69 Short Fleet; oops.... guess I should have read your post before posting my reply....



I started to post this last night and then deleted it. I believe these bolts are 5/16 X 18. Grade 5 bolts of this size (what GM used from the factory for this application) have minimum tensile strength of 120,000 PSI and grade 8 150,000 PSI. There are a lot more factors to consider other than the grade of the fastener such as proper torquing, rolled threads vs. cut threads, the condition of the threads, whether a washer is used, how many times a fastener has been used and re-used, etc. The stamped metal seat bracket would likey tear through the bolt head before either a grade 5 or grade 8 bolt would fail. One thing is for sure, don't use bolts with no head markings like Home Depot and Lowes sells since these are grade 2 bolts with about half the tensile strength of a grade 8 bolt.