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I forget which thread I saw this on, but as an engineer I might be able to shed a little light on the bolt strength issue. I realize that we are talking metric grade bolts, but the analogy is the same for SAE hardware. A grade 8 bolt is stronger in tension and is harder than a grade 5 bolt, however; it is also harder and more brittle. A grade 5 bolt is in fact stronger in shear because it will deform slightly before is shears off. A grade 8 will usually form a hairline crack and break right off. I completely agree that the bolts are backing off and then shearing, but moving to grade 8 hardware will not solve your problem if the bolts continue to back out. ... edit...