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I wonder if it would work to drill out the block and put a bigger steel bolt, nut and washers in. Or would the shear forces kill any standard bolt? I guess they are not designed to cope with that kind of force so maybe that wouldn't work.
Do the shear forces increase a lot when the bolt gets even slightly loose or are they an issue even when it is bastard tight?
frank9755-
I can't remember exactly how much material there is to play with, obviously you wouldn't want to drill it out so much that there's not enough left to stay rigid under the compression from the bolt.
My guess is that if you got an appropriately sized M8 shoulder bolt you would improve the situation. Hi tensile bolts are actually worse under shear, but if it's done up tight enough and stays that way then I guess you can reduce the shear loads anyway. I'm sure if we got @gbj_tester involved he'd have a valuable opinion. Although I suspect it might just be to fit some stays. At some point, the additional weight of an aluminium stay and an M5 bolt will be less than one crazy overbuilt bolt.
chiroshi
I think it's less the threads, more that bolts are rubbish with handling shear force, especially in fatigue. Which when you cantilever a load on it, will inevitably happen. A single stay can take a lot of this stress, it may be enough to attach the stay even higher to still allow some flex in the rack.