most pcb are drilled for 4mm and/or 8-32 bolts i believe. it would depend on that size and what size nut/bolt/washer combo you supply. minimum screw/nut/washer size is going to be 3mm. and using my rads as an example, 3mm threads are weak. good 8-32 should work for bolts because i think thats what swiftech supplies with their uni blocks.
you really should slot the 53(.2) holes to allow for actual 54mm mounting. you dont want someone complaining because they used your bracket and chipped, possibly cracking their pcb while tightening your bracket down. a 7950/70 would be expensive to replace.
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