I don't know the metric conversions. But there probably is one.
As a general rule, the Nut's Wieght Retention = Tinsle Strengh of bolt (Providing they are the same classification)
Finer threads allow greater torque to be applied, not neccesarily greater wieght capacity. There is a piece of literature called "The Riggers Bible," that has bolt characteristic charts. I'm havivg a little internet trouble so I can't help you with a link. Sorry (Mabe Google "Bolts")
Also "double nutting" prevents the stress nut from loosening due to vibration or heat fluxuation. As far as I know it does not add to the strength rating as 1 nut is designed to oppose 1 bolt. from past expierience I find a split spring washer works just as well, is less expensive, and easier to work with. Hope this helps.
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