Depends how big you are. I am 150 lbs and I use a 4" allen key. I tighten them as tight as I can- put them down for 5 minutes and then I put some real effort into tightening them. 1st mistake is thinking you can overtighten them- Second mistake is trying to take the bolts out after a weeks use.
That is on SPD's by the way- Just in case "Proper" cleats need more care.
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How long was I in the army? Five foot seven.
Spike Milligan