Cottered Cranks - slightly loose - how to tighten up?

a hammer would be enough, just wack it, job done

You cannot get a cotter tight enough by tightening the nut, you must use a press or a hammer. Cotters are made of fairly soft steel, so that they won't damage the axle. If you try to tighten them just by turning the nut, either you won't get them tight enough, or you will strip the threads.
After a cottered crank has been re-assembled, the bicycle should be ridden for a few dozen miles, then the cotter re-tightened with a press or hammer. If this is not done, the cotter will probably loosen up. If the bicycle is ridden with loose cotters, the motion of the crank will cause the axle to cut notches into the cotters, and it will become impossible to tighten them.

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