"Unscrew this bolt" is correct. I think "do up/undo" is British. " I'd say that in AmE, unscrewing the bolt is tantamount to removing it.
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NewguestBut is it better to say: Do this bolt up OR Do up this bolt.I'd rather slash my wrists than say either one of them, but I think "do up the dishes" is more common than "do the dishes up" - but I've heard both.
Clive What does it mean?The old folks were still saying it in New England when I left in the 60's. How about, "Do up the supper things?"