On 10/20/2014 12:35 PM, michael@c... wrote:
> But to my mind an
> existing or old wood screw and nut should be correct, so unless they are
> coated with something jamming them up either the screw or the nut has
> got moist.
>
> Michael

More likley both. I believe that as old wooden clamps shrink or expand, 
you can have a differential effect. The hole in the jaw only shrinks and 
expands significantly in one dimension ( across the jaw) , but the 
threaded rod shrinks and expands both ways radially. Plus, they are 
frequently made of different woods.

FWIW
Don