On 29 May, 15:41, dg wrote:
Apart from reasons of practicality, is there any reason for not using
coach screws instead of coach bolts for fixing large timber sections -

They're entirely different devices, for different purposes. One's a
bolt, one's a screw.

Bolts are plain-shanked dowels used to resist shearing (sideways)
forces. For convenience in installation compared to a plain dowel, the
ends are threaded so that a locking nut can be attached.

Screws are compression fasteners intended to compress two pieces
together. They shouldn't be loaded sideways in shear, as they're
relatively thin and poorly attached, thus unable to resist this well.

Obviously there's some crossover: bolts in particular can apply
tensile forces, although screws are much less happy in shear. In
principle though, use each for its intended purpose and don't confuse
them. This applies equally to design in wood or metal, although wood
is usually a little less fussy about applying shear to screws than
metal would be.