Dave Plowman (News) wrote:

In general you can't. The shank of a bolt is smaller than the correct size
to tap a thread on.

The unthreaded portion of a bolt is bigger than that of the thread. If
it's smaller, it would be a screw.

You can roll threads on screws (this is how they get to be bigger in
diameter) but bolts are generally either cut threads or a two-diameter
dowel with a thread rolled onto one part of it.

A bolt is basically a dowel pin in shear, with a loose threaded nut to
fasten it. It may also provide compression, but it's capable of
resisting a radial shear load. This requires a plain shank that's a
snug fit in the hole, i.e. bigger than the thread.

A screw is a clamping device into a tapped hole. It's not intended for
major shear loads and so the shank diameter, or its smoothness (i.e.
non-threadedness) isn't so important.

Many screws are loosely called "bolts", but they aren't.