On 7/2/2013 8:00 PM, wrote:
(In the spirit of "It's better to ask a stupid question than to make a
stupid mistake...)

These two screws are both sold as 4mm:

https://plus.google.com/photos/11272...CLGTvbqC-5LWMQ

(links to four pictures, showing measurements on digital caliper of
shaft & thread for each of the two screws. Hosted by google, no adverts
other than what google stick in, if any)

Am I right in my understanding that 4mm is the maximum size of the thread
diameter, i.e. they are likely to be smaller than this measurement, but
they will not be bigger?

Secondly, for a pilot hole, I understand that the drill bit used should be
the diameter of the shat of the screw, so it is basically the threads that
are biting into the wood fibres and the screw shaft is not having to be forced
in. But as the photo's show, the shafts are not of an available drill size so
it is better to go for the next drill size up or down? Or round up or down
either side of the half measurement?

Sorry if these questions seem trivial to you all, but this sort of thing
gnaws away at the back of my mind, so it's nice to get some professional
input!

Thanks in advance,

David Paste.

The size of a screw is determined by it's complete diameter, including
thread. If they are smaller, then it's not really what it's supposed to
be, is it.

Drill size is always smaller unless you have no intentions of biting
into the material. For example, if fastening a mount to an engine block,
the holes for the mount would be larger than the screw, but bite into
the block to draw the mount into the block. Of course, this is using a
machine screw, but the same concept applies even with wood screws.

I can assure you, there is a proper drill size for your need, you simply
may not have it. If not, go one smaller. The main objective is to
eliminate splitting/tearing of the wood. Even a smaller pilot hole will
eliminate splitting. If you go larger, you risk less biting and thus,
the screw would be more prone to loosen.