On Tue, 2 Jul 2013 17:00:07 -0700 (PDT), 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.

To answer your first question, yes.

For your second question, yes, but it's slightly more complicated than
that. McFeely's is an excellent fastener supplier and has a good
explanation of pilot hole sizes. I'm sure there are other sites that
will support their information as well as others that will dispute it.
YMMV.

http://www.mcfeelys.com/tech/wadb.htm