These two screws are both sold as 4mm:
(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,