Basic screw questions.

(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/112724770564213352605/albums/5896168001404313601?authkey=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.
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On 7/2/2013 8:00 PM, snipped-for-privacy@gmail.com wrote:

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.
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On Wednesday, July 3, 2013 3:40:29 AM UTC-7, SBH wrote:

1+ What he said.
Although when I read the headline question I assumed the answer would be "lay her on her back, climb on and get to it."
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On 7/3/2013 12:27 PM, SonomaProducts.com wrote:

The 3.91 can arguably be called 4mm, the 3.7 NOT. I think if you used the thick part of the jaws to measure the threads near the head you might be closer to 4mm..
That's just my opinion, not based on any formal knowledge.
--
Jeff

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On Tue, 2 Jul 2013 17:00:07 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

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
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On Tuesday, July 2, 2013 6:00:07 PM UTC-6, snipped-for-privacy@gmail.com wrote:

.com/photos/112724770564213352605/albums/5896168001404313601?authkey=CLGT vbqC-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 th at 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, fo r 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 i nto 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 eithe r side of the half measurement? Sorry if these questions seem trivial to yo u all, but this sort of thing gnaws away at the back of my mind, so it's ni ce to get some professional input! Thanks in advance, David Paste.
Quick tip for attaching two pieces of wood together, drill the pilot hole o n the board farthest from the head of the screw,with a drill bit that is sm aller than the diameter of the screw. I usually go with a drill bit as clo se to the shaft (minus teeth)as possible without going over it. Drill the pilot hole of the board nearest the head of the screw with a drill bit that is the diameter of the teeth of the screw. This way when you screwing in the screw, the screw will float thru the first board and grab the second bo ard, thus creating a tighter joint.
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Read
http://en.wikipedia.org/wiki/Screw
all (or almost all) your questions will be answered.
ck

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