Good Screw Sizes fo General Use

[Mr V]

Originally Posted by Michael Hill

Whatever the job requires. #10-24 is generally the first bolt we reach for (if something can't be riveted). We'll use 10-32 if we need more thread engagement, but they just take to long to screw in and out. Our one general rule is absolutely no metric unless we're forced to.

The rule of thumb for thread engagement is in hard materials like steel you want 1x the bolt diameter and for soft materials like aluminum you want 2x the bolt diameter. That does not change whether you are using coarse or fine threads. If you look at standard steel nuts you'll find that their height is 1x the bolt diameter. So going to 10-32 to get more thread engagement isn't really gaining you anything in fact if it is in Aluminum you are more likely to strip the threads out of the item that you have tapped.

The only real advantages that find thread have are the the bolt itself is slightly stronger and that it is slightly less prone to loosening from vibration.

http://www.bestbolt.com/white_papers/white_paper_08.pdf