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Generally, how much bigger should a new screw be so that it doesn't interfere with the crossed threads that have already been made but sits as if it had been tapped freshly?

People generally seem to go just one size larger but wouldn't one ideally want that the "solid shaft" of the larger screw exceeds the diameter of the "solid shaft + thread height" part of the older screw for best results?

Sorry if my terminology is poor - but I hope it conveys what I'm asking.

  • What is the screw going into? (wood, metal, plastic). The real answer kind of depends. So to make it simple, a given proof load corresponds to how many threads you have engaged to make it stretch a bit. So it really depends on how much load you are applying to the screw and in what direction; however, for it to be "Fully effective" as if the prior hole wasn't there, then you would need a new hole big enough to completely take out any prior threads – Eric F Jan 22 at 20:22
  • It's going into metal. Is there a rule of thumb for this? It seems the normal advice is just go one size bigger or am I wrong? – user1936752 Jan 23 at 20:24
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    Well the one size bigger rule isn't always true as the jump between sizes is not constant. The best way is to ensure the prior threads are completely drilled out – Eric F Jan 23 at 20:51

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