As I discovered to my cost, the "core" of a threaded bolt is actually a lot less than the actual bolt size by the time you take into account how deep the thread is. As I recall, for M6, it is less than 5mm. Considering that the strength is related to the area, and that is related to the square of the radius, the thread reduces the strength considerably.
A bit of a rambling explanation but you get the gist...
Cheers, Robin
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