John B. Slocomb wrote:

The normal engaged thread length depends on
the diameter of the bolt and the "standard"
(strength) For example, an M5 screw (grade
6h/6g) will have a minimum stressed
(threaded)area some 4.0103 mm long. This is
usually felt to be a sufficient thread
engagement to provide some 100% of shank
strength. There are innumerable tables
available giving the required data.

M5s with flat, round Torx (T25) heads are the
most common. Well, they are always M5 what
I can remember but sometimes they are
Pozidriv (PZ2) instead of Torx.

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