In article , DoN. Nichols says...

1) Hex has more surface area on the flats, and is harder to round
off with a wrench.

2) Hex uses less material relative to the major diameter than
octagonal.

3) Hex stock stores more compactly. No waste space.

4) It takes fewer cuts to machine the flats, if not working
from stock already that shape. This saves machining time.

5) Hex is easy to grip with a three-jaw chuck, which is the best
for quick gripping of round stock. Octagonal would require a
universal 4-jaw -- which does exist, but which is uncommon, and
a slight irregularity with 4 jaws means that one will not be
gripping firmly.

6) Because wrenches are harder than the fasteners, it's possible to
double the number of points on the wrench - from 6, to 12. 12 is
bigger than 8, and it's probably not possible to double from 8 to 12,
given the common metalurgy.

So by chosing hex heads, it increases the number of index points on the
fastener from 8 to 12, over octagonal.

Jim


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