On Sun, 3 Feb 2008 17:22:03 -0800 (PST), "
wrote:

whats better for strength? i have four fasteners to choose from:

8-32

10-24

10-32

1/4"-20

If diameter is fixed, whats stronger, more or less threads?

The load will be shear, i.e., it will try to shear the bolts off,
putting stress on them in that direction.

If the thread is in the shear plane, the screw with the largest root
diameter under the thread will be strongest. For a given OD, a finer
thread will have a larger root diameter. If the shear is carried by
the unthreaded portion of the screw, the thread pitch won't matter.

If the fasteners are torqued tightly enough that load is carried by
friction in the joint rather than shear, the fine thread again has the
advantage because its greater root diameter makes the bolt stronger in
tension, allowing it to apply more force to the joint.

Among your choices, the 1/4-20 is strongest; a 1/4-28 would be
stronger still.

--
Ned Simmons