On 17 Jan 2006 16:28:33 -0800, "John2005"
wrote:

I would appreciate any opinions on the best thread diameter and pitch
to use. Unfortunately, I cannot just go with a longer thread.

Re best pitch, I agree with tomcas. Put a nut on it. Read on...

Even if the "necked down" part is machined, you don't have much
bearing area in compression on the bottom of the rod and compression
force there is high for given moment because the radius is small. So
only a bit of yield there transfers all load to the thread.

Stress on the thread then varies with distance from the axial
horizontal plane with some parts stressed more than others.

Thread the rod a bit more and put a nut on it, then torque the nut
down tight against the mount. That gives you a larger bearing area in
compression resulting in more evenly distributed stress on the
engaged threads, thus reducing maximum stress on the thread.