[olddog]

Aluminum is a poor metal choice for rods. It is light and if money doesn't matter they make a great race engine that can rev quickly. However you need to replace them often.

Aluminum has a nasty habit of creeping. It is used commonly for electrical entrance service in houses. When you torque the wires down in the lugs, you need to do it right once. The connection will loosen over time as the aluminum flows out from under the lug. If you check the lug a few months later they will not be tight, but you do not want to keep tightening them up or you will eventually cut the wire in two.

The point is aluminum does not hold its shape in high stress applications. The rods are likely the highest stressed part in an engine. It is a great metal for polishing. It is easy to machine. It is light weight. It is a superb conductor of heat. It is reasonably corrosion resistant. It is inexpensive compared to other light weight materials, such as Titanium (which is much stronger and lighter). However it has a large linear coefficient of thermal expansion (it grows a bunch when it gets warmer). That makes pistons get closer to the head, when the engine warms up. It creeps under high stress. Those nice round bearings do not stay that way very long.