For about ten years I ran a world products aluminum block and Mickey Thompson aluminum rods with a set of Ross flat top pistons. The old rule of thumb is .040 for steel rods and .060 for aluminum rods. When I first built the engine I had my son who is a mechanical engineer do the calculations, he was very precise, I had to get the alloy of the aluminum being used etc. HIs calculation on that engine was .058 at 300 degrees, so I would say the rule of thumb is pretty close.