I think the rule that with steel rods, .040 is ideal and .060 for aluminum rods is pretty much spot on, first from my personal experience. And a few years ago I built a 512, with a world products aluminum block, aluminum rods, and aluminum heads. I knew the .060 rule for those rods, but I pondered about the aluminum block and it's affect on the clearance. WELL, my son is a mechanical engineer and I called him and asked him to calculate the required clearance, he said he needed the aluminum alloy numbers, and some other measurements, after he did the calculations, he came up with .058 for my engine. So I just figure the .060 is a pretty iron clad result.