[bergy]

Walls SHOULD wear less than iron. The silicon precipitated out of solution before the matrix aluminum solidified; leaving lots of extremely hard silicon nodules. After the bores were machined, the walls were etched to eat away the aluminum matrix and expose the silicon nodules. So, the piston rings actually contacted the silicon & not the aluminum.

That's the Cliffs Notes version - it's a little more complicated. I worked with Jon Jorstaad (Reynold's chief metallurgist) in development when I was at Warren R&D. Also interesting that we received the aluminum in LIQUID form at the Messina plant.