Well, here's what I found on the aluminum vs cast iron history: ""Pistons [on the '23 model] were slotted aluminum alloy but reversion to cast iron (as in the Single Six) was made in December when aluminum was found to suffer from warpage, piston slap and wear." Packard, A History of the Motor Car and the Company, Beverly Rae Kimes, ed., copyright 1978, Automobile Quarterly, Inc. p. 238.