i know the 4100 is aluminum........im wondering if this one is also..no not aluminum heads, aluminum block. Cadillac 4.5L DFI V8 Vin 5 155hp 4000 RPM 240tq 2600 RPM 3.62 inch bore, 3.31 inch stroke 9.0:1 compression ratio Aluminum block, cast-iron heads Cadillac 4.5L SPFI V8 Vin 3 180hp 4300 RPM 245tq 3000 RPM 3.62 inch bore, 3.31 inch stroke 9.5:1 compression ratio Aluminum block, cast-iron headsAluminum Block Cast Iron Heads !!!! with floating cast iron cylinder sleaves...Yes, the whole thing is aluminumyes it is aluminiun heads cast blockOther related question