Tim:

That amount of aluminum shrinkage is why some casting molders like my Father became/was used aluminum alloys to create objects where shrinkage and on the other side, expansion when the product was put into use was a factor from aluminums he called piston grade aluminum where their alloys shrank so little under the various conditions where heat or vibration was involved so castings could me made tighter fitting than others resulting in less machining to fit objects that could be mated to. Similarly when the product was put in use expansion was so little leakage and alignments were not problematic.

It was at nearing his retirement when great interest was being shown in an offshoot of casting, plastic injection molding where high heat resistent thermoplastics were being used in automotive and other fields where of all things, carbon fiber was being looked at as an alternative for many metals for coverings and different structural purposes where light weight and rigidity aspects were part of the desires. Similarly great strides were made in very light weight catalyst cured epoxy plastics that would be combined with different fillers to form structural properties similar to what thermoplastics could exhibit but with less capital intensive machinery to make objects that rivaled metal castings for the purposes they were made.

My father taught me the basics of so many aspects of good basic casting practices and basic methods and know how it was as if it was one gift he gave as a reminder of who he was and what he did to be as a part of his life he wanted to be remembered for. It is only in these recent times like with what your doing is calling up those teachings for thought and use. Projects like your efforts and results provoke innovation most would not venture to. What your doing is great and at the same time very educational for those so motivated.