[Christ]

Originally Posted by shovel

Forgive me if I'm retarded here, but a typical engine block is effectively a 2-layer affair, with cylinders that hold the explosions in and a shell that holds the oil and water in, around the outside of the cylinders. that shell has no compelling reason to be cast+machined out of metal does it?

You're not retarded, first off.

Secondly, engines now are typically made as you describe, with sleeves. The block actually has coolant passages and such machined into it.

Even with a plastic block, there will be metal parts that need to be machined, though. There have to be sealing surfaces and the like, and those have to be milled to specific tolerances in order to create a positive seal. Bearings, as well, have to be post-machined after casting to ensure a proper dimension and that conditions are ideal for use as an engine.