[olddog]

It's not just valve lash.

With an aluminum block, the piston is much closer to the head, when cold. You loose compression ratio as the engine heats up.

The Cady North Star engine had issues with carbon building up. Then on a cold morning the piston tapped on the head, making quite the knocking sound.

Thermal expansion of different materials is a real thing that must be designed around.

OBTW the bearing clearances are changing too.