I see your point - also, my heads were iron, not aluminum.Assuming the engine is truly cold, I'd not idle-run any engine with no coolant for longer than 120secs, regardless of block and / or head...

Depends on ambient temp?
Long ago, equipped with an Meziere HD H2Opump (>48GpM if you ask Meziere), parked atop a long steep hill, I tried something stupid on a 23°F / -5°C winter's day.

I...