I've never heard that one before and I've been into engines since the '60s.

Spend a couple of hours going through piston manufacturers' websites and you'll see they differentiate clearance by piston material(s).

Mahle has been doing a lot of work on steel pistons (not cast iron) because the coefficients of linear expansion are so close they can run tighter piston to wall clearances for better ring seal. Once again the piston material determines piston to wall clearance. Interestingly Smokey Yunick mentioned steel pistons as something worth looking into about 20 years ago. Steel has much higher hot strength which could be useful in some applications.

R.