I was told by a knowledgeable man (Moog ring engineer) that the gap should be 0.013 in.
Your knowledgeable friend is spot on. When I got my ring package from the guys at Ross Pistons - the compression rings provided fit the bores perfectly with zero clearance. The ring manufacturer (Hastings) documents the minimum clearance based on a formula published here
Hasting Service Tips. It calls for no less then .0035 clearance per inch of bore diameter. A few minutes of filing and they are right to spec . . .
So I am with Blackwater - the manufacturer of the rings must specify the gap based on the expansion rate of the materials they are employing.
In the case of Hastings rings a clearance of .030 would be perfect for an engine with 8.57" bores!
