When the first RailKing BigBoy came out, I determined that 5.5" was the minimum track spacing for small tinplate curves I measured it on (O31, O42 and O54). My club's O72 modules have 4.25" centers on straights but our curves are not concentric. We use straight track to offset the start of the curve to allow additional spacing. Check out the photo Steve posted here.
We also have wide curve modules with O120 outer diameter that maintain the 4.25 centers throughout the curve. We often use these modules as S-curves and do not have clearance problems on them.
