Now thanks to production equipment that measures as it goes etc., poor fits are a rarity.
The best stuff - for years -- the OS line - I did .61abc (actually not abc) pattern engines -for me and others - I could purchase pistons and liners - separately and drop a piston into a liner - --they all fit and stopped at almost exactly the same place in the bore . and the same finger pressure to move to "solid" fit.
the current line of OS four strokes are still a favorite .
powerwise the Saitos beat em for power to weight but the OS 70/91 four cycle engines are bulletproof
I ran em on 40% nitro and 20% oil for hours - no undue wear and simply fantastic power for the size. None of the gasoline engines have hit this mark of overall precision----yet-- but they are rapidly getting there ---