Blueprinting is building an engine to exact, verified dimensions rather than to production tolerance — measuring and equalizing every clearance, volume and weight so each cylinder and each rotating part matches the next.
Engine Builder describes the actual work: align-boring so deck heights are equal and the crank stays parallel, bores cut perpendicular, combustion chambers cc’d with a burette and a plexiglass plate then ground to match the largest, connecting rods set to identical center-to-center length, and bearing clearances verified rather than assumed. Ring gap gets set deliberately — the magazine cites a minimum of .0035 in per inch of bore and recommends .002–.004 in of ring side clearance. Balancing the rotating assembly is a separate operation that usually travels with it. None of this makes power on its own; it makes the power repeatable and the parts live longer. In sealed crate classes the rulebook takes most of it off the table.
Two engines built from the same parts list can differ by real power and real reliability, and blueprinting is the difference between the one that repeats and the one that surprises you.