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You're close, Bob. Blueprinting means machining an engine very accurately, and to specific dimensions.
For instance, the factory specs for certain bearing clearances might be .0015" - .0030", and as long as each bearing falls within that range it's considered accurate enough. On a blueprinted motor, we might specify .0028" for those, and the clearance for each bearing in the set will be machined to precisely that. Similar improvements in precision apply to setting endplay, parallelism and perpendicularity throughout the motor.
In factory motors the rods may vary a few thousandths in length. That's no big deal with the large piston-to-head clearance found in stock motors, but we're trying to reduce it to essentially zero at peak RPM and operating temperature. Any variance in piston height relative to the block deck can obviously be disastrous in this scenario. Many machine shops are not equipped to correct this.
After all that, everything gets balanced by removing metal. Like reciprocating parts all weigh the same, and rotating parts don't have a heavy or light side (there's more to it, but you get the idea).
The result is a motor that's vibration-free, runs with reduced friction and stress, and is uniformly strong. I'd say blueprinting and balancing adds $500-$800 to the cost of an "average" rebuild so it's generally done only to motors that will be receiving expensive performance mods, but any motor will benefit.
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