'Hang around a bunch of motorheads during a bench-racing session and within the first five minutes you're nearly guaranteed to hear someone claim to have an engine that's been "balanced and blueprinted." It's one of those terms that's been thrown around since your grandpa was tuning on his Model T, and yet few can explain exactly what it means. Despite what you'd believe based on the frequency of its usage, the label of balanced and blueprinted actually refers to an engine treated to far more than a simple rebuild.
The basic premise is that the production engines we tinker with were designed to be produced on an assembly line by an army of automated machines and fast-moving technicians, and as such, some flexibility in specifications has to be tolerated by manufacturers to allow for variances in quality. This flexibility can be seen on engine-specification charts in areas like combustion-chamber volume and piston-ring endgap where proper measurements are given in ranges rather than pinpointed figures. The original equipment (OE) manufacturers knew their engines would run for years without trouble in whatever vehicle they were installed, even if every spec were on the less favorable side of the scale, but that's where they left room for improvement.
Since you don't have to be as lenient when building your engine, you have the option of making all specifications exactly what the engineers thought they should be. This means each combustion chamber can be manipulated to displace the smallest allowable volume, each piston ring can show the minimum amount of gap, and each bearing can hug its journal at the tightest tolerance recommended. In addition, all of the moving parts in the bottom end can be weight-matched, and then the rotating assembly can be spin-balanced to remove any hint of vibration.
In theory (and usually in practice), such attention to detail should translate into increased power and durability, but you still have to know what deserves attention and how to determine if it is in fact exactly where it should be. For that we've assembled a collection of the primary areas that should be scrutinized during your next engine build, and what you should do about determining and assuring the accuracy of each spec. Some of these items require specialized equipment most of us may never actually own, but the information is still valid, as it will allow you to communicate with and pester your machinist to get the results you're looking for. Read over our advice, and if you're getting ready to build, follow it up with the further reading we've recommended
How to Read a MicrometerIf you're going to blueprint an engine, you'll need to learn to read a micrometer. You could use an electronic digital mic, but a real gearhead should know how to read an old-school micrometer. Here's how to read a mic accurate to 0.0001 inch.
Start by verifying that the mic reads zero when the spindle closes to the anvil. If it doesn't read zero, adjust the mic until that is achieved. If you look on the spindle, the long horizontal line should line up with the zero line on the rotating barrel. The large hash marks along the long horizontal line with numbers represent 0.100 inch (0.100, 0.200, and so on). Within each of the large numbers are three smaller, vertical hash marks on the horizontal line that represent 0.0250 inch. Around the rotating barrel is a series of numbers from 0 to 24. This means that each full rotation of the barrel equals 0.025 inch. Finally, you'll see a series of long parallel lines above the main horizontal line that break down 0 to 9. These lines represent ten-thousandths of an inch (0.0001 inch).