Briefly answered, but I encourage you all to read about balancing and blueprinting - I'm certainly no expert. However, I have built a few engines in my life and this is how I understand it. The engine has a number of revolving and reciprocating parts. The crankshaft revolves and the pistons and rods reciprocate. Like getting your tires balanced, the crankshaft and everything connected to it that goes round and round with it, is subject to the weights and balances of the parts. This includes the clutch, flywheel, and front pulley / dampener. You know what a tire feels like when it is out of balance; the same is true of the revolving crankshaft in your engine. The reciprocating pistons and rods also affect this motion and their weights react to the balance of the revolving parts. Because of the RPMs involved, this becomes a very accurate situation. Other things come into play, like harmonics, but keeping the explanation simple; the closer one can get to perfect revolving balance to the revolving parts and equal weight to the reciprocating parts, the less power will be consumed making up for the imbalance and less wear on the engine. The engine will rev faster, have more horsepower, and will be smoother. The designers of the engines specify a certain tollerance of weights and balances. But every time you change a clutch, for example, you technically throw the engine out of balance. I just did the engine for 26th and my machinist said it would feel much smoother. The top of the block deck was stamped with numbers that correspond with the piston weights. The pistons were stamped accordingly, and they were within 3.5 grams of each other. They are now equal weight. Same for the rods, rod caps, and bolts. The crankshaft was balanced with the front pulley, flywheel and clutch plate assembled. I'm supposed to notice the difference. On a side note, the crankshaft for an early engine like mine was subject to a vibration within a certain RPM range. This "flaw" was later corrected with another crankshaft design. You may notice a metal plate welded to the side of the oil pan on an early engine. (26th's engine number is L24-2338) This is because the oil pans cracked in this area due to the harmonic vibration of the early crankshaft design.