The traditional engine used for American style Oval track racing, a pushrod V-8, has certain imbalances inherent in the design. Pistons are driven down eight cylinder bores by exploding combustable gases. Rods are fastened to the pistons and they drive a crankshaft with arms (throws) which revolves along its rotational center. The pistons travel a few inches and are forced to stop by the limits imposed by the crankthrow. The other pistons are inroute to the limits imposed by the displacement of the arms along the axis of the crankshaft. The pistons are either up or down or some place in between in their cylinder bores. At 8 thousand revolutions per minute each piston starts and stops 16 thousand times per minute. There are tremendous forces involved in starting or stopping the pistons. If the piston assemblies were of differing weights some would stop or start more readily than others. This would cause undue stress. The stress would soon start cracks in the rotating and reciprocating pieces. Cracks become tears and tears cause breakage.

Balanced engines rotate with less resistance than an unbalanced one. Ease of rotation makes for a smoother running engine and that makes horsepower. There is less wear on the reciprocating parts and less uneven pressure on soft parts such as bearings. A balanced engine will give better performance over its lifespan.

I took Automotive Machine Shop the first time I went to Technical School. I learned how to do it right. (After school I worked for Oldsmobile and learned a lot about how to do it wrong.) This is part of what I learned.

Balancing consists of weighing a related parts group, finding the lightest one and making all the others weigh the same as the light one. If grinding is to be done, ONE part (the lightest one) must not have any grinding marks. You're not trying to make all the parts lighter, just trying to make them all weigh the same.

We'll take rods with bolts torqued to spec as a general example (don't forget to weigh the bolts). As a general rule, most factory rods weigh about +/- 10 grams within each other. The rods are weighed at the factory, usually by machine, and sorted into bins. (The guy doing the weighing isn't paid to take a lot of time sorting parts, usually he's paid by how many he weighs and not how well he sorts....it's easy to miss the right bin.) When an engine is assembled all 8 rods are taken from the same bin. No notice is taken of big or little end weight. As long as the rods are within 20 grams of each other the factory specs are satisfied. This is done for the sake of economy. However, big ends can weigh quite a bit more or less across the range of 8 rods, so balancing the big ends is more important than balancing the whole rod. After the big ends are balanced then the little ends can be done. Just remember: One big end and one little end must not have evidence of weight removal. +/- 2 or 3 grams difference is a great weight to shoot for. Less than a gram is difficult because you could easily go too light and have to grind on the lightest rod and that is not allowed.

Taking rods as the example we can mention pistons. Pistons should be balanced with the pins installed, as pins are hard to balance. One piston must not have grinding marks.

Crankshafts are difficult to balance without a lathe or a crankgrinder but it's not impossible to do a reasonable job with a good set of V-blocks. I personally balanced cranks with the balanced flywheel, clutchdisk and pressure plate installed and torqued. Spin the crank and mark the lowest stopping point by chalking a mark on the flywheel. Do it several times. If the marks all fall to approximately the same place then that side is heaviest. Pick a crank counterweight and grind a little metal off the heavy side, just a little. Spin again and mark the flywheel. Repeat the spin and marking several times. Pick another counterweight and grind a little. Eventually the spun crank will not stop in the same place but all around the clock. This is about as good as it gets!!

It's easier to have an Automotive Machine Shop balance the crank but come with a fat wallet.
If you have really GOOD friends in the parts business it's possible to weigh pistons, rods, lifters and pins out of several sets and get a lot closer than factory specs. There's usually parts that might come within +/- 2.5 grams and that's a lot better than factory.

Remember...you're using OEM parts. I'd even weigh the ARP bolts.

There's not a lot of sense in Balancing without "Bluprinting."

The engineers designed the engine to an "ideal". If you can make yours fit that ideal it will run better. Engine blocks used to be seasoned. The unmachined block was stored outside in all kinds of weather for 2 or 3 years. The cast in stresses were "softened" by the elements and the block was then machined. Even seasoned blocks have problems. Heat and cold cause the direction of the iron molecules to change, generally to magnetic north. Finding a used block from a vehicle driven by some weirdo who always parked his car perfectly aligned to magnetic north would be nearly impossible...but the block would be stronger.

Remember, when dealing with parts made by piecework the paycheck is regulated by how many and not perfection.

The problems you will encounter include crankshaft and camshaft misalignment, tilted cylinder and lifter bores and mismatched deck height. Precision align boring of crank and cam bores, multimachine boring of cylinder bores and careful resurfacing of deckheight are all possible in a quality shop: But you can't do it at home. These guys do great work but they're paid for perfection... Bring money.