How To Balance An Engine - Engine-Balancing Basics

Internal vs. External Balance
Packaging is also an important issue. During the design of the 400ci small-block, a major engineering hurdle was insufficient real estate inside the small-block crankcase for the larger counterweights demanded by the 400's supersized 4.125-inch piston. This was especially difficult in the rear of the engine because the rear crankcase area on a small-block Chevy is restricted by the placement of the oil filter. The solution called for external balance weights placed on the harmonic balancer and flywheel/flexplate. One advantage to external weights is that they are generally lighter because they are positioned at the extreme ends of the crankshaft. The disadvantage is that these offset weights impart their own twisting forces back into the crankshaft, which is not good. This same situation occurs with the 454ci big-block Chevy, which is also the only production big-block that uses external weights to balance the engine.

Small-block Fords have always been externally balanced, but because Ford is a name synonymous with change, the Blue Oval engineers altered the amount of the external balance when the engines morphed to a one-piece rear main seal. Early small-block Fords used 28 ounce-inches as the external weight amount, changing in 1981 to 50 ounce-inches. Like small-block Chevys, parts can interchange between early and late engines, but to guarantee smooth engine operation, the crankshaft, balancer, and flexplate/flywheel must all be kept within the same balance family.

Bob Weight Blues
When it comes to any kind of crankshaft purchase or if you are considering adding a set of new, lightweight pistons to an existing engine, balancing that rotating assembly can be challenging. One of the references you will run across is something called bob weight. This spec refers to the mass located on a single crankshaft journal that includes both the reciprocating and rotating weight portions of the entire assembly. Rotating weight refers to the mass of the bottom half of the connecting rod that is attached to the crankshaft. The reciprocating weight is basically everything from the middle of the connecting rod upward, including the piston, wristpin, and rings.

Every high-performance crankshaft has a specific bob weight value, such as 1,800 grams. A gram is a metric measurement equivalent to 11/428 ounce. An 1,800-gram bob weight is basically the amount of weight in each of the counterweights. However, you don't just add all the values up to determine bob weight. The formula is 100 percent of the rotating weight plus 50 percent of the reciprocating weight. Let's use an example to see how this breaks down:

Before we go further, remember we have a pair of reciprocating weights (two pistons) per crank journal. The math looks like this:

Half of reciprocating weight is 750/2 = 375 x 2 pistons per journal = 750 grams, while total rotating weight is 470 x 2 = 940 grams. Thus 750 + 940 = 1,690 grams.