Piston and Rod Balancing

The first step in balancing an engine is weight-matching all the pistonsand rods. To do this, all the components are weighed. The entire pistonassembly, complete with rings and locks, can be weighed together, butthe rods must be weighed in two stages. The small end and the big end ofthe rod must be weighed separately, since the small end falls under thereciprocating weight category while the big end is rotating weight. Seethe accompanying photo for how this is done.

Now, compare the weights of the same components (for example, the smallend of the rods). If they don't all weigh the same (within the range ofa gram), the heaviest rods must have material cut away until they arethe weight of the lightest rod. The same goes for the pistons and largeend of the rods.

Fortunately, this practice only applies if you are racing with stockcomponents. Today, almost all race-quality aftermarket components comeweight-matched. Other than a quick check on the scales to make sureeverything is OK and determine your bob weights, there is nothing foryou to do. "With lightweight racing pistons and rods, there really isn'tany place you can remove material without harming the integrity of thecomponent," Dorton says. "If you have a rod or piston that is way out ofline on the weight, what you really have is an issue with yourmanufacturer."

Crank Balancing

Once the pistons and rod ends are all equal, the bob weights can bedetermined. With the bob weights attached to the crank's rod journals, abalance machine is used to specify exactly where the crank needs to beworked to bring it into balance. Whether they are forged or billet, mostracing crank manufacturers can ship you a crank that's almost perfect ifthey know the component weights ahead of time. Weight is almost alwaysadded on the outermost throws because it makes the most difference inthose locations.

A crank is brought into balance by either adding or removing weight inspecific places. Since you don't want to increase the dimensions of thecrank's counterweights, adding weight is done by drilling out holes inthe counterweights that run parallel to the crank's centerline andinserting slugs of "mallory metal." Mallory metal is a tungsten alloythat is 1.5 times heavier than lead, and a little goes a long way whenbalancing a crank.

The other method is to reduce weight. Many shops do this by drillingholes into the ends of the counterweights. This works fine, except manyengine builders feel that the unevenness created in the ends of thecounterweights increases windage. Dorton prefers to turn the ends of thecounterweights down in a lathe so that the outside diameter (o.d.) ofthe crank is actually reduced. Final adjustments are done with a handgrinder and a light touch. This takes more time, but the end result is aracing crank with a smoother outer surface.

Calculating Bob Weights

Calculating bob weights isn't difficult, but it does require a littletime at the scales. Here's the complete list for determining your bobweights for zero-balancing a crank.

Rotating Weight
* Big end of rod (including fastening hardware)
* Bearing
* Oil (normally estimated at four grams)

Reciprocating Weight
* Piston
* Wristpin
* Pin locks (if used)
* Small end of rod
* Piston rings

Bob weight = Rotating Weight + (Reciprocating Weight x .50)

Internal versus External Balancing

So far, all of our discussion on balancing has included only the crank,pistons, rods, and the other various pieces that go into thoseassemblies. This is known as an internally balanced engine. The otheroption, an externally balanced engine, also includes the flywheel andharmonic damper. In this situation, both the flywheel and damper areinstalled on the crank when it is dynamically balanced on a balancingmachine. Instead of modifying the crankshaft to achieve balance, weightsare welded to the flywheel and/or the damper.

An externally balanced engine runs just as well as an engine that hasbeen internally balanced, but this method isn't nearly as popular inracing. First, Dorton points out that if the damper were to break orcome off, the main bearings could be damaged in a very short period oftime. Then, instead of replacing just the damper, you would be lookingat a complete engine rebuild. Second, because both the damper andflywheel are integral to the engine's balance, if either need to bereplaced, the entire engine must be rebalanced. This isn't necessary inan internally balanced engine.

Automotive Specialists