If the bob weight figure for your new crankshaft is more than the bob weight of your new rods, pistons, and rings, then weight must be removed from the crankshaft in order to balance the assembly. This is a relatively simple task of precision-drillng a few holes in the crankshaft counterweights. If the bob weight figure for the crankshaft is lighter than the piston-and-rod combo, Mallory metal must be added to the crankshaft. Mallory is a very dense metal, more than twice the density of steel. A 11/42-inch-diameter, 31/44-inch-long slug of Mallory weighs 43 grams. The problem is that drilling a hole to place the Mallory in the crank removes roughly 24 grams, so the net gain is only about 19 grams. Thus adding 50 grams can be expensive because Mallory metal isn't cheap. It also means that researching your stroker-engine package carefully to avoid major balancing headaches and expenses is well worth the effort.
Many car crafters prefer to assemble parts for a stroker package from several different sources to save money. Where this can cause major difficulties is when mismatched components are assembled haphazardly and the balance shop must make major changes to the crankshaft in order to balance the engine properly. A simple way to avoid this problem is to purchase a complete rotating assembly from a single source. Most of these stroker kits have already been properly matched so that only minor machine work such as drilling the crank must be done to complete the final balancing.
Here is the Professional Products balancer's external weight for a small-block Ford. To reduce the amount of drilling necessary to balance this combination, Ishigo removed some of the weight from the balancer.>
Custom Balance Shortcut
Oftentimes, high-performance stroker applications can get a little complicated. As an example, we're working on a 331ci small-block Ford stroker engine buildup that includes a Scat forged steel crank, Scat I-beam rods, and a set of Mahle forged pistons. When we first assembled this package, we chose the Mahle pistons because of their high quality and light weight. Unfortunately, this caused a problem when we took the assembly to the balance shop because an excessive amount of weight would have to be removed from the counterweights to balance the engine. That's when we asked Tom Lieb at Scat for some guidance. His suggestion was simple: "Take the weight off the balancer and the flywheel." That sounded easy, and he offered to show us how it's done.
At Scat, the company's lead technician, Craig Ishigo, showed us the procedure. Our situation was made more difficult because the custom flywheel required for the engine could not be used on the engine dyno. This meant that Ishigo could not reduce weight by drilling the flywheel because a different flywheel with the standard offset weight would be used on the dyno when we tested the engine. After weighing all the components, it appeared that more than 560 grams (20 ounces) had to be removed from the entire assembly. That's quite a bit of metal.