How to Stroke Any Engine - Stroker Science

Bob Weight

Balancing a stroker crankshaft assembly is usually an easy affair, but it can get complicated if you have purchased a separate crank, rods, and pistons. The problem is that crankshaft counterweights are designed to accommodate a given amount of weight for the piston and connecting rod. The big variable is the length of the connecting rod. Longer connecting rods weigh more, which can affect the balance. Crank builders and balance shops use something called bob weight to manage these combinations. Bob weight is the weight attached to each connecting-rod throw on the balance machine that simulates the amount of weight for a pair of reciprocating piston assemblies. Often, you will see a particular crankshaft listed for a given stroke with a given amount of bob weight, expressed in grams, such as 1,800 grams.

To boil it down to the essentials, bob weight is 100 percent of the rotating assembly weight plus 50 percent of the reciprocating weight. Rotating weight consists of the big end of the rod, the bearing, and perhaps a few grams of oil. The reciprocating weight is equal to the small end of the rod, piston, wristpin, rings, wristpin locks (if used), and also a bit of oil. Let's say our rotating weight for a small-block Chevy is 474 grams and the reciprocating weight measures 872 grams. To find the bob weight, we add 474 plus 436 (half of 872) to equal 910 grams. We multiply this by two because we have to balance the crank for a pair of rods and pistons to come up with 1,820 grams. If the crankshaft has a bob weight of 1,850 grams, it's easy to drill a small hole in the counterweight to reduce the counterweight to equal 1,820 grams.

Mallory metal is a tungsten... 

   

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Mallory metal is a tungsten alloy that is 1.5 times heavier than lead. It's also verrry expensive, so don't plan on using a lot of it.

If the crank counterweight is only good for 1,800 grams, we either have to lighten the mass of our piston assemblies or add Mallory (heavy) metal to the crank throw to increase its weight up to 1,820 grams. Enthusiasts get into trouble when they assemble mismatched parts that require expensive machine work to balance properly. This is where balancing can more than offset any savings from these parts.

Bore and Stroke Chart
To find displacement, merely follow the bore from the left to match up with the stroke amounts listed across the top of the chart, match the up and you have your displacement. For example a 4.50-inch stroke with a 4.50-inch bore equals 572 ci.