Bobweight Calculator

When you need it

When sending a crank out for balancing, and any time you change pistons, pins or rods on an assembly that was already balanced.

What the balancer is simulating

The bobweight is the mass bolted to each rod journal to stand in for the parts that will hang there in service. The crank is then spun and material drilled from or added to the counterweights until it runs true against that mass.

Why fifty percent of the reciprocating weight

On a ninety-degree V8, two rods share each journal and the opposing piston is moving the other way. Half of the reciprocating force cancels, so the convention is one hundred percent of the rotating weight and fifty percent of the reciprocating, with both rods counted.

Inline engines and other bank angles use different conventions, which is why the reciprocating factor here is adjustable rather than fixed.

Weigh your parts, balance to the lightest

Do not use catalogue weights. Weigh every piston, pin, ring set and both ends of every rod on a gram scale, then remove material from the heavy ones until they match the lightest. The balance job is only as good as the consistency of the parts going into it.

Changing anything afterwards — lighter pistons, different pins, a rod swap — invalidates the balance. That is a crank back out of the engine, so decide on the final parts before it goes to the balancer.

Internal or external

Some engines carry part of their counterweight in the damper and flexplate rather than on the crank. Those parts are matched to that specific crank and cannot be swapped for neutral-balance items. Know which you have before ordering either.