this is the crank with the bob weights attached to the rod pins spinning at a balance speed the bob weights are filled with lead shot that equals the exact weight of the pistons,pins,rings,bearings,locks,connecting rods and some oil allowances.We like to balance old school on a non computer machine with floating sensors so if the crank is out of balance you will see imediately it will shake like crazy.It takes quite a bit longer but you no its right when it taken off the machine.