Balance in mechanical terms describes the
relative weight of moving components that operate in relationship with each other. According to
Newton,
for every action there is an equal and opposite reaction. Consider a
V-8 engine. At any one moment, there are eight
pistons, and eight
connecting rods moving together. The
crankshaft is always designed so that at least two cylinders operate in opposition, in order to cancel
vibrations within the engine. The moving mass of the opposing part, cancels the motion of its
compliment, preserving
Newtonian balance.
The closer in weight of all moving parts, the greater will be the vibration cancellation. The greater any mismatch in mass, the greater the vibration. The inertia of the heaviest part -- in the case of a flywheel, the heaviest section--- will pull the entire engine in the direction the heaviest part is moving.
It may be possible to balance an engine by taking all matching, moving parts and weighing them. Each part is machined until it weighs precisely the same as the lightest part. This reduces moving mass and vibration, making the engine more reliable at higher RPM. Engines that have been balanced can rev higher and make more power reliably.