Mike Patterson: I agree with everything Harry wrote up below, but would add that a balanced assembly is important. If you've ever seen the calculus equations describing the reciprocating mass in an internal combustion engine, you would understand it IS rocket science. (My teacher, a woman, skipped over the example I would have liked to have spent two weeks on to focus on financial statistics instead.)
There has been an extensive discussion on engine vibration recently (search on Brian Wittman
); apparently the engine's balance (and hence vibration) is affected by the speed at which it is turning, i.e., its RPM. Many users report more vibration at medium RPM than at low or high RPM. I'm not sure of all the science behind it, but I do know that the tolerances allowed in a typical manufacturing environment can be improved upon. Dave Kirk, of Kirk Engines, the creator of the "Killer Kohler" (see the link on the "Better than New section) will take your crank and add balance weights to it that are permanently attached, or at least are in little danger of destroying themselves or your engine. But again, most of us don't need them. The balance becomes a concern with the greater reciprocating mass of the 12 HP, 14 HP, and 16 HP motors, and even then, as Harry mentioned, the factory simply did away with them. So unless you're building a puller, trying to get the last HP you can out of your engine, or have a vibration problem that can't be solved in any other way, balance gears, or a balanced crankshaft aren't really necessary; and the balance gears especially are undesirable.
Again, not your worry, because your engine doesn't have them.
