pastor, you have to know how things get "balanced". So lets start with basics. Each crank has counterweights on them. They are strategically placed to offset the weight of the small end of the rod, and the pistons, rings, pins, oil, etc. In a stock engien, at least in a Mopar from the early 70s, most cranks were forged steel. It's heavier and more dense than cast. So it's stronger, but it also makes for a heavier counterweight. When any engine is rebuilt, certain wear parts are replaced. It is common for the performance work to replace stock type pistons and rings with lighter weight parts. Lighter is this case is always better. And ususally more $$. Stock replacement parts can also be used. However, the oversize stock parts (pistons) will be heavier than the factory std stuff. SO, the shop weighs everything, matches weights on everything, then sticks weights (called "bobweights") on the crank, to simluate the rods, pistons, rings, pins, and oil. Then the crank is spun. If the shop has the harmonic dampner, or the flexplate, or the torque convertor bolted to it, the crank is called externally balanced. That's because you are taking into account the weights of the parts bolted on it. It's usually cheaper, because in most cases, the additional parts carry extra weight to offset the bobweights. If the crank is spun by itself, the engine is called internally balanced. Because the only thing carrying weight to offset the bobweights are the crank's counterweights. I the case of almost all cast cranks and some forged (depending on how heavy the pistons are) the crank may need to be made heavier by adding "Mallory" metal to get things right. If it is internally balanced, the wrong balancer or torque convertor will make it shake. If it was externally balanced, and you didnt get the convertor it was baalnced with, it's a crapshoot as to what weight it needs to be good. If I had to make a call, I would say your 340 has a cast crank or heavy pistons, and the wrong harmonic dampner and a nuetral convertor on it. The paperwork for the engine should say what was done. If not, you are sentenced to changing parts until it's fixed (trial and error) or pulling it out, and having the lower end internally balanced to run any nuetral dampner and convertor. You issues are why I internally balance every engine, regardless of cost, and I supply records of all the work. It sure beats what you're going thru. I agree, the reason the thing is leaking is the imbalance. I was throwing out a 360 years ago in a parts car. I put a 904 trans with no weight on the convertor behind it just so I could move it around. The thing would rattle your fillings lose at 2000 rpm. The leak is the small symptom. Teh bearings will be another when you look at them. Out of balance in an engine is nothing like tires. Tires wont destroy themselves unless you push them in that shape. Engines hurt themselves even when you cant feel a problem.
