mhjo,
I'll do my best to help relieve some of the confusion.
> Early 289 & 302 engines (until, I think, 1981) had a crankshaft imbalance factor of 28 ounces
> Later 302/5.0 engines (I believe 1981 & later) had a crankshaft imbalance factor of 50 ounces
> Each of these engine groups need to have
A. The correctly balanced/weighted vibration damper (also referred to as a harmonic balancer) fastened to the front of the crankshaft and your front crankshaft pulley bolts to it
B. The correctly balanced flex plate/fly wheel that bolts on to the rear of the crankshaft and your torque converter bolts on to it.
C. These parts need to match each other AND the crankshaft to provide proper crankshaft balance,
> If one or the other, or both, of these pieces is incorrect it will cause vibration issues much as you are describing. It will also cause serious damage to your engine in the long (or maybe short) run.
> When only one or the other of the parts is incorrect, the vibration is often less evident/noticeable and more difficult to track down.
> One difficult part of the issue is that the 50 ounce parts will "bolt on" to a 28 ounce engine and also the 28 ounce parts will "bolt on" to a 50 ounce engine.
> What many people who are unaware of the balance issue have done, is to install an AOD or T5 (from a later model car/engine) onto the back of an early engine, utilizing the flex plate/fly wheel from the late engine that likely came with the transmission. There are several other scenarios of mixing the wrong parts, but seeing that you have an AOD, we will not address those at this time. All (or at least, most) AOD transmissions that came with the 5.0 engine with a factory/OEM flex plate are for a 50 ounce imbalance, but I'm not sure about an AOD from a 351, (see next bullet)
> 351 engines don't fall into the same specifics and as long as you don't have a 351, we won't worry about it for now. If I recall correctly, all 351 engines have the same imbalance factor, I just don't remember if it is 28 or 50 ounce. The only thing that may or may not be pertinent here is that the flex plate and vibration damper from a 351 will physically "bolt on" to both early and late 289/302/5.0 engines.
What we need to do to help you, is determine what engine you have and ESPECIALLY what year it is. We really can't do this by identifying the "bolt on" damper or flex plate, as one or the other, or both of them may be wrong. We also don't want to try identifying from numbers on the heads or intake, as who knows what has been swapped in the last 45+ years. I believe there are some numbers stamped and/or cast into your engine block, I'm just not sure where or which ones will help us, except for the numbers/letters cast onto the block just above the starter. If you can see these (from under the car) please post them so that we can at least get a start on this. I am hoping that someone else will speak up to help with ways to make this identification, as my knowledge in this is a little lacking.
I realize that this all can sound confusing, but rest assured that the parts are available to make this right and are not terribly expensive, once we have all of the detective work done. Once we have the engine identified, we can work on identifying the vibration damper and flex plate and determine if they are right or wrong for your application.
Hope This Helps,
Gene
