So if I have a internally balanced 350 with a weighted flex plate, will this cause a pretty good vibration?

 

Depends
What type do you have?
1 pce seal crank engines are ext at rear and neutral at front
2 pce seal engines are true internal ; neutral at both ends

Yes
but you would be struggling to fit the wrong one because the cranks have different bolt patterns
( unless you had a 400 flexplate on 2 pce seal 350 )

 

absolutely correct.

 

Did a ton of research last night and found out exactly what was stated above. 1pc rear mains have the counter weighted flyweel/flexplate.

2pc don't.

I ask this because I have a 87 350 bored .40 over. 1pc rear main and weighted flexplate for my 700R4. I'm getting pretty good vibrations right at the back of the engine which lead me to think it was the wrong flexpate.

What could this be? Maybe the torque converter is throwing it off?

 

Are you using a solid mount under the back of the 700R-4? If so, switch to poly or rubber.

 

The vibrations are at idle and they would make your teeth chatter if you were in the car. I have yet to actually drive the car yet.

 

I'll unbolt the torque converter and push it back and make sure my engine isn't the issue.

 

Well it's not my torque converter

 

Older, 2-piece rear main seal 305 and 350 cranks have no extra counterweights on the flexplate. But newer, 1-piece rear main seal 305 and 350's have a small counterweight on the flexplate only. It is smaller than the counterweight that you would find on a 400 flexplate.

The reason for this is that on 1-piece rear main seal engines, the crankshaft flywheel flange has a slightly smaller bolt circle than on the older cranks, and it is perfectly round. On the older cranks, there is an offset cutout, or maybe a kind of "ear" shape to the crank flange that provides a slight counterbalance that is necessary for these engines. It is a smaller conterbalance than what is needed on a 400, and it is only needed on the crank flange or flexplate flywheel, not on the harmonic balancer too as it is needed ona 400.

 

So I am in the middle of a motor swap, from a 1 piece 305 motor, to a 2 piece 350 motor. Will my torque convertor for the 1 piece rear seal bolt up to the new 2 piece 350 flexplate???

 

It should there are 2 bolt patterns that i'm aware of, and most plates have both patterns. Also some after market cranks come with intentionally heavy counter weights so you don't have to add heavy metal to them while ballancing. If this is an aftermarket crank and you didn't get a ballance job there is a chance that this problem is internal

~Couch