I am going to have to change my flexplate on my balanced 383 and the flexplate that I am replacing has a total of 5 holes that are not on the other flexplate that I am swapping . The replacement flexplate is for sure that of a 400 and so is the cracked one on the engine( only thing is I've got some holes that I barely noticed , that aren't on the other flexplate). Is it that when my engine assembly got balanced they used the now cracked flexplate to balance the assembly?
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Well, when i had my engine balanced i gave them the flex plate i was going to use and they balanced it all together. I would think though that if everything is already balanced a new flywheel should be balanced itself. That is not a for sure answer, but it would make sense to me. IMHO, use it.

Daniel