Learning about the Cast 340

72DMag said

Yes I have the dampner. When I googled the cast # it told me it was from 340 or 318.

Yes 318 are internal balance but that's what my question is about. Did mopar take the internal balance bobweight of the 318 (same crank) toss it in the 340 and add weights to flexplate and dampner to account for heavier pistons. It looks like they did based on this engine I got.

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Don't believe everything you read on the internet.
THIS is a cast crank 340 balancer:

Notice it says "340 cast crank only". VERY easy to ID. If it doesn't say that, it's not a 340 cast crank balancer. Period.
If it doesn't say that, you either have a mismatched crank and balancer, or some previous owner swapped out the entire rotating assembly to get away from the one and a half year only external balance so they could use more common internal balance parts.
And quit basing your assumptions on crank casting numbers. A 318 and a 340 crank could very well have come out of the same mold, but the finish machining and balancing of the cranks make them totally incompatible with each other, which resulted in two different Chrysler PART numbers.