I am not convinced that is true. I have switched them without problems, except for the location of the timing mark. Both balancers are "neutral" balance, and are very close in total weight. It is not like on the V-8's, where the cast crank motors use an offset balance damper. I know the Mopar performance book says not to interchange, but that book has other info for the slant six, that does not apply. Only applies to the V-8's. Aftermarket balancers, do not differenciate, between the cast and forged cranks. Also Chrysler had several different part numbers for dampers in the same model year, depending what trans.
