340 ballance

chrisII said

look behind the right engine mount. the casting date of your block can be found there. best wat to tell if its internaly or externaly is by the vibration dampener as already stated. IMO external balance is just stupid! i NEVER will rebuild a 340 with an external balanced crank (unless its a # match resto). i will use either the forged 340 crank or a 318 cast crank. either is internaly balanced and have the same demenshons, however the 318 crank is about 3 lbs lighter. in some racing applications 3lbs of rotating weight is huge.

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The bigger pistons and rods is why you can't just use a 318 cast crank to make it internal balance. As you stated the 318 crank is lighter so it matches the lighter pistons and rods. If it was as easy as putting a 318 cast crank in to make it internal then Mopar would have done it. A 340 cast crank and balancer are special and can't be swapped for other cast cranks or balancers unless some balancing work is done. I have a '73 340 that I put a '65 poly 318 crank in and it required some metal to be added to the counterweights because the poly 318 pistons and rods are lighter than the 340 stuff. I even used the lighter SIR rods and still had to add metal.

Chuck