I have a 73 340 block, getting engine vibration at about 1500 rpms, not sure if the crank is cast or has been changed to forged. It has what appears to be a 360 external balance dampner on it, It is connected to a neutral 904 converter. Question is will the 360 ext balanced dampner work on the 340? If it is a cast crank, DO I need both the correct dampner and weighted converter (or flexplate),or just the dampner with the neutral converter, in order to balance the motor? Not sure exactly what I have got a hold of, everything I have touched on this motor has been cobbled together so far, so not really sure I can trust anything on the ext of the motor to be correct. Any help will be greatly appreciated

Don't know about the converter, as I am a 4 spd guy myself, but a 360 balancer will not work on a 340 cast crank. A 340 cast balancer says for 340 only cast into the weight, which should be on the motor side of the balancer.

Not absolutely sure, but I think IF you have a cast crank you will also need external balance on the converter and/or flexplate as well as the damper. This is the case with 360's. I agree that a 360 damper isn't the same as a 340 one. FWIW '73 340's had cast cranks, but since your engine seems to have been hacked together, who knows what's in it now.I don't know what your plans are for this engine, but IMO external balance dosen't have any place on a performance engine. It's fine for trucks and everyday cars.

The correct balancer for a cast crank 340 says "For use with cast crank 340 only" on it IIRC. Regardless - if it has a balancer that is for a 360, it has some kind of offseeting weight - which means it's externally balanced and will need the external balanced flexplate too. Question is - which one? Can you get a part number or clear picture of the balancer?

Good old Ma Mopar!

These are different from each other and WILL NOT interchange:

340 cast crank
360
5.9Mag
440 heavy rod

These are different from the above, will not interchange with the above, but will interchange with each other no matter what is cast onto the damper:

383 cast crank
400 cast crank
440 cast crank

So look at the list and figure it out.

R.

Thanks for the info. I will post pics of the dampner later today. Am I correct in assumimg that even though the block is a 73 (ext bal), if it has been changed to a forged crank it is now internally balanced and need neutral converter and dampner?

You would NEED to confirm your assumption... The only way of doing that would verify what the crank is..

Pulled the oil pan last night, confirmed it is a forged crank # 2532457. It has a neutral flexplate, converter and dampner. There is a steel "plug" sticking out of the front counterwieght on the crank. anyone know what this is, and if this has anything to do with the balancing? Could it be the converter itself, is there anything else in the rotating assembly of the engine I can check?

Front of removed dampner

Back of removed dampner

I`m no expert but it sounds like someone may have added mallory metal to the crank to balance it. Is it coming out of the hole as in loose? Mallory metal is heavier then steel so they usually drill a hole and put the mallory metal in the hole to add weight.

Mallory metal is often used to internally balance a crank, usually on cast cranks since they are generally less dense than forgings. If mallory was used on a forging I would have to think that the rod /piston assembly was unusually heavy since as stated the denser forgings seldom require extra weight to achieve proper balance. In any case the mallory should be inserted perpendicular to the counterweights otherwise centrifugal force will tend to "spit" them out. In that event I can guarantee nothing good can possibly result from this.