Ok so today I go to the local auto parts store to order myself a new harmonic balancer for my 383 SBC build. I bought myself a INTERNALLY balacned rotating assembly that did not include neither the flexplate nor the harmonic balancer. I was under the impression that I could use any old flexplate and harmonic balancer off a 350 since they are neutrally balanced and used on an internally balanced engine, correct?? I know if I had it externally balanced then I would have needed the flexplate and harmonic balancer for the balancing. Anyways the guy at the store said I need to know specifically which balancer and flexplate since they need to match but I told him it was internally balanced and he didn't seem to have a full understanding it appeared to me. Now I am not sure what to buy or do becasue I have been told that I can just use a harmonic balancer off a 350 from a scrap yard and it will work since my rotating assembly has already been internally balanced. Could anybody please clear this up for me because right now I am uncertain what to buy.

Thanks,
Keith

You already know the answer. Internally balanced means you dont need anything other than a neutral balancer and flexplate. But if the rear crank flange is a 1 piece round type, it will be externally balanced in the rear and neutral balanced in the front. If its the old style 2 piece seal, a neutral front and rear is used.

And don't use a scrap yard balancer.

You do realize that many, many of the popular "internally balanced" stroker crank kits are not actually balanced as you get them in the box.
The crank has the right counter weights to accept a internal balance job
But you have to do the actual balanceing. Even then you want to give the machinist that does the balancing the correct damper and flywheel you will be using so he can check them.

I realize that F-Bird but I spoke with the company I bought the assembly from on the phone and it cost an additional $200.00 to have them balance it for me prior to shipping. The balancing spec sheet was included with my assembly and i can see various places on the counterweights where it has been drilled so I am positive they did balance it. The question here isn't about if it has already been balanced or not it is simply if I can use a harmonic balancer and flexplate off of a Gen. 1 350 that had a 2-peice rear main. The guy at the store still seems to think I need the balancer and flexplate for balancing even if it were already internally balanced because there are many different styles and diameters of flexplates used. I will probably end up using the harmonic balancer and flexplate off of a 1980 350 I have sitting here as the rubber is not cracked.

Something sounds funny, here. Just because it is an internally balanced assembly, it doesn't mean that it is balanced. It simply means that it doesn't need the 400 flexplate and balancer.

If they want to charge you 200 bucks to balance it, it's not balanced. Did you ask them if it was actually balanced? They should have explained to you that it was, if indeed it was. At this point, it is unbalanced, and will shake FOR SURE. Many cranks have initial balance holes drilled into them, it doesn't mean they are balanced. It would serve you well to take it to a good shop that balances and at least have them spin the crank with the bobweights configured to the spec sheet you have. If it turns out good, then you know FOR SURE that it is ok. It it turns out that it needs work, then you saved you self a real headache.

BTW, I have checked some "balanced assemblies" that were WAY out to lunch.........

Is this a one-piece or two piece rear main crankshaft?

tom

There is 6.75" and 8" plus a host of custom sizes. You have to make sure that your pointer matches the dia.

Higher end balancers for internally balanced engines should be balanced when you get them. In other words, they should not affect the engines balance.