I just purchased an eagle 383 stroker kit for my car that was already internally balanced. I have heard many stories about how people have boughten "balanced" assemblies that weren't actually balanced that well. Should I have a shop balance my assembly or should it be alright?

 

whose kit is it

 

Internal/external balanced has nothing to do with reciprocating weight balance.

If the kit included crank, rods, pistons, bearings, rings etc then it's probably balanced. If it only included crank and pistons etc then it should be balanced.

For an engine to be properly balanced, the machine shop needs to know the weight of the rods plus how much each end weighs with rod bolts and or nuts, the weight of the piston, the weight of the wrist pin, weight of the rings, weight of the rod bearing. From all that, a bob weight is used to properly balance the crank to match the weight of the reciprocating assembly. Depending on the crank itself, weight may need to be added or removed to balance it out.

A good kit will have all the pistons the same weight, all the wrist pins the same weight and all the rods matched on both ends to the same weight. You could have 2 rods that have the same total weight but if one is heavier on one end than the other one then they're not matched even though they weight the same.

 

I had my "balanced" kit balanced with the flexplate as well.

 

I bought the kit from Eagle and it included the crank, rods, pistons etc. Do you think it should be balanced then? If so, how much should I expect to pay?

 

It sounds like a complete rotating assembly so you shouldn't have to send it out to be balanced. They didn't include a balance sheet with it? A balance sheet would be helpful if you ever had to change something. Damage a piston and reorder a new piston with the proper weight. Even something as simple as a wrist pin can have many different weights. Simply ordering by part number doesn't mean you get the exact same part.

The cost of balancing depends on what needs to be done. I paid around $500 to get my engine balanced. Heavy mallory weights needed to be added and the crank was spun up 12 times before the balance was perfect. All that extra work costs more.

In your case, having it balanced may not require any changes which would cost less and you would get a balance sheet showing all the weights for future reference.

 

I got the kit today and they was a balance sheet with it. Should it still be balanced for when I buy the flywheel and harmonic damper also.

 

Yes and no.

Since it's an internally balanced crank, the damper and flexplate/flywheel are neutrally balanced. They can just be installed and used. For a high reving engine, it's recommended to include those when getting the engine balanced. My neutrally balanced flexplate was a few grams off. The downside is that if you ever take off the flexplate, you need to make sure it goes back on in the same position it was installed when balanced. Mine uses a dowel pin. I also have to replace my SFI flexplate every 3 years. The new neutrally balanced flexplate I buy won't be exactly the same as the one I take off.

For what you're doing, the rotating assembly is balanced as close as you need it. Paying to get it rebalanced with a damper and flexplate/flywheel isn't worth it.