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Old 01-02-2013, 10:18 AM
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383 sbc rotating assembly balance question

Hi guys... i am interested in your thoughts on the importance of having a machine shop balance the rotating assembly on a 383 sbc. The reason I am asking is because I put 1 together using a 400 crank that I had around, and an old 350 that a buddy had. We took the block to the machine shop and told them we were building a 383 with it and we're going to use the 5.7 rods with the old 400 crank. We got the block back Bought the right pistons And put it together. We never weighed any of the rods or pistons or did anything to balance it at all other than using the correct balancer and flex plate. The engine seems to run fineI which makes me ask the question how important is it to balance the rotating assembly?

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Old 01-02-2013, 10:23 AM
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balancing 383

I think its good insurance to have any "combination"of different parts balanced by machine shop. Eliminates "strangr problems" that COULD occur later.
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Old 01-02-2013, 10:46 AM
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why would you build any engine with performance in mind and not balance the assembly? A 4,000 rpm stock 400 rebuild would likely run fine.A good 383 kit that advertises its prebalanced would likely run fine at low rpm.
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Old 01-02-2013, 11:01 AM
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How did you get the 400 crank to spin in the 350 block?
If you used a 400 flywheel and balancer, and happened to use a piston that resulted in a bobweight near the stock 400 bobweight, it should be balanced fine for street use. Turn it up to about 6400 RPM a few times and the lack of balance might show up.
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Old 01-02-2013, 11:55 AM
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You already answered your question when you said it runs fine.Didn't You?. So if I tell you in my opinion you should balance it,are you going to take it apart??. This balance vs unbalance debate is as old as the one about 5.7 vs 6" rods.

If unbalanced the range long term is from pounding out rod bearings to either cracking or breaking crankshafts.
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Old 01-02-2013, 06:40 PM
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Did you use pistons that were made for a 383 with a 400 crank they may have been the correct weight. Now it may only be close but that should allow it to run 5000 rpm without issues and maybe a bit more.

But turn the un balanced engine to 7k rpm and hold it there it will come ungluded. Best to be sure its at least factory balanced when you do a 383 and fully machined shop balanced when its not. But it will still work most pistons will be close to the correct weight since they all have the same purpose. Now grab some aluminum pistons and rods and you can throw the weight so far off it will shake its self to pieces and actually make less power.

Is it a stock balancer? If so watch it closely it will let go first usally. Once the balancer starts to spin and throw the rubber out it can snap the end off the crank.

Not sure how to explain calculating the bob weights but you can call thepiston manufacturer and get the standard weight of the pistons. Then figure out how far off you are. If its way out then you will need to fix it if its even close to a tolerence then let it eat and stick a good balancer on it.
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Old 01-02-2013, 08:07 PM
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Eagle has a very good explanation of balancing and figuring bob weights on their website.

Understanding Balancing
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