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C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

Are zf6 flywheels balanced?

 
Old 05-11-2019, 10:31 AM
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64Scout
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Default Are zf6 flywheels balanced?

I'm looking at buying a 383 short block from Summit https://www.summitracing.com/parts/mll-bp3834 and it says it needs to be externally balanced. A 400 harmonic balancer is easy enough, but then it says it needs an externally balanced flex plate. I need to know if I can use my fuel mass flywheel as is, or if it needs to be balanced.
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Old 05-11-2019, 03:51 PM
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Is there an advantage to buying this engine vs one that is internally balanced?
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Old 05-12-2019, 01:35 AM
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Originally Posted by ihatebarkingdogs View Post
Dual Mass Flywheels for the L98 and LT1/4 are externally balanced.


And to simplify, ALL SBC with a one piece rear main seal are externally balanced in the rear...on the flywheel. The weight isn't a lot. A machine shop could easily balance any flywheel to meet the requirements, but I agree that if you could get an assy that works with the parts you've already got that would be the simplest solution.
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Old 05-12-2019, 09:02 AM
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Originally Posted by MatthewMiller View Post
Is there an advantage to buying this engine vs one that is internally balanced?
The issue is that to make a 383, you use a 400 crank in a 350 block. The stroke is as long as it can be, and no room for internal weight like a 350 crank. So you have to balance it externally by the harmonic balancer and flywheel/flexplate.
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Old 05-12-2019, 09:05 AM
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Originally Posted by ihatebarkingdogs View Post
Dual Mass Flywheels for the L98 and LT1/4 are externally balanced. I have no idea whether the amount of "imbalance" of a ZF flywheel and one for a 383 are the same. Good question for the engine builder. Another consideration is that the legacy 383 that used a 400 crankshaft was a 2-piece rear main seal. All SBC's since 1986 have been 1-piece rear seals. The flywheel hole pattern, index size, and dowel pin location are different for the two crankshaft / seal types. If the 383 in question doesn't utilize a 1-piece rear main seal, the ZF flywheel won't even bolt up to it.

Avoid problems. Get a flywheel for the new engine that is known to work with it. Even better if the supplier will balance a new clutch cover plate to the engine/flywheel assembly. But its doubtful that that can be accomplished.
Good points. I have more to consider than I thought.
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Old 05-12-2019, 11:23 PM
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Originally Posted by 64Scout View Post
The issue is that to make a 383, you use a 400 crank in a 350 block. The stroke is as long as it can be, and no room for internal weight like a 350 crank. So you have to balance it externally by the harmonic balancer and flywheel/flexplate.
The reason I'm asking is twofold. First, I have a 396 (1/8" more stroke than a 400 crank) that is internally balanced - didn't even require any heavy metal in the crank to make it happen, but it does have light pistons that probably help. And the few times I've looked online, I see lots of stroker kits that are internally balanced. Second, I'm wondering if there's a tradeoff for that. Is there a disadvantage besides the cost, or is that the main reason to avoid it? I'm actually asking here, because I didn't build my engine and I don't hold myself up as an expert on building or specifying these things.
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Old 05-13-2019, 10:02 PM
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I've not heard of internally balanced strokers. It could be something new, I used to be into hot rods and stuff and read all the magazines. Then about 20 years ago I got into 4x4s, specifically IH Scouts and didn't pay attention to much else, so its probable that there is an internally balanced stroker. However, the long block and short blocks I have been looking at are externally balanced.
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Old 05-13-2019, 11:09 PM
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Originally Posted by 64Scout View Post
I've not heard of internally balanced strokers. It could be something new, I used to be into hot rods and stuff and read all the magazines. Then about 20 years ago I got into 4x4s, specifically IH Scouts and didn't pay attention to much else, so its probable that there is an internally balanced stroker. However, the long block and short blocks I have been looking at are externally balanced.
Not new, but maybe newer than 20 years. My engine was spec'd out 15 years ago, and it's internally balanced. Since you were looking at Summit, here is a list of internally balanced Gen-II 383 kits. You'd have to double-check each one's details to make sure they are "internal." One or two at least are called "internal/external." Also, again, I don't know if there's a drawback to going internal or not, so hopefully others would chime in here regarding that. But if you are going to replace your whole rotating assembly anyway, it is readily available and doesn't look especially expensive.
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Old 05-18-2019, 10:56 AM
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If the crank is cast steel or forged 4340 steel will have a big effect on balancing. Cast cranks, the counterweights aren't dense enough generally to internal balance without a lot of Mallory metal. I tried about 20 years ago to internal balance a 391 (offset ground 383) and it took so much Mallory metal the balance job alone was over $600. I could have bought 4340 crank for that kind of money. A 4340 crank is much more dense, so internally balancing is much easier. Usually, the bigger the stroke, the more effort it is to internal balance. So generally speaking if you have a cast crank, you want to external balance it. If it's 4340 forged steel, you can internal balance it easier.

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