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Discussion Starter #1
I can get ahold of a 406 sbc but the guy says it is balanced for an automatic, my question is does this totally prevent me from running it with a 4spd? Thanks

Scott
 

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Nope.

Once it's balanced, the rotating assembly (damper, crank, pistons, rod, flywheel/flexplate) all need to work together. Stock 400 SBC's are 'externally balanced', and all other stock SBC's are internally balanced. In simple terms, all that means is that the stock 400 damper and flywheel/flexplate have a counterweight on them (externally balanced), and the other stock SBC's do not (internally balanced.)

Since you said it was a 406, that means it's been rebuilt, so it's not stock any more, and it could be a mix of stuff, so you can't just assume it's using a counterweighted damper and flywheel/flexplate.

The only thing you have to worry about when switching from an automatic (flexplate) to a manual (flywheel) application is that the new flywheel is balanced the same as the old flexplate. If the flexplate is a neutral balance type, then you should use a neutral balance flywheel. If it's a counterweighted flexplate...you need a counterweighted flywheel.

Beware though, "balanced for an automatic" doesn't make any sense (should be internally or externally balanced) and might mean the seller is full of crap.
 

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Discussion Starter #3
Thanks, the seller was going by what the guy who built the engine for him said. He was told that it was balanced for his turbo 350. Do you think that he may have just been refering to the flexplate?

Scott
 

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He was told that it was balanced for his turbo 350. Do you think that he may have just been refering to the flexplate?
Could be...you don't balance to a transmission. The damper/crank/rods/pistons/flexplate(or flywheel) all need to add up to a neutral balance. The transmission (and everything downstream of it) is neutral balance also.

Again...as long as you replace the flexplate with a flywheel that has the same balance style (internal/external)...you'll be fine. There's a slight chance that the flexplate was modified by the builder to tweak the balance, but that's a real longshot. Look up the part number of the flexplate that's on there, it'll either be a 400 external balance style, or a 350 internal balance style, and go from there to order the right flywheel.
 

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you might ,,

i'm pretty sure they have to balance a 400 crank if its a stocker with the flywheel mounted to the crank. Worst you would have to do is get the crank re-balanced to the flywheel your using. If its internally balanced now or externally is something you'll need to know. Be nice to find the person that did the balance job and speak with him. Sometimes they'll keep records what they had to do in the balancing procedure. He might have done some work to it to change it to internal as well so you might be able to use a 350 flywheel instead of a 400 external wheel.
 

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Typically, all of the balancing work is done on the internal components of the engine. If the engine is externally balanced then yes, they spun it up with the flexplate/flywheel and damper bolted to it but they generally don't modify either. That way if you ever need to replace either the flexplate/flywheel or damper you won't need to tear the engine back apart and have it re-balanced.

As long as you're dealing with a quality damper and flexplate/flywheel, the balance tolerance is help pretty tight. Just get the correct corresponding balance type (internal or external) and run. If you're at all worried about the new part matching the balance of the original, you can take the flexplate/flywheel that you're replacing and the new replacement down to the machine shop and have them match the balance.
 
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