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Discussion Starter · #1 ·
I bought a 55 post from a couple of hillbilley's in Tennasee that has a built up 400 for power. They say it's freshly built. I'm an Idaho potato and do not beleive every thing I here, but I owne it anyway. The question I have is that it feels like the rotating assembly is a hair out of balance. Is it possible to get the wrong dampener or fly wheel or even get the right ones on wrong. They say the crank is bran new and maby they all need to balanced together?? I actually think they did put the parts in that they say as I have verified the block and head numbers also looked under the valve covers to see the roller rockers. You can pick it up only when in the driver's seat , both driving and running it up at idle. I would say between 16 17 hundred rpm's to 2600ish, then smoothes out. Any thoughts, thanks sharkey 5557.
 

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The balancer will have a lump of metal missing compared to a standard balancer (if I remember correctly), and the flywheel/flexplate will have a weight on it.

Look at the backside of the balancer. The 400 balancer is the only one made that way (not neutral balance) for a SBC so you can't have that wrong unless it's a standard balancer.

The flywheel or flexplate is a different story. The 400 flexplate/flywheel has a slightly smaller weight than a 454 flywheel or flexplate, and either one will bolt up.

I have a new 400 flexplate here, it is GM p/n 340298. (This is an old number and may have been superseded.) The weight is 6-3/4" long, 27/32" wide and 1/8" thick. I don't have a 454 flexplate here, but the weight is a bit bigger but not a lot.

I'm guessing you have a 454 flywheel if it's not shaking too bad.

The dowel hole in the crank should line up with the dowel hole (extra hole) in the flywheel/flexplate.
 

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I bought a 55 post from a couple of hillbilley's in Tennasee that has a built up 400 for power. They say it's freshly built. I'm an Idaho potato and do not beleive every thing I here, but I owne it anyway. The question I have is that it feels like the rotating assembly is a hair out of balance. Is it possible to get the wrong dampener or fly wheel or even get the right ones on wrong. They say the crank is bran new and maby they all need to balanced together?? I actually think they did put the parts in that they say as I have verified the block and head numbers also looked under the valve covers to see the roller rockers. You can pick it up only when in the driver's seat , both driving and running it up at idle. I would say between 16 17 hundred rpm's to 2600ish, then smoothes out. Any thoughts, thanks sharkey 5557.
Sounds like they bought a new crank and did not bother to balance it, And if it has stock rods it will be extrernally balanced on both ends weighted flywheel and balancer.

Make sre its not a skip or misfire as sometimes that will feel like a vibration.

Good luck
 

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Discussion Starter · #4 ·
I wondered about a missfire causing it to feel like that.This engine does in fact have a ( hard to describe) limp through exactly that RPM range that i've been struggeling with. I guess the first thing is to eliminate the limp or slight miss and maby that will take care of both issues. I hate to give up on this engine and yet donot want to get socked into it than could buy a crate. It is supposed to have after market rods and pistons but I would have to open it up to see and i may just do that. Thanks for the input. Sharkey 5557.
 

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Discussion Starter · #5 ·
By the way the balancer does have some metal machined out of it (of caurse on the outer edge by th;e timeing mark). Maby I can see the weight on the fly wheele, should be on the outer edge. Thanks again. Sharkey 5557.
 

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The balancer will have a lump of metal missing compared to a standard balancer (if I remember correctly), and the flywheel/flexplate will have a weight on it.

Look at the backside of the balancer. The 400 balancer is the only one made that way (not neutral balance) for a SBC so you can't have that wrong unless it's a standard balancer.

The flywheel or flexplate is a different story. The 400 flexplate/flywheel has a slightly smaller weight than a 454 flywheel or flexplate, and either one will bolt up.

I have a new 400 flexplate here, it is GM p/n 340298. (This is an old number and may have been superseded.) The weight is 6-3/4" long, 27/32" wide and 1/8" thick. I don't have a 454 flexplate here, but the weight is a bit bigger but not a lot.

I'm guessing you have a 454 flywheel if it's not shaking too bad.

The dowel hole in the crank should line up with the dowel hole (extra hole) in the flywheel/flexplate.
I have seen this same problem on another car using the 454 flywheel instead of the 400 as Rick says. I have also seen a 350 balancer used on a 400 and it was vibrating across the entire rpm range. Since the vibration sorta goes away at a certain rpm it looks like the flywheel/clutch assembly may be it.

Joe:anim_25:
 

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A lot of machine shops tear down and just toss the rods into a bucket. Then when they "freshly build" they just grab 8 rods and start hanging them. Although factory tolerances are there - the odds are that some of the rods are at one end of the tol and some at the other. I've seen far too many grab bag builds to discount this possibility.

I had a sharp friend once that had a major brain ****. He "balanced" his rods by immersing in a beaker of alchohol and measuring the displacement - then ground them to match. When it went on the track and the drivers fillings were loosening up I inquired as to how much the rods weighed at each end, and his total weight including pistons (knowing where it should have been for the crank I sold him) - he told me how he had balanced and - when I said so a soccer ball and a bowling ball weight the same? - He went white, red then stammered the obligatory well duh.... lol.

The bottom line is that if the parts were bought as a matched set - they should have all weighed out right and very little balance work is needed - but if they pieced the parts based on - this is my friends favorite rod, my uncles favorite piston, the rings I have always had the best luck with, and this crank cuz it comes with the coolest sticker - if it's counter weighted for different totals - then you could be way out of wack. For me - if it does not come with a build sheet and specs from a reputable shop - its just a core.
 
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