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OK, I have looked over older posts, but am going to ask this question any way. I have a 400 that was rebuilt with new crank, rods and flat top pistons before I got it. I do not believe it was balanced when this was done. It had a vibration starting at about 4000 rpm and low compression in one cylinder. I decided to pull it apart and found a flat cam lobe in the same cylinder. I replaced the cam, lifters, heads and timing gears. I verified the harmonic balancer was correct for a 400 and checked to be sure that 0 degrees was at #1's tdc. The flywheel is factory for a 350, but has the 400 counterweight plate between it and the crank. A professional mechanic looked at it when apart and could not see anything out of the ordinary. Now that it is back together the vibration is still there above 4000 rpm in neutral or driving. It is fairly smooth at highway speeds and the vibration is not really noticeable when wide open through the gears. Probably due to the noise and adrenaline. Question #1, am I going to damage anything to continue to drive the car. It is only driven on weekends in the summer time, no serious miles. Question #2, how well do these 400 balancing plates work and could that be the problem?
 

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Chris,
The 400 / 454 Balance plates only get it close. They are really meant to be used along with a complete balancing of the motor.
Are you absolutely sure it is a balance problem, and not a "miss" of some sorts ? Run the motor up through the gears one time, and ignore the adrenalin and pay attention to see if the problem is there at higher rpms. If not, it could be a lean miss, or other problem.
If it is an actual vibration, eventually it will wear out the main bearings on the crank. That is usually the first thing to go.
This is a hard one to advise you on without having the car to look at.
Hope this helps,

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
 

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One thing you might check is excessive end play in the crank. Grab the front balancer (trans in neutral, no pressure on clutch pedal) and try to move the balancer in and out. IF you feel any play except very, very slight, you may have a bad thrust main bearing, which could cause some vibrations you describe.

[This message has been edited by tom3 (edited 09-21-2000).]
 

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Hey Chris,
I agree with Bill about the balance plate.But here is a different thought for you,I saw a guy put a 400 flywheel on backwards one time and yes it vibrated.What if the balance plate was on backwards from the get go and you just put it on the same way you found it when you put it back together.

Here is my thought you will almost need to pull the trans to check it,go ahead and invest 30/40$ in a new flywheel for the 400 engine and put it in while it's apart.

Larry
 

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Discussion Starter #5
Thanks a lot. You told me what I suspected, that the ballance plate is not close enough. I don't think it can be installed backwards, it is not flat. The flywheel is also oriented correctly. At WOT there is so much noise and other vibrations it is hard to notice. This is a fairly radical street 400, 500hp. If anyone has a 400 flywheel that will work with an 11" clutch, let me know, I would like to buy it. It would have to have the same number of teeth as a stock 350 flywheel for a 79 Z28. I have an extra one of these ballance plates if anyone needs one. It cost me $25.
 

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Hey Chris,

Sorry I didn't know you were running a standard trans.If you don't get any response in someone having a fly wheel G.M. sells them new,I forgot what size you said but they only come in the larger diameter, so if you have the small you should get by with a starter change and every thing else should fit.

Larry
 

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Chris,
Northern Auto has 168 tooth flywheels for 265 to 350 nuetral, 400 and 454 balance for $90.00, $100.00 and $100.00 new.

Northern Auto Parts, 1-800-831-0884, Fax, 712-258-0088.
They are at 801 Lewis Blvd., P.O.Box 3147, Sioux City, IA 51102

Good luck with your vibration and post back if the new flywheel helps. pdq67
 

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Hey Chris,

Devil's Lake makes a valid point,before you tear any thing down take all the belts off and start it and try it in netural and see what happens.

One other thing comes to mind. The diaphram style pressure plate (PP) has flexable straps in several places around it to allow the center to henge,so to speak,so it can move away from the clutch disc in order to release.

I had a guy who road raced his Corvette,which had a small block that would turn 9500,and down shifted so hard at such high rpm that it bent all these straps/henges and the whole inside part of the PP became off center and developed a bad vibration.

Here is what I would do, if you pull all the belts and still have the vibration,when you take it apart to change the fly wheel,stop after taking the clutch and PP off.Start the engine with only the flywheel to see if it still vibrates.CAUTION with the weight of the clutch/PP now gone the engine will rev up MUCH quicker than normal,so bring it up easy.

If it still vibrates change the flywheel,and if it don't replace the PP.

One other thing check the pilot bushing.

Larry
 
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