Inliners International Forums General Discussion Engines Vibration at high RPM

I just installed a new crank and harmonic balancer on my 250 chevy motor. Now over 3500 RPM I get a deep vibration in the motor. No vibration at low RPM at all. The crank was miked and plastiguaged when I put it into the truck.
Anybody have any ideas?

Dear DD;

Well; something is loose or way out of ballance.

Did you mark the clutch pressure plate cover when took it apart?

Larry/Mike; Is there more that one type of flywheel for those etc.

What about the fan clutch??

John M....

Reply per John's question: The 250 is internally balanced so any flywheel for that inline family, or SBC V8 up to 1985, that bolts on should work. The exception is the flywheel from the 400 cube small block Chevy as that engine was externally balanced. You can use either the 12 3/4 inch 153 tooth wheel or the 14 inch 168 tooth wheel no problems. The flywheel size of course depends on the starter and clutch size you are using.

Assuming you have a correct flywheel for an internally balanced 6 or SBC V8 I can only think of a couple of reasons that relate to it. First is someone had that flywheel balanced with a crank attached to it and they did all the weight changes on the flywheel. Dumb yes, but it happens. Second is the pressure plate is way out.
Generally it is best to have the flywheel and p- plate balanced as an assembly minus the clutch disc. Once balanced the machinist should have marked the 2 pieces so they can be reassembled in the car in the same relationship they were balanced in.

If your truck has an automatic try unbolting the convertor from the flex plate and indexing the studs around to the next holes. You only have to do this twice to cover all 3 locations that can make it better or worse.

Putting the engine aside for a minute - is the vibration at 3500+ only while driving or 3500+ at any time driving or parked?

Toss some more info to us before you go ripping into the engine. Driveline problems need to be ruled out.

Mike G

Yes a little more info can Be helpful. Because a AT convertor Will do what Your saying I know BEEN THERE had That Happen. Also Is Their any Type Of Rattle at start Up Or Shut Down??? If So this could also Be a Lose FlyWheel Issue.
More info Please But hope all This helps so far For things to be testing/Looking first.}[oooooo]

Ps What do you have for a Dampener?????

Reply per John's question: The 250 is internally balanced so any flywheel for that inline family, or SBC V8 up to 1985, that bolts on should work. The exception is the flywheel from the 400 cube small block Chevy as that engine was externally balanced.
The other exception is the 400 SBC balancer you cannot use, it is externally balanced.

The truck is a 4spd and has never been ballanced. The truck starts and runs fine at low RPM and the more I drive it the vibration seems to be going away, It still vibrates just not as bad as when the motor was first reassembeled.

Dear DD;

Okay, that leaves the harmonic balancer as the "most likely".

Do you have your old one??

JM.....

No. The old one is gone. it flew off the front of the motor while I was driving down the road. Boy did it make a lot of noise when it left.
Ken

Dear Ken;

Perhaps you got a 400 SBC dampner by mistake.

Pull it & check against another/the part # etc.

I got (by accident) an externaly ballanced flywheel once. It caused the same problem.

JM........

The SBC dampers will fit right on the 250 crank whether they are from an internally balanced SBC or from the externally balanced 400 SBC however none of the SBC dampers of either type have any belt pulley grooves machined into the outer ring.
If the damper you have on the engine now has 1 or 2 belt grooves in it then it is correct for the inline and the external balance problem is not a factor. If it is the right damper for the 250 it still may have a balance defect but it normally wouldn't be as severe as if it was the 400 SBC externally balanced damper.

Another question is did the original damper fly off before or after you had the crank work done?
Knowing this might help focus on the problem you are having now.

Mike G

now that i know it is a stick. here is a problem i have had 3 times in 20 years. the center recess where the flywheel bolts to the crank could be off center. the last one i did took a .120 cut to get it on center.

Dear Ken;

If Bob is right, you will be able to see that with the engine running & the clutch cover off.

Or you could use a piece of chalk & see if It's 'out of round' by rotating it by hand.

John M....

The flywheel is the same one that has been on the truck since day one. It never gave me any trouble before. What would I do with the chalk to tell if it is out of round?
Ken

Dear Ken;

Bob mentioned that the crank centering for the flywheel may be 'off center'. We now know the flywheel is good.

To check this: Hold a piece of chalk (stationary on bell housing) next to the flywheel where it just touches it. Then rotate the flywheel & see if the line/mark made is the same for 360 degrees.

Easy to do with plugs out by hand. Don't try with starter motor.

You could get fancy with a dial indicator or run the engine & watch (carefully) the distance between the flywheel/bell housing.

An 'off center' flywheel will make a heaver mark on the closer side & a lighter/no mark on the wider one.

This is a rare condition but does happen.

Good luck, John M.....

Thank you for all of your help and advice. If and when I find the problem I will post what it was.
Ken

Ken,

Was the crank brand new? or was it a re-grind?
What brand?

Having balanced thousands of crankshafts, the factory specs aren't always what we would like to see. Many of the older 6's, whether Chevy, Ford, or Dodge, have been as far out as 40-60 grams, and this will give you a vibration at different rpm's.

Ken
www.CustomDesignPerformance.com