EXPLAIN THE CAUSE OF THE VIBRATION IN MY 383

Engine, Trans & Driveline

  1. soundhd

    soundhd Well-Known Member

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    Have a vibration in the 383 in my 70 Roadrunner. (383 auto). Not the original motor but dated in Nov 0f 69. Vibration can be felt revving the engine some when in park. Has always been there but been dealing with other vibration issues but getting down to the nitty gritty... So far have replaced the driveshaft, had the new wheels & tires "re-balanced" (previous dealer did a really lousy job). Anyway looked into it some (first time I had hard about this issue with some 383's) and the the vibration has to do with if the crankshaft is cast or forged, if it has the "correct" torque converter in it or if it is balanced correctly, if the engine is internally or externally balanced and maybe the harmonic balancer is defective. I am not an engine guy but am getting it looked at and fixed later next week and the repairman seems to know about those issues with the 383's but I did not want to spend a lot of time with him on the phone having him explain it all to me (pretty busy guy) so I thought maybe I would see if anyone here would be willing to explain it all.

    thanks
     
  2. Potts

    Potts Well-Known Member

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    No time?
    So he is that guy.
    At idle?
    In gear?
    What speeds?
     
    Last edited: Sep 22, 2021
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    • 69Bee

      69Bee FBBO Gold Member FBBO Gold Member

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      Doesn't really matter what year the block says; it depends on the internals. Cast crank gets a counterweighted balancer and converter/flexplate, while a forged crank has a neutral balancer and converter/flexplate. I say flexplate because there could be a neutral converter on a Milodon "Cutout" flexplate used.

      crankshaftid.jpg
       
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      • moparedtn

        moparedtn Ed on the Ridge FBBO Gold Member

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        Simply stated, there are harmonic balancers and flexplates/flywheels for cast crank engines
        and they cannot be used on a forged crank engine - or vice-versa.
        You will get harmonic vibrations if they do get mixed up - and given the age of our engines
        AND the fact a lot of folks "building" them these days don't know enough to differentiate one
        type of crank from the other, it's quite possible someone has messed up without realizing it.

        Of course, there could also be something as obvious as a balancer coming apart/defective or
        a flexplate/torque convertor throwing balance weights off, too.
         
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        • Don Frelier

          Don Frelier FBBO Gold Member FBBO Gold Member

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          Perhaps but first thing is does it fire on all 8 cylinders?
           
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          • matthon

            matthon Well-Known Member

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            Food for thought.
            I pulled my hair out for months over a vibration, have an entire thread here.
            Everything was new (part of the problem), everything was checked and checked again, and again.

            The culprit, bad new engine mount.
            I had pulled them both out a number of times and nothing visible, also didn't show any signs of being defective while running, revving.
            Both were actually bad, but one way worse.
            Swapped my ancient original decrepit mount in and haven't had a vibration since.
             
          • Grabbergreendream

            Grabbergreendream Well-Known Member

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            Honestly, the guy you didn't want to spend time on the phone with has about a clear a picture of what we have to go by...
            Pics of your balancer, flywheel, & converter "might" help determin if you have mix-matched parts, but it really depends on what crank is in your engine( & if it was ever balanced for replacement parts)...
            The more info the better.
            Do you have any build specs(or know for certain the crank is forged or cast)?
            Plenty of help here, but not accurate advice based on the info given
             
            Last edited: Sep 23, 2021
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            • kiwigtx

              kiwigtx Henchman #4 Staff Member FBBO Gold Member

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              It could be a bad universal joint.
               
            • MoparLeo

              MoparLeo NRA BENEFACTOR LEVEL LIFE MEMBER FBBO Gold Member

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              Harmonic. a particular vibration/pitch frequency. In other words harmonic vibrations will be at a specific rpm. Always the same. If it is a loose, broken, worn part it can vary.
              Engine....driveline...........tire/wheel/drum. Pretty basic to narrow down the problem.
              Engine ? Don't need to move the car.
              Driveline ? Accelerate, coast, decelerate.
              Tire/wheel ? Smooth asphalt road, slowly increase speed until it vibrates.
              Of course this is after a good visual inspection for obvious missing, loose, broken parts. Mud, debris on rims, inside or outside etc...
               
              Last edited: Sep 23, 2021
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              • 1STMP

                1STMP FBBO Gold Member FBBO Gold Member

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                Most here know what to look for. Specifics
                would be a good starting point. Please
                explain in not so generic of terms. Your
                frustration is/has been shared.
                 
              • soundhd

                soundhd Well-Known Member

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                Thanks for all the above replies. The info on the forged vs cast crank sounds what seems to be the issue but when the mechanic looks it all over hopefully the problem will be found.
                The vibration happens when the car is in "park", very small when idling and gets worse when revved up but can also be felt when moving.
                Will post an update when fixed.
                thanks again
                 
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                • AR67GTX

                  AR67GTX Well-Known Member

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                  Always a good idea during the process of elimination to drop the belts off the engine accessories to make sure it’s not an alternator, PS pump or AC compressor if equipped.
                   
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                  • Potts

                    Potts Well-Known Member

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                    Ok. So now we know the vibration has nothing to do with drive train or wheels.
                     
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                    • khryslerkid

                      khryslerkid FBBO Gold Member FBBO Gold Member

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                      • AR67GTX

                        AR67GTX Well-Known Member

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                        Someone above mentioned the possibility of an engine miss is what you feel as a vibration. Very possible - I had a BB Chevy motor with a week plug wire connection at the plug terminal and the cylinder wasn’t firing. Felt like an engine balance vibration. Finally located it by grounding out each cylinder looking for rpm fall off. Also showed up as arcing to head in dark garage.
                         
                      • WileERobby

                        WileERobby Well-Known Member

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                        Hah ! I remember the first valve job I did on a 67 Olds. It was by buddy's uncle's car. Buttoned it all up... fired up... vibration. Wut ? Now I'm almost in panic mode. Hah... the #8 spark plug cable wasn't firmly snapped on the plug. The first of many flawless, superb auto repairs.....:rolleyes:
                         
                      • kiwigtx

                        kiwigtx Henchman #4 Staff Member FBBO Gold Member

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                        Oh well....back to basic....good plugs, clean air, clean fuel, good spark....no shit in the carb.
                         
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