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
