the feathering spindle is the shaft that goes through the head block that the blade grips fit onto,it has a spacer on it that seperates the grips from the head block dampers.you need to measure the distance from each end of the blade grip to the center of the head block,then do the same for the other side,make sure you measure accurately and both sides should be the same,if they are not,then you need to carefully file down one of the spacers slightly to make the distance equal.the spacer to file will be one of the small ones seperating the grip from the head,not the ones inside the blade grips.you will be able to tell if this is what is causing your vibration by spinning the main blades as fast as you can by hand and then looking at the tail,if the spindle centering is off,then you will see the tail jerk as each blade slowly goes round past the tail.the problem with the shaft centering is caused by the damper not fitting properly into one of the sides of the head due to inaccurate molding,so it is likely that more than one shuttle will be affected.