My original posting on this subject was slightly in error.  Everyone else's postings made me remember.  (It was ten years ago, by the way...)

 

1)  Yes, a mark is placed on the back of one propeller blade.  A piece of reflective tape, as I recall.

 

2)  An electronic gizmo is temporarily affixed to the cowling.  Don't recall how -- tape or something.

 

3)  An RPM input is provided to the gizmo.

 

4)  The electronic gizmo measures acceleration (vibration).  The gizmo has an optical sensor which detects when the reflective tape on the back side of the prop goes by.  The RPM signal lets the gizmo figure out how many degrees to attach the weights.

5)  I was wrong when I said that the screws holding on the spinner are changed.  What he really did was affix a screw and washer and self-locking nut to the spinner backing plate.  If the gizmo said he needed 1.2 grams at 90 degrees past the reflectively taped blade, then he went into his box of parts and found a screw and washer and nut whose combined weights added up to what was needed.  Yup, boxes of washers in 0.1 gram increments

 

- Rob Wolf