As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE TACHOMETER GENERATOR FAILED UNDER NORMAL OPERATING CONDITIONS. THE SUSPECT CAUSE IS A SMALL O-RING FITTED ON THE DRIVESHAFT AND INSIDE THE ROTOR SHAFT IS AN UNSPECIFIED PART. (DOES NOT APPEAR IN PARTS BREAKDOWN.) THIS O-RING RESTRICTED THE SELF-CENTERING ACTION OF THE DRIVESHAFT WITH MATING DRIVE ON THE AIRCRAFT. THIS CAUSED THE DRIVESHAFT TO FAIL.