As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING ENGINE REMOVAL FROM A TWIN OTTER ONE OF THE TOP FOUR ENGINE MOUNT BOLT SHEARED OFF. THE BOLT SHEARED OFF AT THE AREA WHERE THE SHANK DIAMETER IS REDUCED. THIS BOLT TURNED APPROX. 1/4 TURN AND THEN SHEARED THE HEAD OFF. THIS BOLT WAS POST MODIFICATION (DH)6/1811.