As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE CONVERTING AIRCRAFT CONFIGURATION FROM SINGLE PILOT CONTROLS TO DUAL PILOT CONTROLS FOR PILOT TRAINING, MECHANIC FOUND COLLECTIVE CONTROL SPRING, SPRING WASHER, AND COVER LEFT IN COLLECTIVE CONTROL BOX. PREVIOUS MECHANIC HAD CONVERTED AIRCRAFT FROM DUAL PILOT CONTROLS TO SINGLE CONTROLS ON 10/10/21 BY PULLING THE PIT PIN AND FAILED TO RETRIEVE THE SPRING, SPRING WASHER, PLATE. AN OCF WAS PERFORMED BEFORE FLIGHTS ON 10/11/21 WITH NO ISSUES NOTED.