As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INSPECTED THE STARTER ADAPTER SHAFT GEAR AND CRANKSHAFT GEAR ON BOTH ENGINES OF A CESSNA 421B IAW TCM SB CSB94-4. FOUND THE GEARS SPALLED AND PITTED BEYOND LIMITS. REPAIRED ENGINES BY INSTALLATION OF TCM KIT NR EQ6642. THE STARTER ADAPTER AND CRANKSHAFT GEARS THAT ARE SUPPLIED IN THIS KIT AND ON CURRENT PRODUCTION TCM GTSIO520 MODEL ENGINES UTILIZE AN IMPROVED TOOTH PROFILE.