On June 24, 2023, about 1615 central daylight time, a Cessna 421 airplane, N123SM, was substantially damaged when it was involved in an accident near Rockport, Texas. The pilot and three passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he had flown the airplane from the West Houston Airport (IWS), Houston, Texas, to the Corpus Christi International Airport (CRP), Corpus Christi, Texas, where he picked up the airplane owner and his friends for a return flight to IWS. He stated that they loaded 130 gallons of fuel while at CRP. Recorded ADS-B data indicated that the airplane departed CRP about 1545 and headed northeast. The pilot reported that once the airplane reached 6,000 ft msl, the left engine tachometer generator went offline but the engine still operated normally. However, about 2 minutes later, the left engine lost power and the airplane banked left. The pilot attempted to stabilize the airplane with aileron and rudder control, but the airplane entered a spin. He moved both throttles to idle, applied right rudder, and was able to regain control of the airplane. After leveling off, the right engine had stopped and the propeller was not windmilling. The left engine was not producing power but the propeller was windmilling. The pilot activated the left engine boost pump to the high setting and looked for areas to land while turning toward the nearest suitable airport; he then executed a forced landing to a field. After the accident, a test run of the right engine was performed; the right engine was able to run and produced rated engine power during the test run. A teardown examination of the left engine revealed that the starter adapter gear had failed. The failure stripped the teeth off the accessory gears, including the camshaft drive gear. The starter adapter was the subject of FAA Airworthiness Directive 2007-05-15, which was issued “to prevent failure of the starter adapter assembly and or crankshaft gear, resulting in failure of the engine and possible forced landing.” The AD required compliance with Continental Motors Mandatory Service Bulletin (SB) MSB94-4H. The SB detailed inspection and replacement procedures for the starter adapter assembly and crankshaft gear. Part 1 of the SB required various inspections if rough engine operation was detected. Part 2 of the SB detailed a visual backlash check of the viscous damper installed on the starter adapter while the starter adapter was still installed on the engine. Part 2 was to be complied with every 100 hours of engine operation and was to ensure that excessive wear was not present on the gears of the crankshaft or starter adapter gears. Part 3 of the SB required removal and visual inspection of the starter adapter and gear at 400 hour intervals. During the engine examinations, the backlash check described in part 2 of the SB was performed on the airplane’s intact right engine and excessive backlash was not detected. Damage precluded performing the check on the left engine. Examination of the airplane’s maintenance records showed that part 2 of the SB had been complied with on June 2, 2023, on both engines during the airplane’s annual inspection. According to recorded engine monitor data, the airplane had completed 7 flights and 10 hours of flight time since the annual inspection. On the right engine, part 3 of the SB was complied with by replacement of the starter adapter on June 10, 2020. On the left engine, the starter adapter was removed, inspected and placed back in service on June 20, 2020, about 100 hours before the accident. There was no maintenance record for 2022 in the maintenance records reviewed. The mechanic that performed the SB reported that on the most recent annual inspection he performed part 2 of the SB and the readings were well within the specified limits listed in the SB. He noted that he had performed this check many times in the past and did not find the procedure difficult, just that it required patience to perform the checks.