SR-22 Lead Technician reported a failure of starter engagement. After further inspection they found the accessory drive gear attach bolts sheared.

Narrative

Pilots reported failure of starter engagement; initial decision was made to remove and replace starter and starter drive adapter. Upon removal of starter and starter drive adapter; an inspection was made due to excessive difficulty installing replacement parts. Inspection revealed slippage of accessory drive gear in engine accessory case. After this the airplane was removed from service and a factory rebuild motor was ordered as replacement. Once the inoperative engine was removed from the aircraft; an in depth inspection of the accessory case was performed. This inspection revealed that all 6 bolts attaching the accessory drive gear to the back of the crankshaft had sheared due to excessive rotational force. It should also be noted that all 6 attachment bolt heads were safety wired together as a single group rather than two groups of three as shown in the Continental Standards and Practices manual; M-0 Appendix C. Corrective Actions: Removed and replaced inoperative motor with a Factory Rebuild motor and will return aircraft to service once replacement is complete.Plausible Cause: It is my opinion that the bolts being overtorqued at time of engine assembly combined with a single group safety; and the high stress on the bolts from starting the motor from the rear led to the failure of all 6 bolts. Once the core engine is returned to Continental; I will be requesting a teardown report and evaluation for a confirmed cause of the failure.Engine Model: IO-550-N(43B)