Technician reported the aircraft's engine was disassembled after magneto failure and it was found that the required magneto bearings had not been reinstalled after overhaul.

Narrative

The aircraft had a propeller strike [over 5 years ago] about 1;100 hours time since new. The engine was removed and sent to an aircraft engine shop for disassembly inspection and reassembly. The aircraft/engine was returned to service and operated for about three hundred hours on the magneto (6350) which then failed; the magneto was repaired/yellow tagged and returned to service. At that time the magneto adapter LW19096 did not disengage so the replacement of the magneto was routine.About 500 hours time in service; the (6393) magneto was removed for service bulletin. The LW19096 part disengaged from the magneto and the drive gear. We removed the LW19096 noticing that it did not match the picture; which should have been included bearing 67542. We then assumed we screwed up and searched the oil sump with camera; magnets; suction without any bearings or fragments found. We pulled the quick drain and used a camera to search inside; no joy. We pulled the oil screen; and no fragments were found. Finally; we cut the oil filter and again; found no metal. Frustrated; we pulled the other magneto and disengaged the LW19096 and found no bearing 67542. We have concluded that the engine shop removed the bearing for NDT (Non-Destructive Testing) and never replaced them; the bearings are press fit and must be removed with a puller. No 67542 bearings were on the new parts list in the logbook.The O540 and IO540 require bearings to support one end of the magneto drive gear. I am aware of one other instance where bearing 67542 has been omitted and the engine was full of metal fragments from the LW19096 was vibrating against the case.