Need inputs - broken gear

[CubFly]

Requesting inputs from powerplant wizards, A few months back, my C90 in the Cub backfired during run-up. It was on the left magneto. I taxied back to the mechanic and the engine ran fine during taxing on both magnetos.But then the prop got locked during re-start (hand propping) The magneto gear drive had missing teeth. Pictures below. Nobody can convincingly answer the root cause and how it happened. Also, did the backfire due to the broken teeth/gear or the broken gear caused the backfire (?)is also a chicken n egg discussion. Since then, the engine went through a full IRAN, new mags, gears, cylinder valves, etc (technically an overhaul except brand new cylinders). Its almost ready for test flight. What are your thoughts? What caused the damage?

 

[Jeff Fontenot]

 

[Jeff J]

 

[nrpetersen]

 

[CubFly]

 

[Jeff J]

 

[JimC]

 

[nrpetersen]

 

[bob turner]

 

[dieseldriver47]