Normally owners/pilots are not allowed to oversee such work in many places...
And we all know why that is, don’t we.
I told you how I knew the magneto to engine advance: The Timing Fairy told me.
This is your cue to say the problem never happened.
And are you
sure about your 20 degree +/- 1 BTC magneto to engine figure for all IO520s?
Really sure?