If it was safe, Cessna would have put it in their POH.
If it were in the POH and someone cocked it up, the'd sue Cessna.
Matey
everything you described sounds fine but the only Cessna I have ever flown fitted with IO-520 AND a co-pilot's door is a C185.
I'm too fat, too old and too lazy to climb in the window of a C206!
The only Cessna with 520's that I have flown in that Doesn't have a Co-Pilot door is the Cessna 402/404!
206/207/210/310 All seem to have 'em
