Is one able to change a certified aircraft, say a Cherokee 140 for example, into an "Experimental" aircraft? I appreciate this would devalue the machine, but I'm wondering if one could fit uncertified avionics/engine etc to a certified aircraft if it were deemed experimental?
If you are looking at a certified Cherokee 140, you will find quite a few with uncertified avionics/engine etc. in them anyway. Your logbook check will find some of these things but not all of them.
There was one Cherokee 140 that a mate owned which had an O-320 E2D engine with the data plate on the engine carefully changed to read E2A so it would be covered by the RAM 160hp STC upgrade as the STC does not cover the E2D engine upgrade in Cherokees. By the way, this upgrade turns the plane into a much safer and willing little rocket ship.
Another one had Metco-aire Hoerner wingtips installed, which lowers the stall speed, increases the cruise speed, increases the climb rate, gives more positive aileron control(so they say) - all good - but absolutely nothing in the logbook saying when, where or by whom they were installed.
Welcome to GA in Australia.