I agree that an o-ring is not an "airplane part" exclusively, but you better know what you are doing when sourcing things like o-rings from suppliers other than Rotax. The identical size o-ring (diameter and x-section) comes in probably a dozen different "rubber" materials and several different durometers (hardness). Some of these material differences can be identified by color, but others cannot. If you do not know the material specs for the o-ring you are replacing and just order a standard buna N 60 durometer o-ring, you are taking a chance I would not care to take on something like an engine that can get very hot and has several kinds of fluids that must be compatible with the o-ring material in some very critical locations. Just 2 cents worth of thoughts from an old hydraulics engineer.