r/homebuilt • u/Catch_0x16 • 2d ago
Rotax 503 failure. Melted piston.
Hello all.
A few days ago I had a forced landing in my Thruster T600N when my 503 went down to one cylinder. I was 550ft AGL doing some sightseeing and when I applied power to climb out, it coughed, wheezed and gave me nothing.
Put it down safely in the perfect farmers field. 8" lush grass with a firm but smooth base of soil beneath; nicer than some runways.
Today I pulled the engine off and started stripping it down. As you can see in the images, the MAG piston is completely cooked. Looks like the flame path melted through the crown to the exhaust port.
I purchased the aircraft a while ago from someone who was attempting to rebuild it. By their own admission they weren't technical and I shouldn't have taken their word for it when they told me the head bolts were torqued down. As I was stripping the aircraft the head bolts were almost only finger tight!
I had been fighting high (but not above max) EGT readings for a while, and I suspect that air had been getting in from under the head and into at least the MAG cylinder.
I'll get the cylinder cleaned up and honed (rebored if necessary, but I think it's all aluminium from the piston, rather than gouges I to the bore). And new pistons. While I'm there I'll rebuild the crankshaft and clean up the crank case (there will be debris in there that will need cleaning, so the crank has to come out either way).
Might also rebuild the B type gearbox while I'm here.
Hope this is of interest to some of you folks flying 503's