Interesting (and Scary) surprise!

Wow. That's no good.

 

Rusty rot of that kind doesn't happen in 13 hours and not often even in 13 years. It happens from the inside out, I'm glad it didn't cause you any inflight problems. The mufflers on Piper Pacers, and PA-11's are bad about it too and have a similar AD.

 

Could some kind of excess moisture be getting into the inside? Unburned fuel?

 

This happened to me twice in my long life. Once, a cracked manifold in a six cylinder Chevy truck. I had a 1000 mile trip where I battled extreme fatigue, headaches, and nearly passed out several times. A mechanic discovered it when I got an oil change before starting back home. The first incident was in the air force during the war when I was aboard an engineering check flight after heavy maintenance. A connection was left open in the exhaust pipe just forward of the cabin heater cuff and we were getting exhaust into the interior of the airplane. The pilot made a lousy but successful landing. All of us were obviously pretty sick and an alert officer decided to have the airplane re-checked and found the open exhaust pipe connection. Sneaky and deadly stuff.

 

What engine do you have in your 120?

 

Just the same as any muffler, volatile and corrosive gasses, condensation, heat cycles, the material they're made of and they just don't last forever even if they look good on the outside.

I take them off the plane, remove the heat shroud and blow air through them from a shopvac exhaust and plug the other end and immerse a half at a time in water or spray with soapy water. I have a small borescope that helps too.

Pain in the butt , but worth the effort if it keeps somebody from being asphyxiated. They just can't be thoroughly inspected with a mirror and flashlight. If one holds ones hand over the end just right it is possible to make a muffled fart sound.. Sorry, crude mechanic humor.

 

Much better! Looks like it should last a while