The exhaust system on my Cessna 120 requires inspection every 25 hours, according to a placard on the dashboard.
I inspect it every 15 hours.
This last time, I did the inspection after 13 hours and got quite a shock!
The port side muffler was blown out as seen in the picture. When I inspected it 13 hours ago, there was not a single hole or crack. If fact, I tap it with a screwdriver handle to check for soft spots.
This is the side that is attached to cabin heat, which fortunately I have not used recently.
Bob Z.