Hi Sub-sonic,
I am trying to understand and learn how the flame tube failure caused pin holes. Pin holes are a very specific type of corrosion and it has been about 25 years since I had a class in corrosion engineering. I am just curious about the mechanism. Do you think that it is erosion breaking down a passivation layer with the new flow path that allows the start of pits or is it strictly a thermal issue with a change in grain structure?
Some folks on this site have intentionally removed their flame tubes and this is the first I have heard of it causing a pitting issue with the shell. Again, I am just curious about the mechanism.
Hi dogday,
I'm no engineer, but I continue to seek knowledge. Heat is the enemy. Excessive heat will cause the metal to erode and change the metal itself, making the metal weak and susceptible to failure. Heating and cooling cycles, along with moisture will cause the corrosion to begin. On this design or more specific my muffler the pin holes started at the end seams. The purpose of the flame tubes is to keep the exhaust flame from directly hitting the outer corrugated can. The exhaust flame is like a torch and the flame tubes buffer that flame. Most designs, say a Citabria or a Scout, muffler you cut the end off, weld in a new flame tube and close it up... not this design, seems like you can do it one time and then there's not enough of the corrugated material. When I started as an A&P looking up the exhaust was the first thing you did because turn around time on a muffler repair was a week or two. Now you can get a $20 endoscope hook it to your cell phone and take a look, easy. But, inspecting the muffler is important, that's what I'll do. FAA FAAST team has a good video on line ALC-498, Husky SB35 and AC91-59-A are good references. Hope that make sense.