The Cessna 206, while a great airplane, does seem to have a couple of week spots. Yes I know, let the jokes begin... The cylinders, nose wheel drag link, gear boxes, horizontal attach, door switch, and slow flap motor are all gripes of mechanics and pilots to one degree or another. Add to that list, exhaust hangers on the firewall. This is the third airplane that I have worked on fixing this problem. For those who want to look at the pictures and don't care about the details, skip the next paragraphs.
The exhaust tube is supported by a hanger which attaches to the firewall. Vibration is transferred from the engine, to the exhaust, to the hanger and to two small AN365-1032 screw which are held in by nutplates and 2 small AN426-3 rivets through the firewall and a .040 bulkhead. The vibration causes everything to flex which eventually creates cracks in the firewall and bulkhead. The material is not strong enough to take the vibration. Interestingly enough, I have only over seen cracks on the pilot side, although I know cracks have been found on the co-pilot side exhaust hanger as well. This airplane had a patch at one time, but it was not enough to transfer the loads and vibration without failing.
The repair one can effect, is to either replace the bulkhead or make a sandwich patch. Since a bulkhead is $400 and a long ways away in the USA, we decided to make a patch. I cut out the damage with a cut-off wheel, stop drilled one crack that went even farther outside of the affected area, and began working on making a patch.
Some .030 stainless was used on the firewall, then a smaller piece of stainless and a piece of .040 aluminum were used to "fill the hole", then on the backside of everything (inside of the airplane, behind the rudder pedals) another piece of .032 aluminum was used for the back of the sandwich patch. A 90 degree angle drill comes in real handy for drilling out rivets and making new holes for the patch.
Some fuel tank sealer goes behind the stainless patch and is riveted "wet" so that a good seal is made and no exhaust gasses would have the chance to enter the cabin area. The rivet spacing was as close to uniform as could be, given the other ribs, nut plates, and form of the bulkhead. Also the front patch is slightly larger than the rear patch to help eliminate stress points. Total time spent on this repair was about 3-4 days. Just getting at the area can be a chore as it involved removing the rudder pedals, brake master cylinders, and also bleeding the brakes once completed. I was quite happy how it came out in the end.