Stainless Zenith exhaust notes, poor Internet advice.


(If you are having trouble seeing the pictures in this article, it is because I am a computer troglodyte. I will have the Brains and Looks of the outfit correct this shortly, check back in the afternoon and hit F5 or Refresh. -ww)


Our friend and 601 builder Russell Johnson sent the following photos of a fit issue with a stainless Zenith exhaust we made in our jigs for him.  The issue he was having was that when bolted up, the pipe touched the base of the firewall. He was asking for advice on this. I covered the simple 5 minute solution for him in a private e-mail the same morning he sent the note.  Russell also asked the same question on an Internet discussion group for Corvairs which I can read, but I am banned from posting on. (I lack civility at times.) The response he got is posted below the pictures, and it is a fair warning not to listen to some advice you get on the Internet, even when it come from seemingly credible sources. I have no issue with Russell, I am just highlighting the point that customer service issues on parts we sell are always better answered by us directly than by looking for a quick answer on the Web. Discussion groups serve a purpose, but answering installation questions on products we sell isn’t one of them, especially if I am barred from responding on that group. If you’re new to homebuilding let this serve to open your eyes to the limitations of quick answers from strangers.


Above, a photo of our stainless Zenith exhaust fitted to Russell’s 601.  First let me point out that we have made nearly 250 of these exhaust sets in the 9 years we have been putting Corvairs on Zeniths. You can see on our Web pages the yellow 60-pound jig they are made in, and see that the CNC bent pipes are absolutely unchanged since 2005. Additionally the blue Zenith motor mount jig we have that makes mounts from CNC pre-cut tubing sets has also been in place, unchanged for nearly 300 mounts. We have more than 60 Zenith 601s flying with mounts and exhausts that came from these tools and work together. The logical question is what is different about Russell’s aircraft that it has an issue?

Several small things that the person who responded on the Internet blew right past. First, we didn’t build Russell’s engine mount. It is a very good job, but it is his craftsmanship. Second, he is using the gold washers under the red mount bushings. We generally do not use these; we put the red bushings directly on the mount. Beyond these points there are other factors that come into play. The Zenith firewall is set in the plane at 13 degrees leaned back. It takes a very small variation in the installation to affect the fit of the exhaust because of the length of the pipes. There are also variables on how the mounts are torqued up. There are also variables in the fit and thickness of the exhaust gaskets and the condition of the pipes coming out of the heads. But either way, there is a very simple solution to resolve any of these variables.


Above, a close up of the fit at the head. The pipe isn’t square because the other end is touching the bottom of the firewall and preventing it from laying flat. This view also shows the bottom gold washer we do not use. The graphite exhaust gaskets are Clark’s part number C-479C. There is some variation in them, and if you have a thin one near the firewall, it will make the exhaust pipe close to the bottom of the aircraft. Additionally, these gaskets seat on the flange welded onto the pipe extending from the head. Many heads have this flange corroded or bent, allowing the exhaust pipe to be displaced. If you have one of these in your heads, it needs to be replaced. We have also seen a front stack that was not seated in the head make the exhaust too close to the firewall. Issues with the stacks are easily seen by observant builders, as when they are correct, they are all in a line and the same height. Remember that you cannot change a stack with the head bolted on the engine, because the pipe will not pass the upper head stud to come out. Heads that went to Falcon have already had these things checked out.

Above, sharp eyes will notice that the engine here is just in the mock-up stage, it does not have lower cylinder baffles nor any pushrod tubes. For the sake of fitting the exhaust, I am assuming that Russell bolted the heads down firmly. The mount bolt appears to be torqued down the correct amount, where the top red bushing is the same size as the top gold washer. Many of the points I bring up here make a small difference, but two of them can add up to thinking that the system is wrong. In Russell’s case, I think that the largest variable is the fact that we did not make the mount, but either way, it is easily resolved.

The correct clearance from the bottom of the fuselage is only 1/2.” This may sound tiny, but the Corvair does not move much in flight. The most the engine moves is actually when the starter is cranked on the ground. If it doesn’t touch while cranking, it will not touch in flight. Why so close? The pipe is 1.5″ in diameter and it is going through a small cooling slot. If it hung down 2″ it would not only have more drag, it would also be touching the cowl. The smooth running Corvair does not need the same pipe clearance that is required on a 200hp angle valve 360 Lycoming. If the installed pipes don’t clear, the simple solution is to bolt them up tight to the head, insert a 1-3/8″ wooden clothes hanger dowel from Home Depot six inches in the end, and gently bend the pipe to the required shape. Wont that crack it? Absolutely not. Get a look at the pipe: it starts out life as a straight piece of tubing 20 feet long. It isn’t heated when it is formed, it is bent cold. It is a particular heat-treat of 304 stainless specifically designed to bend at room temp and never crack. This isn’t just some stainless I picked out at a muffler shop. These are formed by the same company, of the same alloy and on the same machines that make every Power Flow exhaust. I have gently bent 10 or 12 of the pipes we have installed on aircraft. Many of these have been flying hundreds of hours without issue. This is the proven and simple way to have the exhaust fit your aircraft exactly without having to make the cowls with a big giant hole to account for all possible variables.

On the Internet advice issue: Below is a response written by a guy on the Web to Russell’s question. Notably, the guy did get an exhaust and a mount from us six years ago. He put it on his aircraft. Is he in a good position to offer fabrication advice? Not really, his career was in architecture, and most importantly, his plane was wrecked on one of the first flights it made. His test pilot made an error, but when you read the reply below, maybe you can get the picture that this man’s decision-making also played a factor in the accident. His choice of welders and test pilots as local “experts” isn’t good.

The Internet solution:

“I had the exact same problem.  I had a stainless steel fabricator friend of mine try to bend it without success.  He finally cut the pipe and mig welded it back to an acceptable configuration.  I think the cut was at the last bend.  It was a real geometric puzzle, but we made it work.”

Really? Does this sound easier that a wooden dowel and a few minutes? Let’s take it step by step: I can bend these pipes with 20 to 30 pounds of pressure on the dowel. Next, only an idiot would MIG weld a 304 exhaust. 304 is only welded with high-end TIG welders after we back purge the whole pipe with argon. We post flow all the welds for 30 seconds and pay a lot of attention to weld sequence and heat build up. Next, mig welds are brittle, the last thing you want in an exhaust. We have carefully studied the Zenith systems over hundreds of hours, and I came to the conclusion that they were better off without any clamps or supports on them. They are a specific length and stiffness that they will not resonate, nor crack, but they do flex a tiny bit just where this guy and his buddy put a brittle MIG butt weld that isn’t purged, so the inside of the pipe is certainly charred. If the pipe broke at that spot in flight in a Zenith, you would have a very good chance of having a fire. If you don’t fly sitting on a parachute, think that one over. We specifically have these tubes bent in one piece so that there is no chance of that type of failure. Here is a guy telling other people to do something very dangerous. … and totally unnecessary.

Hey William, why are you such a jerk about a small detail like this? What happened to live and let live? Can’t we all be friends? Airplane building isn’t tee-ball or junior soccer. Score is kept here by two very impartial referees, Physics and Chemistry, and when one of these guys decides to eject you from the game, you’re likely to get benched for eternity.

Yeah? So what. I’m not going to follow that guy’s advice when I build my Zenith so why make a big deal out of it? What is the cost to me if the guy gives bad advice if I don’t listen? Go back and read my story I wrote 2 months ago called “If only someone had told him.” It was about Guy A’s Zenith getting destroyed by Guy B when Guy A decided against my advice to play flight instructor. Ready for this? The person offering exhaust advice here is “Guy B” from that story. His own Zenith was totaled by an “Expert,” but “Guy B” had it insured and got a big pay off. On round two he was at the controls when Guy A’s 601 was also destroyed. Three more and Guy B is going to be an Ace. How does this affect you? Planning on buying insurance? Would you like to know how much the rates go up when 2 aircraft in a small pool are destroyed?

I have almost 10 years of work in the Corvair/Zenith combination. We did it the right way, and bought a new kit for ourselves from the factory, worked with them, not against them, did all the testing, developed the parts, wrote an Installation Manual, assisted many people, traveled far and wide and made countless house calls that no one ever was asked to pay a dime for. We have stood behind everything we have done, and never considered using an LLC as a legal loophole to run out on builders. In this endeavor we have been assisted by many Zenith builders who made the process easier and offered encouragement when the light at the end of the tunnel was far away. Does this entitle me to some special award or praise? No, it was just a challenge that I willingly took on, and if some Zenith builders choose it, that is compliment enough. However, what I do not deserve, and our builders don’t deserve, is dangerous advice from Internet experts.-ww


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