Exhaust Valves and Exhaust Risers

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RMarkSampson
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Exhaust Valves and Exhaust Risers

Post by RMarkSampson » Sun Jan 06, 2019 1:34 am

Guess my DA-20 does not know how to exhale. Another failed compression at Annual - #2 cylinder this year. Last year it was #4. So off to the engine shop for that cylinder. Now I'm wondering how common is it to have exhaust valve issues on the IO-240-B. I have put 1500 hours on her since an overhaul so she is getting toward the TBO end of things. She still runs smooth, develops full power, burns little/no oil and give no indication of a exhaust valve issue - but the compression check was 40/80 on cylinder #2, and upon pulling the cylinder there was clear indication that the exhaust valve has asymmetrical deposit marks indicative of the exhaust valve failing. I have an EDM-900 EMS so looking back at the data I could see some lower EGTs on #2 at lower RPMs - but above 1500 RPM the EGT was on-par with the other cylinders. Suspect the higher pressures at higher RPMs was enough to fully seat the exhaust valve during its cycle - but at lower RPMs it was not fully seating itself. I also have a subscription to Mike Busch's SavvyAnalysis service - they provided me with some feedback on spark plugs and injectors - but did not highlight any exhaust valve issues. I provided them feedback on the issue and they confirmed their "FEVA analysis" did not detect the issue but will use my data to see if they can improve their software for the future.

Another issue was finding that cylinder #2 exhaust riser flange had broken in two - and cylinder #3 riser had a crack in it. I would not be too upset about these failures except I had replace both of these risers last year when I had pulled the exhaust system off to repair cylinder #4 and both flanges were broken in two. These are Diamond parts, and at $300 a riser, I'm not exactly happy that I am paying another $600 this year for parts I replaced last year. Anybody else's DA-20 having broken riser repeats?
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Re: Exhaust Valves and Exhaust Risers

Post by RMarkSampson » Mon Jan 21, 2019 2:17 am

So I figured I would recap the whole saga of my exhaust risers for general knowledge. First a cylinder needed rehab for an exhaust valve not seating correctly so that requires the entire removal of the exhaust system. One flange was found broken, another flange-2-riser braze (not a weld) was cracked. These are Diamond-specific parts so I initially went with the "get new" from Diamond course of action (COA). If I would have initially went with a "get it welded" COA, 6CT would already be back in the air - but I didn't. However, over the course of the next week or two - Diamond did not have one of the risers in stock and then their time to restock kept getting extended, now at three weeks. Meanwhile, I did speak to a welding shop (turns out they are up in Canada) that specializes in aviation welding and is very familar with Diamond's risers. That is when I started to learn about the difference between brazing and welding and how Diamond's risers were prone to failure because they braze. I definitely listened to them because the two broken/failing risers are the same exact risers I replaced last year for over $600 when cylinder #4 was rehabbed - now they wanted $600 again?!? So when Diamond said "three weeks" I turned off the new risers and sent the risers to the welding shop down in South Florida who is now rehabing them with quality welds. I asked them to look at all four risers - and if they saw the brazed connection between the riser and the flange as a weak point to just go ahead and weld them all up right. So, this COA takes a week to do - but in my opinion it is better and faster than just replacing them with Diamond new parts.
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Re: Exhaust Valves and Exhaust Risers

Post by Colin » Fri Jan 25, 2019 3:34 am

You said the welding shop was in Canada but then you sent the parts to Florida. Which shop did you use?
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Re: Exhaust Valves and Exhaust Risers

Post by RMarkSampson » Fri Jan 25, 2019 10:35 am

6CT is over with the Diamond Service Center in Lakeland, FL and they have a shop in South Florida that they use. I actually don’t know the name, just relying on them to have developed a relationship with a good welder. I’ll get a name and post some before/after pics when I get chance.

Acorn welding has a very nice website you can google. They are up in Canada but have a page with the DA-20 exhaust system laid out. Calling them I spent several minutes on the phone with them where they freely provided their experiences with the DA-20 exhaust risers without any sales pitch. I just got a positive vibe that they had a good shop and provided experience that any of us Diamond owners could use. I can’t endorse their work but I certainly can tell everyone that it is a phone call worth making if you are working through an exhaust system issue as I am.
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Re: Exhaust Valves and Exhaust Risers

Post by Spinner » Wed Jan 30, 2019 7:26 pm

We (MFC) have spent a lot of money with Acorn on various parts such as exhaust parts for Diamond, Cessna and Piper. As well we have had DA20 steps repaired, worn out canopy arms etc. I would definitely give them a shout. They treat us well.

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Re: Exhaust Valves and Exhaust Risers

Post by Pascal » Fri Feb 01, 2019 2:26 pm

I guess I should be able to provide more empirical data soon. My engine has about 1500 hours on it and so far the compressions and oil consumption have been great. Touch wood. We'll see over the next two years how the situation evolves.
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Re: Exhaust Valves and Exhaust Risers

Post by DragonFly » Sun Mar 31, 2019 8:33 pm

I have 4 DA20s in my flight school. I have been constantly battling exhaust valve issues since shortly after I got the first plane. Specifically, they tend to "cup" really badly and only last about 500-700 hours before compressions get bad enough to pull jugs and rehab them. Typically, all that is required is new exhaust valves. After many conversations with Continental, they say it is normal wear. I tend to disagree on an engine that has only 600 hours on it, since new! That being said, I started looking at history of our different aircraft, and when they started to lose compressions, and what might have changed. The only thing that changed about 100-200 hours before compressions started to drop, was the change of the propeller. Because of the metal leading edge of the MT prop, I eventually change all the planes over to them. The one that went the longest, without even one cylinder change, went 1700 hours from a top overhaul before it started going down. The top was done before I bought the plane, and a review of the logbooks seems like it indicated it wasn't really necessary. I had the engine changed with a factory reman zero time, and added the MT, after 200 hours all compressions were down in the high 50s low 60s, now it has 600 hours and it needs 3 cylinders. Not sure if it has something to do with the MT being so much coarser of a pitch making the engine work harder in the flight school environment (many take-offs and climbs), but we are going to switch back to a Sensenich after a 4 cylinder change to test the theory. Should have a pretty good idea in a few months. On a side note, no one has never noticed anything wrong with how the engines run, not rough, makes full power, climb is normal, normal takeoff distance, nothing that would indicate that the compressions were as low as they were.
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Re: Exhaust Valves and Exhaust Risers

Post by CFIDave » Mon Apr 01, 2019 1:27 am

DragonFly wrote:
Sun Mar 31, 2019 8:33 pm
On a side note, no one has never noticed anything wrong with how the engines run, not rough, makes full power, climb is normal, normal takeoff distance, nothing that would indicate that the compressions were as low as they were.
If you read the writings of piston aviation maintenance expert Mike Busch, he emphasizes that too many Continental engine owners are replacing cylinders prematurely (or performing "top end overhauls" on engines that don't need them) due to low static compression readings. His view is the most of this these low compression measurements are normal.
https://www.amazon.com/Mike-Busch-Engin ... 1718608950
https://www.savvyaviation.com

While he's mostly referring to big-bore 6-cylinder IO-550s like those found in Bonanza/Baron and Cirrus aircraft, his advice might also apply to the Continental in a DA20-C1. Maybe you don't need new cylinders?
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Re: Exhaust Valves and Exhaust Risers

Post by DragonFly » Mon Apr 01, 2019 3:45 am

I agree on the cylinders not actually needing to be replaced. However, as I understand it, no amount of leakage past the valves is "acceptable" and once the compressions go below the master orifice leakage they can not be considered airworthy.
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Re: Exhaust Valves and Exhaust Risers

Post by RMarkSampson » Mon Apr 01, 2019 12:18 pm

Also agree with DragonFly. I've now done two cylinders where the exhaust valve lost its ability to firmly seat and pass a static compression check. One fail was a "0/80" - the other was "40/80." In both cases my EDM-900 data showed the CHTs, EGTs on all cylinders were fine, developed adequate power (RPMs pushing an MT propeller did not change between pre/post cylinder rehab) etc. A fail is a fail so I'm not advocating ignoring that; but low compressions don't scare me much anymore - because what happens in the cylinder at operating pressures is not what happens in the cylinder during a static compression check. If static compressions are low but passes then I intend to declare them good and keep flying the engine.

I'm not sure if lead buildup can lead to an exhaust valve not seating well. I've borescoped my engine and certainly saw lead deposits IVO the spark plugs and valves that may be an issue. With the electric fuel pump on for takeoffs/descents/landings - the IO-240B is certainly running very rich. Will excessive lead deposits around the exhaust valve impact a static compression check? The exhaust valve is suppose to rotate when it cycles - if it does not rotate then the exhaust valve exhibits an asymmetrical pattern when inspected by a borescope. To me all these items need to be viewed when assessing the health of the engine.
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