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cdarza

Piston Head

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What are the piston heads (not brand-new) supposed to look like?  Was playing around with a cheap endoscope and didn't like what I saw - but then again, I also don't know what to be looking for.  

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All our piston heads on heads have some build up. The amount and type is influenced by whether you use just auto fuel or 100LL. 100LL has more build up. Then it would depend on how much idling you do. Whether you are running a little leaner or rich mixture. Engines with dual filters under the cowl mounted right on the carbs will eventually end up with more build up vs an engine that gets cooler outside air that run richer and will have more build up. Unless you somehow get a huge amount of build up which would be rae then I wouldn't lose any sleep over it.

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3 hours ago, procharger said:

Looks like you have a little bug on your piston in last picture😃

It does look like it  !! LOL       and @sandpiper   Normal cruise of 5000rpm (im at 900hrs now).    Although what i havent been able to figure out yet is my low egt of 1100 and 1200.  Im quite sure the temp probe is mounted in the correct spot and distance.   Ive also changed the needle setting to run more lean and didnt get much of a different reading.  

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I have seen 1/8" of irregular buildup on pistons after 2000 of 100LL. Chunks of junk broken off.  Looks like the moon surface.  Really, it is the valves you want to focus on.  Especially the exhaust valve. Look for asymmetry in the face (a hot spot). A little bit of a bulls eye--good.  The rising sun--bad. 

https://www.aopa.org/news-and-media/all-news/2016/november/04/aircraft-maintenance-tackling-top-overhauls-borescope-tests

The exhaust valve is the most crucial component to inspect. Exhaust valves are the weakest link in most cylinders because they are subject to extreme heat as the combustion gasses flow out of the cylinder, and they can only dissipate this heat through the valve guide and valve seat. In addition, they tend to collect deposits of lead and other byproducts of the combustion process on the valve surfaces as the gasses flow by. This can cause a poor seal between the valve and the valve seat. Once a breach begins in the sealing surface of the valve and seat, the hot gasses passing through it will quickly erode the valve and begin the failure process. One indication is to look for uniformity of the coloration on the face of the valve. If the valve is seating properly, the face will typically look uniform with no color variations at a particular "clock position" on the face. Don’t worry too much about the "burnt pizza" look on the face of the valve—it’s the uniformity that matters.

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Build up on the piston is normal. As for using a borescope, I don't think it is really needed, unless there is a problem with the compression. The Rotax is not prone to the same issues as other aircraft engines. 

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According to data published by the Australian Transportation Safety Bureau (the equivalent to the NTSB), Rotax powered light aircraft were the second most common type of engine to have an engine failure or malfunction over the 2009-2014 period they looked at (link below). Worse than Lycoming and Continental. Jabiru was the worst. Would borescope have helped?  Who knows. But the publications I read say it is prudent to borescope, easy to do, and may just save your bacon one day. 

https://www.atsb.gov.au/publications/2013/ar-2013-107_research/

 

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22 minutes ago, MEH said:

According to data published by the Australian Transportation Safety Bureau (the equivalent to the NTSB), Rotax powered light aircraft were the second most common type of engine to have an engine failure or malfunction over the 2009-2014 period they looked at (link below). Worse than Lycoming and Continental. Jabiru was the worst. Would borescope have helped?  Who knows. But the publications I read say it is prudent to borescope, easy to do, and may just save your bacon one day. 

https://www.atsb.gov.au/publications/2013/ar-2013-107_research/

 

I have seen that report , your summary is quite misleading  , given small numbers involved statistically speaking there is no difference between first 3 entries :

 

Taking into account the number of aircraft on both the CASA and RAAus registers and the number of aircraft involved in the above data, this represents an engine failure or malfunction occurrence in the study period in about:

  • one in 10 aircraft with Jabiru engines
  • one in 36 aircraft with Rotax engines
  • one in 35 aircraft with Continental engines, and
  • one in 33 aircraft Lycoming engines.

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My "summary" is not "quite misleading". The facts of the ATSB publication stand as stated.  The point made by quoting their published data was to refute the notion that "Rotax is not prone to the same issues as other aircraft engines", as if Rotax was somehow immune to engine failure. 

If your engine fails and you did everything possible and reasonable to ensure that the engine was safe ahead of time, you will sleep well. I would hate for anyone to wonder on Monday morning if they should have played the game differently on Sunday. 

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13 minutes ago, MEH said:

My "summary" is not "quite misleading". The facts of the ATSB publication stand as stated.  The point made by quoting their published data was to refute the notion that "Rotax is not prone to the same issues as other aircraft engines", as if Rotax was somehow immune to engine failure. 

If your engine fails and you did everything possible and reasonable to ensure that the engine was safe ahead of time, you will sleep well. I would hate for anyone to wonder on Monday morning if they should have played the game differently on Sunday. 

I am the one who said, "Rotax is not prone to the same issues as other aircraft engines". You took my comment wrong in thinking I meant they are less prone to failure. Rotax has ceramic coated aluminum cylinders, so rust is not an issue like it is on Lycoming and Continental. This is one of the reasons for using a bore scope. Second Rotax has liquid cooled heads, so it does not normally see the heat issues with valve that Lycoming or Continental does. This is another reason for using a borescope.  I in no claimed that Rotax was less prone to failure. Rotax engines do fail, but the failure mode is normally diferent than Lycoming and Continental. They typically do not have the cylinder failures thatthe big two have.

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The consensus thinking on exhaust valve failure is that there is imperfect seating of the valve and escape of burning gasses around the valve during combustion causing a hot spot on the edge of the valve at the point of the microscopic leak.  It takes some time for the hot spot to result in a crack and failure. The failure of the compression test is one way to detect the failing exhaust valve, but it only detects the valve that has a big enough leak at the moment of the test and the test is done at 80 psi. The peak pressure in Rotax 914 cylinders during combustion at 4000 rpm is about 435 psi and at 5800 rpm is over 700 psi. (https://www.researchgate.net/figure/Comparison-of-measured-and-calculated-cylinder-pressure-at-5800rpm-4000rpm-Rotax-914_fig3_280656658). It is the hope of the borescope to catch the failing valve earlier by detecting the developing heat damage before the leak gets big enough to detect by differential compression testing. The inspection takes about 2 minutes per cylinder once you are practiced in manipulating the scope.  For those who would like to spend $200 on a borescope and a little time to go the extra mile and learn how to borescope their cylinders/exhaust valves, AOPA has an article with a poster included that gives pictorial examples of normal and sick exhaust valves.  Check it out:

https://www.aopa.org/news-and-media/all-news/2014/june/pilot/pe_ownership

As there are vastly more Continental and Lycoming engines in service than Rotax engines, it is my contention that those who are producing publications on the topic are basing their recommendations and observations based on the legacy engines simply as a matter of volume in the fleet. Rotax engines have exhaust valves and in 8 minutes you have gone the extra mile.  Sit on the couch, or an 8 minute mile.  Your choice.

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What do you save by detecting the exhaust valve heat damage earlier than a differential compression test?

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MEH or anyone else, are you aware of a burnt valve on a Rotax 912 that happened during normal operations, or any any other conditions for that matter?

 

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There is no comparison to Continental and Lycoming when it comes to a 912. They are dinosaurs of the 1930's. Not much has changed and probably won't, just like our GA aircraft. You can thank the FAA bureaucracy and lawyers. I have been dealing with Continental and Lycoming for 47 yrs even worked for them and am amazed at the 912 technology.  The others better take note but I doubt it. Once while working on a serious issue with Lycoming I made a suggestion to a senior design engineer about a better way to solve the problem. He told me that we have been doing it this way for 30 years so why should we change it now. That told me everyting.

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The frequency of exhaust valve failures in Rotax?  Very low. Probably on the order of frequency that the engine starts making metal, but we cut the oil filters open at every oil change looking for that remote possibility.

As to why you would want know that your exhaust valve is in the process of failing earlier than detectable by compression testing?  Same reason people want to catch their polyp when it is small.  We have no idea how long it will be until there are regrets. Presumably every in flight exhaust valve valve failure had a compression test at the last annual.

As to why the industry standard hasn't uniformly adopted the borescope inspection? Same logic as has been highlighted.  We have been doing only compression testing for the last 75 years, why should we change now? 

 

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It would be interesting to consider the implications of compression testing on relatively tight Rotax engines compared to relatively loose L/C piston/cylinder fit.  We may be comparing apples to oranges.

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My point is that there are defects allowed in most engines and just detecting one does not mean you need to do something about it.  

If you see a hot spot discoloration on a valve during a borescope, do you replace the valve immediately? If you see a flake of metal in your oil filter, do you do an immediate teardown and overhaul? I think not.

In the case of exhaust valves, would the limit of acceptable damage be detected with a compression check? Just because something is old does not mean that it is no longer good.

I am sure at least some of those in-flight exhaust valve failures had low compression checks.

It is only recently that camera and lighting technology have reached a point where borescopes are cheap to purchase.  This will greatly aid the adoption by mechanics.

 

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