top overhaul

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ahmebane
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top overhaul

Post by ahmebane »

So I just read the article in March 2020 AOPA regarding the dangers of cylinder replacements. i just bought a DA40 that has recently (50 hours) had all four cylinders replaced with new assemblies. How concerned should I be? The work was done by a Diamond shop, but am I sitting on a ticking time tomb? The cylinders were replaced because the plane sat idle for a couple of years & one of the cylinders came up with a low compression, so they decided to replace them all before selling.

Thank you!
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Re: top overhaul

Post by 40flyer »

Biggest danger comes from poor maintenance procedure during the new cylinder install. Bolts need to be pre-torqued using the appropriate tools to ensure good long-term seating of the cylinder base. Many maintenance shops will not attempt this but rather send the engine off to an engine overhaul facility to get it done correctly. Your mileage may vary depending on how good your Diamond service facility follows procedure.
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Re: top overhaul

Post by Lance Murray »

The biggest danger was to the sucker that paid for the “top overhaul” but it is something to be aware of. Talk to a trusted engine shop and find out if torque can be checked after say 100 hrs. This would take a lot of disassembly to
Properly check the cylinder and case torque. Maybe check with Mike Busch.
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Re: top overhaul

Post by Davestation »

ahmebane wrote:
Fri Feb 28, 2020 7:23 am
So I just read the article in March 2020 AOPA regarding the dangers of cylinder replacements. i just bought a DA40 that has recently (50 hours) had all four cylinders replaced with new assemblies. How concerned should I be? The work was done by a Diamond shop, but am I sitting on a ticking time tomb? The cylinders were replaced because the plane sat idle for a couple of years & one of the cylinders came up with a low compression, so they decided to replace them all before selling.

Thank you!
IMO certain people really love mx fear mongering, especially when it comes to cylinder work - it is IMPOSSIBLE for a mechanic to do it properly and you almost certainly WILL die from their error (I'm thinking of one or two articles in particular from the sources mentioned on this thread).

Having said that, I do think it very odd that they would replace three perfectly good cylinders merely because the fourth was low...

How concerned you should be depends solely on the reputation of the shop that did the work. It certainly wouldn't hurt to have them retourqued by a mechanic you trust though.
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Re: top overhaul

Post by Colin »

I think you're reading those warnings mostly because mechanics are trying to kill pilots all the time, and most pilots have become wise to it and try to stay out of wrench range.
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Re: top overhaul

Post by perossichi »

There are two dangers of top overhauls. 1. Failure to achieve proper fastener preload. 2. Improper break in.

I know of several very serious accidents that occurred because of exactly what Busch was talking about in that article. It is rare but it does happen. BTW, Busch has many articles and several books that touch on this. I believe there is an upcoming EAA webinar on this topic which you can register for and pose your problem to him online. This webinar is in early April 2020.

There is a webinar on the Saavy YouTube channel on this called the Perils of Cylinder Work. At the end of the webinar, there is always a Q&A session. I believe someone did ask your question. I think he answered that checking the torque on the hold down bolts couldn’t hurt but is not likely to help.

One thing you could do is call up the shop and ask them what there procedure is
Do they use new nuts? Do they clean the stud and thrubolt threads? Did they replace the thrubolts? Did they lubricate the thread, nuts, and cylinder base flange and with what?

Unfortunately, it is my understanding that most of Busch’s advice is forward looking not backward looking on this topic.

You can also email him. He is very generous about returning emails in my experience.

Proper break in also a problem. This is easily diagnosed by borescoping the cylinders, though. Again, Busch has a webinar on this called Breaking Good.

Some folks, including some APs, don’t like Busch, but I find him very refreshing. He tells you why and he provides data where available. Between EMT and upset training and Mike Busch’s advice on maintenance and power plant management, I think I’m much safer in an already very safe plane.
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Re: top overhaul

Post by perossichi »

Also, if the plane was really sitting idle for several years without proper preparation of the engine, I’d be concerned with corrosion on the camlobes and lifters which cannot be inspected in a Lycoming without splitting the case. Also, the steel cylinder barrels can rust. I would borescope the cylinders ASAP and perform oil analysis on each oil change. There are a number of firms that do this— Busch likes Blackstone labs. If you find metal in the oil filter, get that properly analyzed. I would expect that you first sample of oil will have high levels of Iron and then drop off.

It does not make any sense to replace a cylinder that doesn’t show any diagnostic signs of wear. I completely agree with Busch that no one should ever pull a cylinder because of a low compression reading. Only if borescope shows problems. The compression test is unreliable. I’d stay away from that Diamond shop that did the top overhaul if what you say is the full context of that decision.
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Re: top overhaul

Post by Davestation »

I should hope there is zero rust in the cylinders now since they were all replaced.

A low reading has to be rectified, it can’t just be ignored because Busch says cylinder replacement can’t be done properly in his opinion. That’s what I mean about fear mongering being dangerous.

The shop should however be able to tell pretty easily what is causing the low compression. A borescope can actually give you very little insight compared to just putting your ear to it when doing the pressure loss test. Is it coming out the intake, exhaust, or breather, and via the head return or the rings? A borescope mostly just diagnoses the latter, but with all four cases there’s not a ton you can do without inevitably pulling the cylinder if it failed miserably. Only if it just barely missed the mark are you going to do more digging and possibly rectify the problem without doing anything invasive.
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Re: top overhaul

Post by perossichi »

It is not correct that a cylinder with a low compression alwaysneeds to be replaced. The compression test is not reliable. At the least, if you have a low compression rating, you should fly the plane for at least an hour and take another compression test with a warm engine. typically, this reverses many invalid test results.

It has been shown that very low compression engines still produce full power.

Both continental and lycoming have revised their standards for compression to much lower acceptable values.

It never makes any sense do a top overhaul in which cylinders that show no abnormalities are replaced.

No one is advocating not replacing cylinders for fear of the risk of a poor install. All we are saying is don’t replace a cylinder unless it is absolutely warranted such as a failed exhaust valve. Note that the only way to diagnose a problem exhaust valve before it fails is to use borescope to check the pattern of deposits on the valve head which shows hot spots. Engine monitor data can also be used. Compression tests only work to detect exhaust valve failures after failure not before.
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Re: top overhaul

Post by Davestation »

I didn’t say always - if it just barely fails then by all means fly it for an hour. I did say if it fails miserably you can’t just chock it up to “the test was inaccurate”.

Only Continental has revised their standards, not Lycoming. Also worth noting that people commonly overlook the fact that even though continental allows lower readings, they specify that this allowance is only for dynamic leak, zero static leaks are acceptable.

The most common thing we’ve found em when sending a cylinder out for repair is a worn out valve guide, flying the plane for an hour doesn’t fix that.
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