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Dangers of Replacing a Cylinder

mfleming

mfleming

Well Known Member
Patron
I ran across this article by Mike Busch on the dangers of replacing a cylinder unless the engine is off the airplane and in the shop. Even then this article says there is still a big risk by doing the job without tearing the engine down :eek:

So, if you needed to replace a cylinder on your RV, do you think this article makes a good case for tearing down the engine instead of pulling and replacing it?

Link to the Mike Busch Article.
 
mfleming said:
I ran across this article by Mike Busch on the dangers of replacing a cylinder unless the engine is off the airplane and in the shop. Even then this article says there is still a big risk by doing the job without tearing the engine down :eek:

So, if you needed to replace a cylinder on your RV, do you think this article makes a good case for tearing down the engine instead of pulling and replacing it?

Link to the Mike Busch Article.
Click to expand...

This appears to be a controversial article. While I see his point about bearings, standard maintenance practice is to replace cylinders when they go bad and no case overhaul is required or routinely done with these replacements. If any significant portion of these cylinder replacements resulted in imminently destroyed bearings, I think we would have heard about it by now. These engines have been in production for 60+ years and a significant percentage of these engines get a top overhaul before a major.

Personally, I pulled 2 cylinders at 10 hours (due to glazing during break in) and then pulled all four at 100 hours for a full re-hone/re-ring (piston pin plugs were throwing a lot of metal). I have 550 hours now and no signs of bearing wear or problems. My oil pressure is the same as it was at 10 hours. If there were any meaningfull bearing wear, this would not be the case.

To add to this, it used to be a somewhat common practice on auto engines to pull the main and/or crank bearing to check clearance if they were suspect. If the clearance was good, they were reassembled and put back into service. Not sure I buy his logic that any disturbance of the bearing shell dooms the engine to imminent destruction.

Larry
 
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Pull a jug if need be

lr172 said:
This appears to be a controversial article. While I see his point about bearings, standard maintenance practice is to replace cylinders when they go bad and no case overhaul is required or routinely done with these replacements....snip

Larry
Click to expand...

I agree with Larry, Though also agree with the article, great caution should be taken if a jug needs to be pulled.
 
This seems to be at odds with a previous Mike Busch article I recall where the topic was if you had one bad cylinder should you replace all of them. He argued strongly that no you shouldn't, just replace the one bad cylinder and carry on.
 
It doesn't seem to be at odds with it at all; he said several times in this one that replacing multiple cylinders increases your chances of trouble.
 
I don?t see in the article where it says anything about the need to tear down the engine to replace a cylinder. It does say that it is preferable to have the engine off the plane when cylinders are removed to better enable achieving proper torque on the through bolts if that?s what you mean. The risks discussed are also stated to be substantially greater when multiple cylinders are removed as opposed to a single cylinder.

Erich
 
Just wondering if the risks become more elevated with certain engine types. I know Mike Busch has lots of experience with big-bore Continentals so perhaps some of his tendency to err on the side of caution might come from engines not necessarily applicable to the smaller Lycomings?

I've replaced 3 cylinders on the same engine, two in one shot and a single one a year prior. That engine seems to continue to soldier on without any challenges. It was a mid-time O-320 which had previously had cylinders replaced with chrome cylinders so by the time it had 1100 hours on it since overhaul it had gobbled up 7 replacement cylinders. None have been replaced now in several years of operation so I strongly suspect the first "chrome overhauled" cylinders were very marginal to begin with.
 
mfleming said:
I ran across this article by Mike Busch on the dangers of replacing a cylinder unless the engine is off the airplane and in the shop. Even then this article says there is still a big risk by doing the job without tearing the engine down :eek:

So, if you needed to replace a cylinder on your RV, do you think this article makes a good case for tearing down the engine instead of pulling and replacing it?
Click to expand...

No.

The article should be properly titled "The Problem of Achieving Correct Fastener Preload" rather than the "Be Afraid" headline more suited to supermarket checkout pulp.

The preload problem is a little easier to deal with on an engine stand...but not much easier.
 
I've replaced more cylinders than I can count, not a single one ever had an issue afterwards. I would be more concerned about a maintenance error removing the entire engine than just a cylinder.

In a world of fake news this is another example of hand-wringing to get attention.

Its not difficult to properly check torque on cylinder and opposite-side thru bolts, even with baffling in the way if one has the right tools.
 
Ya, I tend to agree with Bob that there are dozens of items involved in removing the entire engine.

Jim
 
DanH said:
No.

The article should be properly titled "The Problem of Achieving Correct Fastener Preload" rather than the "Be Afraid" headline more suited to supermarket checkout pulp.......
Click to expand...

Yes, I agree the title is more suited to a tabloid.

He does describe the difficulties of preloading bolts properly in a way that I had not thought of before.

The question that seems to be unanswered is how often does a cylinder replacement cause the problems he's describing. I doubt that question is being effectively tracked, plus the harm he's describing may not show up immediately making any correlation to a cylinder change difficult.

The article does make one think. I haven't heard anyone dispute the main premiss that torquing the bolts to the proper preload is very difficult and almost impossible to tell if one was successful.

Searching the NTSB aviation database, one finds plenty of instances of engine stoppage due to bearing failures.

I find dismissing Mike Busch's observations, just because I don't like the implications, hard to do without a well reasoned counter argument.
 
mfleming said:
The question that seems to be unanswered is how often does a cylinder replacement cause the problems he's describing. I doubt that question is being effectively tracked, plus the harm he's describing may not show up immediately making any correlation to a cylinder change difficult.
Click to expand...

Pretty sure at the time Mike wrote that article, he was arguing with the FAA that a particular cylinder replacement AD was unnecessary, and he was playing the risk card.

I haven't heard anyone dispute the main premiss that torquing the bolts to the proper preload is very difficult and almost impossible to tell if one was successful.

I find dismissing Mike Busch's observations, just because I don't like the implications, hard to do without a well reasoned counter argument.
Click to expand...

Ok, I dispute that torquing to proper preload is "very difficult". Pretty sure Bob is with me. Better? As for well reasoned counter argument, Mike's premise is merely that it is easy to screw up the preload process. I agree with that opinion. It's easy to screw up almost anything aeronautical.
 
DanH said:
Pretty sure at the time Mike wrote that article, he was arguing with the FAA that a particular cylinder replacement AD was unnecessary, and he was playing the risk card.

Ok, I dispute that torquing to proper preload is "very difficult". Pretty sure Bob is with me. Better? As for well reasoned counter argument, Mike's premise is merely that it is easy to screw up the preload process. I agree with that opinion. It's easy to screw up almost anything aeronautical.
Click to expand...

Is there any difference in the designs that makes a Continental easier or more difficult than a Lycoming in this regard? I ask because IMO Mike tends to be more Continental centric in his writing and that make was the subject of the aforementioned AD.
 
Continentals are slightly more difficult to change cylinders on due to the pushrods being underneath and spring loaded (on larger displacement versions.) Other than that really no difference.

I just changed a cylinder on a Continental TSIO-520 on an AgWagon that gets flown so much the oil is changed every few days. Turbocharged and operated full-throttle all the time, slow, hot and heavy, or off to land. The engine has 1700+ hours since overhaul and hardly leaks. Over its life its had a number of cylinders off and on.

Hmmm.
 
Cylinder, Main bearing , WOW!

Anyone who has torn down a split case engine understands that once the case is bolted together and the dowels seat it is not going to separate enough to allow bearings to slip out of place, even with cylinders off. If it did the case would be so badly warped it shouldn't be flown.

Taking a cylinder off or even all 4 has never been an issue. Like Bob, I have rebuilt dozens of Lycoming engines and replaced countless cylinders over the years, NEVER, NEVER have I worried about a bearing being dislodged. As a matter of fact I have never had a bearing that I didn't have to tap out with a rubber mallet to get it free. I think the author of the article is a bit over dramatizing what COULD happen.
 
I agree with all of the comments here. You should not be afraid of replacing cylinders on our Lycoming engines. Just do them properly and under some supervision if you haven't done it before. Remember, these really are ancient engines, and they really aren't too hard to work on.

That being said, Continentals are quite different, especially the big-bore ones, of which Mike has a lot more experience to reference his comments. They are harder to change, but still can be done. Just be cautious.

And I do agree with not removing the engine. There's a lot more chance of something not being connected properly when messing with the whole front end as opposed to just one cylinder, or even all of them.

If you are curious, go down to the local airport, especially if there is a flight school operation. Chances are you can see an engine with a cylinder or 2 off. If not, ask the local mechanic when he/she might be replacing one and if you can come by for a look.

Vic
 
Cylinders

I too have replaced several hundred cylinders and built a couple dozen engines. Don?t believe you could dislodge a main bearing with the case halves clamped together even if you wanted to. I would rather not disturb the crankcase clamping torque on a well built engine, but cylinders need to be changed sometimes. Cylinder hold down torque is pretty straight forward, on some of the Lyc?s with thru bolts the opposite side of the engine gets torqued as well.

Don Broussard A&P, IA ect
RV9 Rebuild in Progress
57 Pacer
 
In 1984 one cylinder was remove from my Bo's IO-520 to repair a broken exhaust stud, about 1300 hours on the engine. This was done by a 25 year old A&P. At 1950 hours on the engine that same cylinder departed the crankcase, the only cylinder ever removed from that engine. Just a few years ago I had some dealings with that very same mechanic. He had the attitude, that feels tight enough. There are many competent mechanics out there. An some not so. In the end I suspect a lot of the cylinder malfeasance is due to not paying attention to details.
 
I?d be willing to bet that the guys on vans airforce are not the target audience for this article. Your average pilot of a certified would never even think of removing a cylinder themselves and would rely on the local A&P . Let?s face it most people here do things themselves, because if they brought it to someone else it probably wouldn?t get done to their liking, 😂


I remember an old merc outboard mechanic watching a guy pick up a set of needle bearings with a magnet just cringing. Everything has its idiosyncrasies the difference is the guys here do research ? before ? they do anything.

Now maybe, I read a different article but the one I read stated most of his info comes from continentals. Right off the bat.

It also said that many mechanics don?t use wet torque, went into detail on differences of frictional losses, most don?t use all new nuts and bolts with perfect plating. (Having built car race engines you always use new bolts And nuts on reciprocating parts and fasteners.) the cheapest insurance you can buy.
He then goes on to say even if the mechanic THINKS they did it right that the MFGs own manuals say nothing of replacing the bolts,nuts etc, toque to click in one continuous motion, or disassemble / remove the through bolts to replace/ retorque the bottom nuts properly. So even the best intentioned likely will not get proper preload even if they try to do it 100% right.

I found the article to be a real eye opener and from what I?ve seen for work from A&Ps I doubt most A&Ps are doing this correctly, I?d bet 9 out of 10 get the clamping force wrong. Heck I?d bet half the over haul shops get it wrong, 😂


Many mechanics I see when in the field, (construction equip, race cars etc) when they get a click stop then re click with a slight move even after the click, that?s probably saving them. I always thought it was a bad practice, but now I?m not so sure.
 
Torque

How many hundreds of times on this forum have we heard the advice-get an A&P to help with that engine or with a pre buy. Not all A&P's are created equal and there are far to many that are not experienced enough or trained properly to remove and replace a cylinder. I would surmise that there are also too many that do not have access to a quality, properly calibrated torque wrench.
 
I can say from years of working in Q.C. and having my A&P rating, all of my measuring tools and any shop measuring tools are calibrated at a regular intervals. Period. A torque wrench is a precision measuring device, so......If people are not calibrating them and keeping certs on file I don't see how they could pass an audit by the FAA or any customer. Ask your mechanic if he has calibration schedules and certs for calibrated tools. Ask if the IA that stamps it off has any idea of the calibration standards.

They should.

I don't see how they could look at themselves in the mirror after assembling an engine and not knowing, for sure, that every fastener that was supposed to be torqued, was in fact torqued properly.

That said, there are many "mechanics" in the world that are really just parts changers. That goes for all fields that have mechanics. Heavy equipment, automotive, aircraft, it's all the same. A good mechanic measures, installs, and tests using the proper practices and techniques. Wet torque is not a new thing to anyone that builds engines. Whether it is a diesel engine from Caterpillar, or a standard Lycoming.

Are there guys out there that are missing regular torque wrench calibration? Do they not take extra care of precision tools? Do some do improper torque technique?

I am sure of it. I personally don't know of one, but I am sure they are out there.

I can honestly say I am not the least bit afraid to R&R any or all cylinders without splitting the case. If you use proper technique and procedure, there is no reason to fear this. But. Is YOUR mechanic using proper technique and procedures?

And new fasteners on critical components and where applicable should not even be a question.
 
This was beat to death once before.

http://www.vansairforce.com/community/showthread.php?t=159309&highlight=cylinder+replacement

I appear to be the only one that says a lot of caution must be used if more than one cylinder is removed from an engine. I have seen one four cylinder Lycoming engine that did NOT have a warped case and NO case fretting have one dowel pin rotate in the case. I believe that the bearing rotated when one or more cylinders was off the engine and the crankshaft rotated. Torquing the replacement cylinders pushed the bearing down on the dowel pin pushing it to one side.

I am the first one in this thread to agree with Mike Bush's warning about pulling a cylinder being potential harmful.

BTW, said engine that had the dowel pin rotate ended up with a crack in the case ~800 hours after 4-cylinders replaced. I am of the option that the 2,200 hour engine should have had a complete disassembly instead of just four cylinders replaced.

I respectfully disagree with all the experts saying pulling a cylinder cannot cause any damage. Pulling a cylinder or all cylinders can and have been done successfully but I caution anyone that does not have experience that there can be issues. I have seen the issues caused on one engine. IS the next engine that this issue is going to be yours when you pull the cylinders?
 
This is crazy, I'd venture to say that easily 90% of engines have top ends replaced prior to TBO. If replacing cylinders routinely caused engine failure the FAA would be all over this and we'd have aircraft engines failing daily.
 
RV6_flyer said:
I am the first one in this thread to agree with Mike Bush's warning about pulling a cylinder being potential harmful.
Click to expand...

The other thread dealt with crankshaft rotation while the case is unclamped, and its potential for bearing shift. Some say the chance of bearing shift is very limited, others don't, including Lycoming. Not the same discussion at all.

Regarding how to obtain sufficient stud tension upon reassembly, we all agree there is a potential for harm. Mike takes it a bit further, by arguing it is very difficult to obtain accurate stud preload, even for a diligent mechanic. It's not a silly argument. The mechanics who do the work successfully have a grasp of the two key underlying physical issues.

(1) If stud tension due to preload is higher than tension applied by the anticipated load, the studs are not subject to stress cycles. Cycles are key to fatigue.

(2) Static friction is always higher than dynamic friction. Put another way, it requires more torque to start the nut rotation from rest than it does to keep it moving. It is why final torque is to be applied in one smooth rotation, without stop and start.
 
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From https://mooneyspace.com [hearsay]

Lycoming said they had put considerable research into this and could say that much of the cracking was due to improper torquing. They said one of the contributing factors was removing more than one cylinder and then retorquing them using the single cylinder torque sequence instead of the multi cylinder sequence. They said the multi cylinder sequence was developed by mounting strain sensors in areas prone to crack and developing a sequence that minimized strain in these areas.
Click to expand...
 
isosceles said:
https://www.aopa.org/news-and-media/all-news/2000/august/pilot/airframe-and-powerplant-(8)
Click to expand...

This article attributes the cracking associated with cylinder removal to improper torquing of the through bolts and studs upon reinstallation. Maybe Mike's article should have centered around finding a competent mechanic willing to follow proper torquing procedures instead of advocating a full overhaul, which can also be subject to cracking due to improper torquing of studs. I still struggle to see how field replacement makes it any harder to properly torque the studs, beyond the added difficulty of using torque plates in an installed engine.

Larry
 
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DanH said:
The other thread dealt with crankshaft rotation while the case is unclamped, and its potential for bearing shift. Some say the chance of bearing shift is very limited, others don't, including Lycoming. Not the same discussion at all.

Regarding how to obtain sufficient stud tension upon reassembly, we all agree there is a potential for harm. Mike takes it a bit further, by arguing it is very difficult to obtain accurate stud preload, even for a diligent mechanic. It's not a silly argument. The mechanics who do the work successfully have a grasp of the two key underlying physical issues.

(1) If stud tension due to preload is higher than tension applied by the anticipated load, the studs are not subject to stress cycles. Cycles are key to fatigue.

(2) Static friction is always higher than dynamic friction. Put another way, it requires more torque to start the nut rotation from rest than it does to keep it moving. It is why final torque is to be applied in one smooth rotation, without stop and start.
Click to expand...

And to add, if not lubricated well, the applied stretch can vary by 20% plus or minus due to thread, nut and washer friction. Hardened washers are required.

Development of (production) torquing methods (and calibration) by my former employer went on for years. They made 50,000 engines a year with dozens of head bolts in each engine.
 
DanH said:
SNIP

(1) If stud tension due to preload is higher than tension applied by the anticipated load, the studs are not subject to stress cycles. Cycles are key to fatigue.

SNIP
Click to expand...

Not quite true - the bolt will still see cyclic loading, but the magnitude of fluctuation will only be a small percentage of the externally applied cyclic load in a properly designed joint. The amount depends on the relative stiffnesses of the bolt(s) to the members being clamped, as well as the tightening torque applied. Additionally, it depends on precisely where the external cyclic load is acting. As Bill noted in his post, this is not a simple topic, but it is a very important one.
 
I appreciate the discussion among some of the very well qualified folks on VAF. It is a shame that someone like Bush with all that experience can’t stay away from the sensationalism and provide useful, helpful, articles instead of chicken little sky is falling all the time. His writing polarizes people and many good points get lost, or are never found.
I have some basic questions unrelated to the technical discussions/arguments that are playing out here.

Many of us are flying with replaced jugs. Some had the work done by a “qualified” mechanic. Some did it themselves. If you believe Bush, we are all in peril. How do we know?

What symptoms should us average humans be looking for? Will we see metal in the filter before the bearings go? Will the bolts just bust and cylinder go flying?

If you replaced a cylinder and have concerns, what can you do about it?

I think this is more important to most of us than having a full understanding of the technical aspects, although it is interesting, and I am always impressed with the VAF brain trust.
 
Sometimes you will catch a single missing fastener before the whole mess departs. If you really did have a ham fisted mechanic that completely ignores the basics of maintenance (as Bush seems to indicate is so common), then its unlikely he has all the fasteners equal. One will likely show signs of distress before the others. If you are really worried about it, then an open cowl preflight is required. Study each fastener carefully and look for missing paint, evidence of relative movement, and of course missing fasteners.
 
JonJay said:
I appreciate the discussion among some of the very well qualified folks on VAF. It is a shame that someone like Bush with all that experience can’t stay away from the sensationalism and provide useful, helpful, articles instead of chicken little sky is falling all the time. His writing polarizes people and many good points get lost, or are never found.
I have some basic questions unrelated to the technical discussions/arguments that are playing out here.

Many of us are flying with replaced jugs. Some had the work done by a “qualified” mechanic. Some did it themselves. If you believe Bush, we are all in peril. How do we know?

What symptoms should us average humans be looking for? Will we see metal in the filter before the bearings go? Will the bolts just bust and cylinder go flying?

If you replaced a cylinder and have concerns, what can you do about it?

I think this is more important to most of us than having a full understanding of the technical aspects, although it is interesting, and I am always impressed with the VAF brain trust.
Click to expand...

I thought the AOPA article, referenced a few posts above, does a good job outlining the risks. There is no mention of spun bearings, but a lot of discussion around case cracking due to improper through stud torque. Unfortunately most of this isn't seen externally (often not visible without zyglo) and also is often not likely to create serious problems. I spoke at length with the Divco folks when I sent my 540 case in a couple of months ago. They indicated that it is pretty uncommon for a case to come in for overhaul without some cracking around the main journal webs. If you are looking to stay ahead of problems stemming from this, close examination of the case for cracking would be prudent. While not an expert in this area, I would deduce that once cracking in the web area extends to the case shell area and therefore externally visible, the problem is quite severe and action is necessary.

Larry
 
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Toobuilder said:
. If you are really worried about it, then an open cowl preflight is required. Study each fastener carefully and look for missing paint, evidence of relative movement, and of course missing fasteners.
Click to expand...

Thanks Michael and Larry. No, I am not really worried about it at all. I am in Walt's camp here and don't believe this issue has caused "airplanes to fall out of the sky" as Mr. Bush would like to scare people into thinking. Horrible article, not for the technical merits presented, but for the title, hype, and unnecessary play on fears to get folks to read your rag....
Reminds me of the "red box" articles he wrote. Good technical data, but nobody blew up by accidently venturing into the "box". I spoke with an engineer at Osh, one of Mr. Bush's clients and the arguable leader in the whole balanced injector thing, and he stated he seriously doubted one could detonate a typical small bore Lycoming even if they tried. No excuse not to exercise caution, but again, Bush's hype gets in the way of what should be sound technical discussions. That is a shame.

Larry, I thought the article was good, but focused again on big bore Continentals, not our small bore Lyc's. Not that it wasn't good info or the same principles apply. Always interested in learning.

I have been diligent in my CI's looking over the case and all hardware and will continue to do so.
 
I think Mr.Bush lost a lot of credibility with this one. As a USAF trained jet engine troop and an A&P, I certainly know about the correct use and care of torque wrenches. Settings, certifications, operation and even hand placement were routinely evaluated by QA for my team's continued certification. Screw it up and you are de certified and run through school again. I do cringe when I see the "technicians" at the tire shop jerk the lug nuts tight and then drop the torque wrench on the floor, but torque wrench care and feeding is A&P 101 as far as I'm concerned.
 
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