Katamarino
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Could these have rotated in service to line up, or would they have been installed this way?
Piston ring gaps - can they rotate in service?
- Thread starter Katamarino
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Could these have rotated in service to line up, or would they have been installed this way?
Yes. To get all 3 lined up, provided they didn't move during removal, still possible but not as common in my experience. Why was it removed?
Katamarino
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Removed for low compression. 5 out of 6 exhaust valves found to be "like a cock in a sock", to quote the Ozzie mechanic.
Daleandee
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I would hope the mechanic did not install them that way. Yes, rings can and do rotate in service on four stroke engines. On some two stroke engines there is a pin in the ring groove to prevent rotation:
Checkout_my_Six
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they move....and likely were not installed that way.
Could these have rotated in service to line up, or would they have been installed this way?
Yes they rotate on the pistons, but...
I found the same thing on 3 out of 4 pistons on a 0-320 motor.
I talked with a long time trusted AP/IA and he said he has seen the same thing before. He said with no technical backing, just guessing that the combustion pressure could stop the rotating and hold the rings in that position??
He was just theorizing.
I spaced my rings apart and put it back together with the old rings and no honing and flew it another 100 hours. The leakdown improved and the oil consumption improved a lot. From 1 qt per 4 hrs to 1qt to 8-9 hrs and the motor ran great.
That's my story.
I have rebuilt a lot car and racing engines and have never noticed this issue until I took my 0-320 cyls off to check it out and the cam.
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nrpetersen
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As a MechEngr I have a hard time accepting they rotate in normal service, but I also know rotation has been proven and accepted by radioactive tracer analysis. I'm forced to accept rotation.
Dan Thomas
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Many years ago as I was shop foreman (and chief researcher, I suppose) in a plant that remanufactured air brake components including compressors. We had warranty complaints of oil-pumping in the compressors. The air brake compressor is engine-driven and runs all the time; it's compressing is controlled by a governor that senses tank pressure, and when the set pressure is reached it sends pressure to a couple of tiny pistons that lift the intake valves off their seats so that the compressor cannot compress anymore and the air just chuffs in and out. That's when oil-pumping becomes a problem. With no cylinder pressure, oil can seep up past the rings and into the chamber, and when the compressor goes back to work that oil is blown into the system, gumming everything up. It also cokes in the hot compressor discharge in the head and forms blockages.
I had built a compressor dyno to test every unit. I started a series of tests, running the compressor with the head off, and watching the pistons. I could see oil slowly accumulating on the piston heads. At 2500 RPM that piston is a blur everywhere except at top and bottom dead centers, but you can see what's happening.
Bendix used cast iron pistons in their compressors. Midland used aluminum, as did Cummins and Clayton. Bendix compressors were normally remanufactured with the cylinders resized and cleaned up and aftermarket aluminum pistons used in them. I found that the aluminum pistons pumped a lot more oil than the cast iron pistons. The major difference was that the cast piston had much less cold clearance than the aluminum, since aluminum expands at twice the rate of iron, and the block is cast iron and expands very little. The connecting rod's sideways thrust component, along with vibration, was able to make the aluminum piston slop sideways in the cylinder, and as it dragged the rings along it also lifted them clear of the cylinder wall on one side of the cylinder so that the oil ring did not clean the oil off on the downstroke, and on the upstroke the thrust was against that side of the cylinder so the oil was pushed upward. The cast iron piston had very little slop and didn't do this nearly as bad as the aluminum. When the compressor is not pumping it doesn't get any hotter than the coolant, so the aluminum piston shrinks and lets this happen. It has to be made smaller so that it doesn't seize when pumping of long periods.
The only solution at the time was to keep cylinder sizing to very close tolerances; none of this quick honing to re-establish a crosshatch like most shops did, and reuse the pistons. Nope. And I also found that bearing clearances had an awful lot to do with the pumping as well; looser bearing fits let more oil be thrown into the cylinders, flooding them and causing the rings to hydroplane over the oil. By regrinding most of the crankshafts and getting them within .0001" or .0002" of spec we got the oil- pumping way down. .0002" is one-twelfth the diameter of a human hair, or one-fifteenth the thickness of a sheet of paper.
I wrote a paper on this and took it to a heavy-duty brake association convention In St. Louis. Never heard anything from anybody, but within a few years Midland started using cast iron pistons. Huh.
So what? Well, the aluminum piston in an aircraft cylinder, when cooler, can slop back and forth, pulling and pushing on the rings, and that motion can also rotate them somewhat, based on the sideways thrust and engine vibration patterns. And the common practice of a quick honing and new rings doesn't help a lot. In fact, rebuilding an engine to the max serviceable limits, whether cylinders or bearings, is asking for oil consumption. It's a false economy unless you're willing to buy more oil and put up with more fouled sparkplugs. Some of these engines are bad enough already without doing that to them.
I should also mention that uneven heating of the cylinder causes it to become somewhat oval, and those rings will tend to rotate so that the ring ends are at or near the maximum diameter of the cylinder. It's their nature. You'll find the rings worn the most at their ends, where the pressure is the greatest. They're most comfortable at one end of the egg or the other.
I had built a compressor dyno to test every unit. I started a series of tests, running the compressor with the head off, and watching the pistons. I could see oil slowly accumulating on the piston heads. At 2500 RPM that piston is a blur everywhere except at top and bottom dead centers, but you can see what's happening.
Bendix used cast iron pistons in their compressors. Midland used aluminum, as did Cummins and Clayton. Bendix compressors were normally remanufactured with the cylinders resized and cleaned up and aftermarket aluminum pistons used in them. I found that the aluminum pistons pumped a lot more oil than the cast iron pistons. The major difference was that the cast piston had much less cold clearance than the aluminum, since aluminum expands at twice the rate of iron, and the block is cast iron and expands very little. The connecting rod's sideways thrust component, along with vibration, was able to make the aluminum piston slop sideways in the cylinder, and as it dragged the rings along it also lifted them clear of the cylinder wall on one side of the cylinder so that the oil ring did not clean the oil off on the downstroke, and on the upstroke the thrust was against that side of the cylinder so the oil was pushed upward. The cast iron piston had very little slop and didn't do this nearly as bad as the aluminum. When the compressor is not pumping it doesn't get any hotter than the coolant, so the aluminum piston shrinks and lets this happen. It has to be made smaller so that it doesn't seize when pumping of long periods.
The only solution at the time was to keep cylinder sizing to very close tolerances; none of this quick honing to re-establish a crosshatch like most shops did, and reuse the pistons. Nope. And I also found that bearing clearances had an awful lot to do with the pumping as well; looser bearing fits let more oil be thrown into the cylinders, flooding them and causing the rings to hydroplane over the oil. By regrinding most of the crankshafts and getting them within .0001" or .0002" of spec we got the oil- pumping way down. .0002" is one-twelfth the diameter of a human hair, or one-fifteenth the thickness of a sheet of paper.
I wrote a paper on this and took it to a heavy-duty brake association convention In St. Louis. Never heard anything from anybody, but within a few years Midland started using cast iron pistons. Huh.
So what? Well, the aluminum piston in an aircraft cylinder, when cooler, can slop back and forth, pulling and pushing on the rings, and that motion can also rotate them somewhat, based on the sideways thrust and engine vibration patterns. And the common practice of a quick honing and new rings doesn't help a lot. In fact, rebuilding an engine to the max serviceable limits, whether cylinders or bearings, is asking for oil consumption. It's a false economy unless you're willing to buy more oil and put up with more fouled sparkplugs. Some of these engines are bad enough already without doing that to them.
I should also mention that uneven heating of the cylinder causes it to become somewhat oval, and those rings will tend to rotate so that the ring ends are at or near the maximum diameter of the cylinder. It's their nature. You'll find the rings worn the most at their ends, where the pressure is the greatest. They're most comfortable at one end of the egg or the other.
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Short answer, yes they can rotate. Some engines and designs seem to be more notorious for it than others.
I wouldn’t install them that way but I don’t lose any sleep over things when I find them lined up.
I wouldn’t install them that way but I don’t lose any sleep over things when I find them lined up.
On some two stroke engines there is a pin in the ring groove to prevent rotation:
You have to understand why they pin the rings in a 2 stroke to stop rotation. If they were allowed to rotate, the end of the ring would end up in the area of a port and get hung up as it passes by. That would turn that cylinder into junk in one stroke of the piston.
There are also concerns with how much port area is allowed. Too much and the ring will bulge in the unsupported area, which will again turn the cylinder into junk with one stroke of the piston.
I bought a new international diesel truck years ago and the air compressor was pumping oil into the air system pretty much from the time it was new.
The dealer was instructed from the factory to install/weld a bung to the turbo outlet and take air from the turbo to pressurize the air compressor inlet.
I did not like that thinking it would take away some boost from the engine. I had no choice as it was the way they were told to fix the problem at the time. Did it work? I guess it did some what? The whole motor was trouble from the time it was new with it's hi pressure oil fired injectors which leak oil constantly somewhere. Those electronic DT-466 were terrible IMO. I hated them, at one point I had 6 of them. Got rid of my last one last spring thankfully. They are Junk IMO. The mechanical DT 466 are great engines and will run for millions of miles.
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Daleandee
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Yep ... I'm well aware of all of that. I remember a fella at our field that was doing his own two stroke rebuilding of a Hirth engine. Somehow he didn't know about the chamfer that was required at the intake/exhaust ports. When he started the engine he quickly realized something was wrong.
Dan Thomas
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That was a common fix. It didn't cost you much boost; the compressor's displacement volume is tiny compared to the engine's. That constant pressure keeps the oil from creeping past the rings.
But it caused another problem. The wrist pins were lubricated by oil thrown off the crankshaft. Only a few of the oldest compressors had a pressure feed via a drilling through the rod to the pin bearing. With the constant pressure on the piston, the pin was always against the bottom of the bearing, and no oil could get between the surfaces. It caused massive wear, especially in Midland compressors where they had the bright idea of using Delrin for pin bushings. Bronze bushings weren't available so I made them. Lots of them. A lot of those Delrin-bushed compressors blew up, especially those on engines with high boost pressures.
Some rods ran the pin right on the aluminum. Had to bore those and bush them.
We rebuilt around 17,000 compressors in my 12 years there. Left it to go into aviation full-time, where I found plenty of frustration at not being able to make better parts for some deficient airplane stuff. STC/PMA processes cost money and time that I didn't have.
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Are the rings all at the 12 O'Clock-ish position? (straight up or down, as seen from the side of the engine)
There are some potential reasons why that would happen, including out-of-roundness of the bore.
There are some potential reasons why that would happen, including out-of-roundness of the bore.
Mxfarm
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cough, cough.... wow, those type of tolerances, just measuring, much less machining is impressive.
Interesting stuff, thanks for sharing.
Snowmass
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How many hours on those cylinders? I would expect more rotation the newer the cylinders because as the cylinders wear out of round due to crankshaft induced side thrust the more the rings be "locked" in place.
Dan Thomas
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I had some really good dial gauging equipment. .0001" was a needle width on the gauge, and the gauges were large enough to display that. The machining of both cranks and cylinders was done by grinding, not by cutting. It's far harder to nail such tolerances on a lathe or boring bar. The crank grinder was an old 1950 Storm Vulcan model 10 much like this model 15:
It weighed around six thousand pounds. The dial gauge was on a mechanism that rode on the journal while you were grinding it, measuring it in real time. Carbide shoes on that thing. If you wanted to get really fussy on the tolerances, you could use an Arnold gauge like this one:
Those are .0001" divisions. A needle width there would be around .00001". Close enough?
The cylinder hone was a serious machine, too, a Sunnen CK-10:
The dial at the top of the head advanced at preset rates and it moved the honing stones outward as the metal was removed and the stones wore away. You soon figured out how much stone you'd lose on, say, an eight-thou cut and would dial the total in; the machine would stop when zero was reached. Most of the resize was done with coarse stones; they're quicker and wear less, then you'd finish the last couple of thou with finer stones. That machine took all the taper and out-of-roundness out of a cylinder. If you wanted a taper you moved the cylinder up or down so that the head spent more time at one end. The power meter on that gray head on the right measured motor load, and if there was taper it made the needle swing upward at the tight end of the cylinder. You learned to correlate that swing with the rocking movement of the machine's head as it worked the rotating honing head up and down in the cylinder, creating the crosshatch. If it was tight in the bottom, you raised the cylinder block a bit to grind it out, or you pushed the dwell button to stop the head at the bottom of its stroke for one cycle.
Nostalgic for me. I can still hear those machines even though I haven't run them in over 29 years.
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Checkout_my_Six
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Now that’s splitting hairs….lol
Katamarino
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260 since new.
Wow, that is a fresh engine. Was it just one cyl ?
My 320 motor had 1700 hrs on it and was 40 years old. Never overhauled.
Your rings gaps were staggered for most of the pistons life. If it was like that for 260 hours there would be plenty of carbon on the piston skirt.
Katamarino
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5 out of 6 cylinders were messed up in the same manner. CHTs were never run over 400 on the hottest, and the vast majority of the time were at or well below 380.
Clip4
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Some where I read that wear of the piston ring grooves allowed ring rotation and to rich mixtures and flooding caused the wear.
nrpetersen
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From wikipedia - Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. People who work in the field of tribology are referred to as tribologists.
The tribology of a boundary-layer splash-lubricated cast iron or chrome-faced piston ring running in a cast iron or steel bore with a fire on one end, a fool on the other, is incredible. Over 120 years of optimization is what has made the piston engine so hard to replace.
The tribology of a boundary-layer splash-lubricated cast iron or chrome-faced piston ring running in a cast iron or steel bore with a fire on one end, a fool on the other, is incredible. Over 120 years of optimization is what has made the piston engine so hard to replace.
Just to add an old round engine guy would say most of the rotation would happen during start up until engine reached operating temps then would be influenced on cylinder condition. He also said if the rings didnt move they would wear a ridge at the ring gap areas. Whether true dont know but always sounded plausible.
455 Bravo Uniform
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I think it’s all the globe-trotting you do with a single engine piston and the varying magnetic field of your position relative to the poles and that influence on the iron rings.
I remember an article a few years ago, I think it was in "hot rod", where they compared total seal rings (which have an overlapping end to make a "gapless" ring), staggering rings the traditional way, and intentionally lining up the gaps. They found no statistical significance, but the interesting part was when they took pistons back out the rings were no longer aligned the way they were during installation.
Tangentially related, Mike busch has a story about continental intentionally filing ring gaps over spec until the engine failed the compression check. It still made rated power. Point being the compression checks we do at annual are essentially worthless beyond their ability to detect a bad valve, and the ring's ability to rotate is part of that issue.
Tangentially related, Mike busch has a story about continental intentionally filing ring gaps over spec until the engine failed the compression check. It still made rated power. Point being the compression checks we do at annual are essentially worthless beyond their ability to detect a bad valve, and the ring's ability to rotate is part of that issue.
Katamarino
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Ironically, the shop that did the work is claiming too lean mixtures (i.e too high EGTs) have caused the problems; despite engine monitor data and written confirmation from Continental stating that this is nonsense.
To clarify, thats 260 TSN? Or 260 TSO? What type cylinders installed? What break in procedure was used? Was the initial engine run-in done on a test stand or in aircraft? If you remember what TT did the oil consumption stabilize and/or what TT was 1st oil change?
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Domenick
Got you beat. My previous O-320-D3G in a 1979 Warrior went 2924 hours. Likely just luck of the draw. It was a Lycoming factory reman from 1994. Nary a problem. When replaced, it had its original Lycoming cylinders. I'm hoping the current engine gives similar service.
Daleandee
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Can't say whether that is true or not but the reasoning sounds correct. I do think the gap areas would cause ridges in the cylinders if the rings were aligned & did not rotate. Then again I've not noticed this on two stroke rebuilds but those are usually done with only a few humdred hours on the engine. For example the Rotax 503 had a rebuild time of 300 hours.
Katamarino
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Since new. Superior Millennium cylinders on an O-470-50 engine. Superior are being much more helpful than the shop that did the work, and are taking the parts back for review (lifters and camshaft are badly pitted too, despite 260 hours in one year). Initial run in on a test stand (I think, I wasn't there). Oil consumption was stable pretty much from the beginning, first change was at 25 hours.
So its TSN on only the cylinders and not the whole engine? Are those steel cylinders? Was the Superior break-in procedure used?
I would find out more about this as it sets the rings.
This should not be associated with your new cylinders. Just a guess, but there was proabaly an existing issue with these at the time of your cylinder replacements.
Where was the location of the cylinder that did not fail?
Awesome run on your engine.
I ran mine to 1800 after I put the cylinders back on. I am sure it would have made it to TBO and well beyond.
I hope your current engine treats you just as well.
Katamarino
Pattern Altitude
The engine was indeed broken in strictly in line with the procedure. The camshaft and lifters were also new. Cylinder 6 was the one that did OK - it has a completely different serial number to the 5 that failed, which are all consecutive numbers.
What exactly was done to the engine and why? Why just cylinders, cam, and lifters changed with new? To get this much damage within that type of timeframe points more to a higher level issue whether operational or maintenance. At least in my opinion. Did you get any oil analysis done? Something just doesn't seem right or I'm missing some info.