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Well, I pulled the cylinder - here are some pics

walkman

walkman

Well Known Member
I ended up pulling the cylinder. It seemed to be leaking past the rings. After I got it off I took a bunch of pictures. Here

The valves and seats aren't great but don't appear to be too horrible. The cylinder walls had hone marks still (900 hour engine) but what appears to be some light scuffing in one area. The rings don't look too bad, are intact, and move freely in their lands. They aren't coked up except the oil control ring is a little. They aren't very sharp any more, but its a 900 hour engine. No sign of spalling on the cam lobes.

However I found some things on the piston I didn't like. First, the wrist pin won't budge. Piston rotates just fine, but it ain't coming out. Need to figure out how to persuade it without putting side load on the bearing. The biggest issue with the piston is some heavy scoring around about 1/5 of the circumference. Its very rough to the fingers, you can feel the grooves. They are pretty deep. Possible signs of uneven heating on the piston skirt.

Last time I pulled a jug it was an O-320 (this is an XP-360) but I recall the exhaust valve having a convex face. This has a concave face. Am I mis-remembering?

Let me know what you think.
 
Oh no you shouldn't have!. The guy with the webinars says so! :p

Looks like it got hot, why I don't know, but stick a fork in the piston, its done.

The cylinders and valves look like they will clean up nicely if they're not scored too deeply. The seats and valves will get pitted like that when too much oil ends up in the combustion chamber. The oil cokes and sticks to the valve seats, and eventually they will erode.

I have a tool to remove the piston pin, email me your address and I will let you borrow it. Stuck pins are not out of the ordinary.
 
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rocketbob said:
Oh no you shouldn't have!. The guy with the webinars says so! :p

Looks like it got hot, why I don't know, but stick a fork in the piston, its done.
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I have to agree with Bob. When I saw the picture of the piston (piston pin view) my first thought was detonation. Always hard on pistons.
 
walkman said:
The biggest issue with the piston is some heavy scoring around about 1/5 of the circumference. Its very rough to the fingers, you can feel the grooves. They are pretty deep. Possible signs of uneven heating on the piston skirt.
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You're saying the other skirt (facing down, unseen in the photos) is not scored?
 
Its normal to see some scuffing but not up to the ring lands like that. Its an area I think definitely benefits from a piston coating like ECI puts on their pistons.
 
Sid Lambert said:
Mahlon would say that scoring on the piston is normal. Mine was worse and he said it looked normal.
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I have seen thousands of pistons from many different engine types and I have a hard time believing this is "normal" for a properly built and operating engine.

I have seen plenty of 100 hr Conti test pistons and they have a nice oval pattern and some polishing, but this is grinding - that blackness in the oil is aluminum dust.

I would pull all the jugs if it were mine.

To the issue, this looks like grinding wear, so one thing to know is the original surface finish of the fresh hones cylinder and ensure it was cleared and cleaned of it's abrasive material before assembly.

It probably generated it's own heat, as heat related scuffing generally has torn metal (AL) on the skirt as the elevated temps resulted in soft material. The piston crown did not look like there was any detonation damage, but it is hard to tell without seeing them in good light.

If my engine looks like this at 900 hrs I will be jumping up and down on someones desk at Lycoming, but that is just me. YMMV

edit - after going back and looking closer at the piston skirt and at the bottom of the stroke cylinder wall, there is micro-metal transfer from the piston to the cylinder. This would definitely be temperature related, but still hard to say it was the root cause. It could just be a result of the higher friction pushing the temps up from another cause. If it was just ignition timing, then experience says it would be greater scuffing and larger metal transfer spots than exhibited. So, the micro scuffing is likely a precipitated result, not the root cause.
 
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I am just pointing out what the experts says is normal. I realize it's the stuff I don't know that stands a better chance of killing me.

For what it's worth...

I rebuilt my entire engine after I had a broken piston. My A&P friend said just replace that piston, an engine shop said just to a top... I'm not one to halfass it so I rebuilt the entire engine. You are the one flying the engine. If it makes you uncomfortable, replace it.

http://www.vansairforce.com/community/showthread.php?t=106379
 
rocketbob said:
Oh no you shouldn't have!. The guy with the webinars says so! :p

Looks like it got hot, why I don't know, but stick a fork in the piston, its done.

The cylinders and valves look like they will clean up nicely if they're not scored too deeply. The seats and valves will get pitted like that when too much oil ends up in the combustion chamber. The oil cokes and sticks to the valve seats, and eventually they will erode.

I have a tool to remove the piston pin, email me your address and I will let you borrow it. Stuck pins are not out of the ordinary.
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ha! Yes, my thought on the piston. One additional data point I did not mention. I have all oil analysis since new. This is a 900 hour engine that has flown a regular 80-120 hours year since new. The oil analysis has been OK each time except for a slightly elevated aluminum number since new. That number has not spiked or really varied much, and was not outrageously high. I'm thinking this might be the cause. Is it possible the cylinder or piston were out of round, or is this more likely one cylinder got hot for some reason? CHT's have been fine.

I just ordered the $80 tool from Spruce, I'll have it tomorrow. Thanks for the offer.

DanH said:
You're saying the other skirt (facing down, unseen in the photos) is not scored?
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Dan, that is correct. The other side looks good.
 
BillL said:
I have seen thousands of pistons from many different engine types and I have a hard time believing this is "normal" for a properly built and operating engine.

I have seen plenty of 100 hr Conti test pistons and they have a nice oval pattern and some polishing, but this is grinding - that blackness in the oil is aluminum dust.

I would pull all the jugs if it were mine.

To the issue, this looks like grinding wear, so one thing to know is the original surface finish of the fresh hones cylinder and ensure it was cleared and cleaned of it's abrasive material before assembly.

It probably generated it's own heat, as heat related scuffing generally has torn metal (AL) on the skirt as the elevated temps resulted in soft material. The piston crown did not look like there was any detonation damage, but it is hard to tell without seeing them in good light.

If my engine looks like this at 900 hrs I will be jumping up and down on someones desk at Lycoming, but that is just me. YMMV
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Thanks for the input Bill. All the other cylinders are at 78 or 79/80. They've actually come up 1 or 2 psi since I purchased in Feb. This cylinder was at 76/80 when I purchased, and has plummeted. I'm disinclined to pull the others based on those numbers. thoughts?
 
walkman said:
Is it possible the cylinder or piston were out of round, or is this more likely one cylinder got hot for some reason? CHT's have been fine.

Dan, that is correct. The other side looks good.
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Pistons are machined out of round...they're actually oval by a few thousands. I don't see how a cylinder can be honed out of round.

Wondering if you had preignition. Sure looks like it by the appearance of the piston crown. What kind of ignition?
 
rocketbob said:
Pistons are machined out of round...they're actually oval by a few thousands. I don't see how a cylinder can be honed out of round.

Wondering if you had preignition. Sure looks like it by the appearance of the piston crown. What kind of ignition?
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It had two Slick mags on it when purchased. Both impulse coupled. I removed the left one in the spring and installed an EFII ignition. I left the right mag, and checked its timing. Its dead nuts on 25 BTDC.

The poor idle problem had come on slowly through the year, did not *seem* to come on with the addition of the EFII setup. In fact, as you might recall, there was some discussion about vapor lock as the year warmed up.
 
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walkman said:
Thanks for the input Bill. All the other cylinders are at 78 or 79/80. They've actually come up 1 or 2 psi since I purchased in Feb. This cylinder was at 76/80 when I purchased, and has plummeted. I'm disinclined to pull the others based on those numbers. thoughts?
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If it were me - - -

Anyway, the reasoning is that if it was a systemic root cause that precipitated this failure, then it may also me occurring on the other cylinders. They won't all fail at one time even - - one has to be the "winner" to the end point. It "might" be better to find it out now, rather than one at a time. It is ripped apart now, so how much additional time is it? If they are all ok, then new gaskets is the minimum investment. I am assuming that it is a minimal risk to reinstall the jugs and not break rings.

RocketBob's view is probably more important as he is current on THIS exact, specific, type of engine.
 
rocketbob said:
Pistons are machined out of round...they're actually oval by a few thousands. I don't see how a cylinder can be honed out of round.

Wondering if you had preignition. Sure looks like it by the appearance of the piston crown. What kind of ignition?
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Bob

Do you mean detonation or pre-ignition? Scuffing at the skirt is a common sign of detonation in high output auto engines. This scuffing on my piston is not symmetric though, only in that one place.
 
walkman said:
Bob

Do you mean detonation or pre-ignition? Scuffing at the skirt is a common sign of detonation in high output auto engines. This scuffing on my piston is not symmetric though, only in that one place.
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You're right its probably detonation as I get the two mixed up easily.
 
LettersFromFlyoverCountry said:
I'm simply in awe of the expertise on these matters.

So, what caused it?
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That is the $64,000 question. Or at least, the $25,000 question.

If the scuffing was symmetric, that is on the "4 corners" of the piston skirt, or on opposing sides of the piston skirt, that is a pretty common symptom of detonation. At least in auto engines it is.

However, this scoring is only on the top of the skirt. There is one very ugly alternative. That is material "raining down" on the skirt from the cam lobes/lifter faces. That means a rebuild/new engine.

I only did a very cursory examination of the cam lobes last night. I'm going to pull the opposing cylinder and turn the engine through and get a really good look at the lobes and lifter faces over the next couple of days.
 
BillL said:
If it were me - - -

Anyway, the reasoning is that if it was a systemic root cause that precipitated this failure, then it may also me occurring on the other cylinders. They won't all fail at one time even - - one has to be the "winner" to the end point. It "might" be better to find it out now, rather than one at a time. It is ripped apart now, so how much additional time is it? If they are all ok, then new gaskets is the minimum investment. I am assuming that it is a minimal risk to reinstall the jugs and not break rings.

RocketBob's view is probably more important as he is current on THIS exact, specific, type of engine.
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Actually no, a lot of times its the hottest cylinder(s) that tend to have issues and the rest can be fine. These engines are simple and robust and give the operator plenty of warning if something's wrong as long as attention is being paid and in this case it is.
 
walkman said:
However, this scoring is only on the top of the skirt. There is one very ugly alternative. That is material "raining down" on the skirt from the cam lobes/lifter faces. That means a rebuild/new engine.
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Nah you would see trash in your oil filter if this were the case.
 
rocketbob said:
Nah you would see trash in your oil filter if this were the case.
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I'm clinging to that hope, Bob!

I would also have expected to see iron way out of parameters on the oil analysis, neither of which was the case in the spring when I did it or any of the others noted in the log books.

However, I will also pull an oil sample and pull the filter and examine both.

I love a good detective story.....
 
DanH said:
Lubrication failure. That's the thrust side, and the dry side for 1 and 3.
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If it was a lubrication failure it likely happened when the first start occurred after the engine was assembled, or from starts with low oil pressure. #4 gets the least amount of oil flow. Might check into oil pressure and for this reason why I like high oil pressure.

This is why Lycoming recommends running 1000 rpm after a start so there is adequate splash and oil pressure.
 
I had similar scuffing on an O-360 from FOD ingestion. Turns out the air box was warped excessively and the filter was not sealing on one side. Dirt/sand was ingested and scored the pistons, some were scored on all surfaces, some only on the lower surfaces of the piston and bore.

If you want to get a good look at the cam you need to pull another cylinder from the same side, that way you can see all of the lobes and tappet faces. Pulling the opposite cylinder does not give any additional access to the cam for inspection.

I also agree with the previous advice of pulling all of the cylinders to inspect the pistons and bores. If they are OK you only spent a few dollars each on seals for the peace of mind to know there are no issues. If the other cylinders need new pistons and rings as well, at least you can do it now instead of taking the engine apart multiple times for wear issues that may show up in the near future.
 
On engines without piston oil spray nozzles (most non-turbo engines) Oil pressure itself dues not lubricate the cylinder walls. The cylinders and pistons rely on windage (splashing) for lubrication which occurs as soon as there is any oil volume spilling from the crank and rod bearings. During start there is usually sufficient oil film in the bores to prevent dry scuffing.
 
"Dry side" meaning 12 o'clock on the piston...the top of the barrel. Thrust side is 6 o'clock in cyls 2 and 4, always wet due to pooled oil.
 
The RV-6 I fly had some similar cylinder problems last year, but no piston scuffing. I reconditioned #2 and 4 (hone, new rings, new pistons, recut the valve seats) but due to the amount of pitting in the barrel around the Reiff preheater bands I recommended only turning on the preheat for an hour or two before flight. I believe a root cause of the problems we had were a result of the turning on the Reiff band heaters for extended periods which heated the cylinders hot enough where the oil became a very thin film and essentially washed off and pooled at the bottom. When the engine started there was little to no oil lubricating most the cylinder bores and so they wore badly even though they were low time since new.
 
DanH said:
Lubrication failure. That's the thrust side, and the dry side for 1 and 3.
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This is #1

rocketbob said:
Or piston slap due to excessive piston/bore clearance.
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So, its either too much clearance, or too little clearance. :D

rocketbob said:
If it was a lubrication failure it likely happened when the first start occurred after the engine was assembled, or from starts with low oil pressure. #4 gets the least amount of oil flow. Might check into oil pressure and for this reason why I like high oil pressure.

This is why Lycoming recommends running 1000 rpm after a start so there is adequate splash and oil pressure.
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Oil pressure is right in the middle of the range. I almost always idle at 1,000-1,200 for scavenging/anti-fouling purposes.

Mike H said:
I had similar scuffing on an O-360 from FOD ingestion. Turns out the air box was warped excessively and the filter was not sealing on one side. Dirt/sand was ingested and scored the pistons, some were scored on all surfaces, some only on the lower surfaces of the piston and bore.
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This is interesting. How did the scuffing occur on the skirt of the piston? I would expect ingested FOD to scuff the bore more. I did notice, when I pulled the FAB off the other day, that the filter E3450 filter had shrunk and deformed, and there was evidence that sand/dirt might have been passing around the rubber seal. I replaced with my spare year old filter off the shelf.
 
Did I miss the answer?

The early posts suggested detonation - which can be brought on by pre-ignition.
Did you check the timing of the electronic ignition? You said the mag was okay, but I didn't see anything about the electronic one.
 
walkman said:
I'm clinging to that hope, Bob!

I would also have expected to see iron way out of parameters on the oil analysis, neither of which was the case in the spring when I did it or any of the others noted in the log books.

However, I will also pull an oil sample and pull the filter and examine both.

I love a good detective story.....
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I would agree with RBob on this one. The oil filter captures most of the large stuff and aluminum is soft so as not to affect the hard steel parts. If you can get a mirror to check the followers, that is typically where the failure begins. They begin as a large diameter spherical radius and wear to flat, then concave resulting in high contact stresses on the edge of the cam lobes, in addition to lack of rotation. A good cam lobe will show some oil varnish on the edge indicating good geometry.

Root cause? Hmm hard to pinpoint without knowing much more on history, fuels, oils, build information, etc.

Well, your aluminum in the oil should show a big drop:D
 
BobTurner said:
The early posts suggested detonation - which can be brought on by pre-ignition.
Did you check the timing of the electronic ignition? You said the mag was okay, but I didn't see anything about the electronic one.
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I think Bob's earlier post mentioned pre-ignition, I asked if he meant detonation.

In my knowledge, pre-ignition can be brought on by detonation, but detonation isn't brought on by pre-ignition. Please correct me if I'm wrong.

Pre-ignition would by definition be ignition before the spark plug fires. Possibly by a bit of carbon, dangling end of a helicoil, or even a very hot spark plug tip heated up by detonation, acting like a glow plug for example. The shock of detonation can destroy the protective boundary layer around the piston and cylinder walls etc allowing them to heat rapidly, which may lead to pre-ignition. But unless I'm forgetting something, not the other way around.

Engines can run for quite a long time detonating, which would happen AFTER the peak pressure at 14 ATDC, and is a short high pressure pulse and dissipating pressure. In fact some auto engines run in that mode a lot. Pre-ignition happens well before TDC, in fact is probably easiest when the pressure in the cylinder is low (and the charge less dense hence lower effective octane) right after the charge is introduced. In order words near BDC. This means that the charge is already burning as it is being compressed, resulting in an extremely high and increasing cylinder pressure rather than a dissipating cylinder pressure as in detonation. Pre-ignition is catastrophic pretty quickly.

I saw no signs of pre-ignition. That would include melted pistons, melted or deformed spark plugs, high and rapidly increasing CHTs. Detonation would have different symptoms including a shot peened, pitted, or swiss cheesed piston face, broken rings or lands especially the second land, broken valves, and symmetrical scuffing of the piston skirt. The typical engine ringing from the sudden sharp pressure spikes would be difficult to detect by ear in an aircraft engine I would suspect, except in a test cell with a stethoscope.

If you mean, too far advanced timing, no I didn't check the EI side - which would require running the engine and using a timing light, but I also didn't see anything to indicate it. My CHT's have been quite low in this engine actually. Rarely getting above 380 even on extended full power climb outs on 100+ degree days.

I'm happy to be told I'm wrong, if it means I'm learning something :)
 
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BillL said:
I would agree with RBob on this one. The oil filter captures most of the large stuff and aluminum is soft so as not to affect the hard steel parts. If you can get a mirror to check the followers, that is typically where the failure begins. They begin as a large diameter spherical radius and wear to flat, then concave resulting in high contact stresses on the edge of the cam lobes, in addition to lack of rotation. A good cam lobe will show some oil varnish on the edge indicating good geometry.

Root cause? Hmm hard to pinpoint without knowing much more on history, fuels, oils, build information, etc.

Well, your aluminum in the oil should show a big drop:D
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I will be pulling another cylinder and rotating the engine. I will check for any pitting of the cam follower faces or other stress. I will also check to see if I can feel any burr or deformation on the apex of the cam lobes. I'll try and post pictures, depending if I can rig up a suitable camera.
 
walkman said:
I think Bob's earlier post mentioned pre-ignition, I asked if he meant detonation.

In my knowledge, pre-ignition can be brought on by detonation, but detonation isn't brought on by pre-ignition. Please correct me if I'm wrong.

Pre-ignition would by definition be ignition before the spark plug fires. Possibly by a bit of carbon, dangling end of a helicoil, or even a very hot spark plug tip heated up by detonation, acting like a glow plug for example. The shock of detonation can destroy the protective boundary layer around the piston and cylinder walls etc allowing them to heat rapidly, which may lead to pre-ignition. But unless I'm forgetting something, not the other way around.

Engines can run for quite a long time detonating, which would happen AFTER the peak pressure at 14 ATDC, and is a short high pressure pulse and dissipating pressure. In fact some auto engines run in that mode a lot. Pre-ignition happens well before TDC, in fact is probably easiest when the pressure in the cylinder is low (and the charge less dense hence lower effective octane) right after the charge is introduced. In order words near BDC. This means that the charge is already burning as it is being compressed, resulting in an extremely high and increasing cylinder pressure rather than a dissipating cylinder pressure as in detonation. Pre-ignition is catastrophic pretty quickly.

I saw no signs of pre-ignition. That would include melted pistons, melted or deformed spark plugs, high and rapidly increasing CHTs. Detonation would have different symptoms including a shot peened, pitted, or swiss cheesed piston face, broken rings or lands especially the second land, broken valves, and symmetrical scuffing of the piston skirt. The typical engine ringing from the sudden sharp pressure spikes would be difficult to detect by ear in an aircraft engine I would suspect, except in a test cell with a stethoscope.

If you mean, too far advanced timing, no I didn't check the EI side - which would require running the engine and using a timing light, but I also didn't see anything to indicate it. My CHT's have been quite low in this engine actually. Rarely getting above 380 even on extended full power climb outs on 100+ degree days.

I'm happy to be told I'm wrong, if it means I'm learning something :)
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"Pre-ignition" means the fuel starts burning, but at its normal "slow" rate of burn, before the desired point in the cycle. The ignition source could be a too hot plug, glowing carbon, or mis-adjusted mag or electronic ignition. As you stated, the piston is now compressing hot, high pressure air/gas, causing temperatures and pressures to sky rocket. (note in normal operation this happens a bit, as typical ignition is 25 degrees before TDC).
"detonation" means a fuel-air mixture in a certain state which results in an extremely rapid burn ("explosion"). Often such fuel-air mixtures will spontaneously ignite, with no spark needed. Common sources of such mixtures are too low octane fuel, excessive pressures and temperatures.

So if you have detonation, it may cause plug tips or carbon bits to glow, resulting in pre-ignition. And if you have pre-ignition (from a too far advanced spark) the resulting high temperatures and pressures can lead to detonation further into the cycle.

Detonation comes in all forms. If the detonation happens very late in the cycle, there is little fuel left to burn, little harm. In cars this is called "mild knock". If the detonation comes closer to TDC it can do major damage, quickly. See the post "check your fuel tanks". (A plane was mis-fueled with Jet A, which lowers the octane. The engines lasted about a minute or two after takeoff. This is pretty typical for severe detonation).

My very un-trained eye was focussed on the cylinder head, which did look shot-peened to me, and missing all the usual deposits (maybe just my computer).
 
walkman said:
I will be pulling another cylinder and rotating the engine. I will check for any pitting of the cam follower faces or other stress. I will also check to see if I can feel any burr or deformation on the apex of the cam lobes. I'll try and post pictures, depending if I can rig up a suitable camera.
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Make certain that you re-torque all thru-studs before turning the prop; you don't want to rotate the journal bearings.
 
Reading this thread I cannot see any reference as to measuring the piston/cylinder. You have what I see as a classic case of piston slap and I suspect you or someone with a real good ear for these engines would have heard it at idle speeds and just above, the slap usually goes away at higher rpms. For some reason you lost lubrication on the thrust side of that piston at some time, heat, clearance, dry start whatever. Metal to metal contact occurred and the rest is history. I suspect if you measure the piston you will find it below limits for reuse. Ideally there should be little if any scoring on pistons even at high time and any reference to normal scoring I think should be just considered as yea I've seen a lot of that, there is no metal to metal contact in an engine that would lead to long life and be normal. I suspect your cylinder if it measures good could be honed and reused, the piston and rings are toast. The valves and seats do look as though they need a dressing maybe lapped with compound or recut but the overall color of the valves looks about rite. The rings have obviously been letting a little extra oil by and thus the sooty look of the head.
 
walkman said:
So, its either too much clearance, or too little clearance.
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Not necessarily a matter of clearances...just lack of sufficient oil film for the applied pressure.

As Mike said, there is usually enough oil film in the bores at start up to prevent dry scuffing. However, let's assume the thing sat in the hangar for a few weeks, or maybe months. The top of the bores get dry, and often pick up a little rust. Now drag it out and crank it up with a bit too much throttle applied. The first two or three combustion events in the right-hand cylinders push the pistons hard against the top of the bore, before splash lubrication can wet it again, and the rust starts chewing the aluminum.

Just an example of course.
 
Jerry Wilcox said:
Reading this thread I cannot see any reference as to measuring the piston/cylinder.
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That is something I would certainly do but most mechanics don't have the tools to do this job. Even with a bore gauge in hand its somewhat of an art to measure effectively with it. I always have a hard time with one because I don't use one day-to-day. So I invested in a three-prong bore micrometer and can measure down to tenths with that.
 
To remove the pins you have to heat up the pistons where the pins are and beat them out. A small propane torch works fine. It's common in non oil squirter engines for the pins to get locked in the pistons from carbon and heat. If you had detonation you'd see it on the piston face, in severe cases detonation will burn a hole(s) right through the top of the piston. Scuffing is usually caused by excessive heat, not enough piston clearance or lack of oil film. This may have happened many hours ago. It would be wise to remove all the cylinders and see what you have.
 
DanH said:
Not necessarily a matter of clearances...just lack of sufficient oil film for the applied pressure.

As Mike said, there is usually enough oil film in the bores at start up to prevent dry scuffing. However, let's assume the thing sat in the hangar for a few weeks, or maybe months. The top of the bores get dry, and often pick up a little rust. Now drag it out and crank it up with a bit too much throttle applied. The first two or three combustion events in the right-hand cylinders push the pistons hard against the top of the bore, before splash lubrication can wet it again, and the rust starts chewing the aluminum.

Just an example of course.
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And a good scenario to add to the list of possible causes/contributors.

Air cooled aircraft engines use a lot more piston/cylinder clearance than other engines. As much as 5X more, so clatter is part of the beast. I have never actually seen scuffing/galling due to too much clearance, typically just a structural issue and the skirts fracture.

The little voice in my head keeps asking about timing advance. But I tell him there is no evidence, so be quiet.
 
rocketbob said:
That is something I would certainly do but most mechanics don't have the tools to do this job. Even with a bore gauge in hand its somewhat of an art to measure effectively with it. I always have a hard time with one because I don't use one day-to-day. So I invested in a three-prong bore micrometer and can measure down to tenths with that.
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This is beyond my shade tree mechanic abilities :D

I'm good enough to do basic things and, hopefully, recognize when its beyond me.

Regarding an earlier post, I have no intention of attempting to reuse this piston. Best case scenario is a new replacement piston/rings and the cylinder sent out for reconditioning.

I'm pulling #3 (at least) tonight and will report back.
 
It's likely that not all readers follow the significance of scuffing on one skirt only, or the mechanics of piston side thrust.

Here's a little morning coffee sketch, with crank rotation as viewed from the pilot's seat. Crankshaft stroke has been exaggerated to make the mechanics more obvious:

dbjvys.jpg


Given combustion pressure, the piston is forced down the bore and sideways against the cylinder wall. The proportion of side thrust to desired thrust down the bore is a function of rod angle, which is constantly changing as the crank goes around. For the rod angle portrayed here, I've sketched in the proportion as vectors at the piston pin.

At normal ignition timing, the highest gas pressures are (hopefully) around 15 degrees ATDC. The drawing shows the crank at about 50 degrees ATDC. By that point in the power stroke, the combustion gas pressure is falling off rapidly. It's a good thing, as the proportion going to side thrust is rising with increasing rod angle.

Now a subtle detail. Ignition timing for start is retarded, so peak gas pressure happens later in the stroke during the first hit, or two or three. How many depends on quickly the engine reaches enough RPM to kick out the impulse coupler weights, or show an EI's brain the programmed RPM for advance.

Note the lubrication difference during start. The side thrust is against the top of cylinders 1&3, but against the bottom of 2&4. Tops tend to be dry, in particular after being parked for some extended period. Bottoms tend to be swimming in oil. Now think about which side is likely to have some rust flakes.

Oil pressure has nothing to do with any of this. The bore walls are splash lube, and there is very little of that during the initial stokes of a start. Nor, within reason, is it a clearance or piston temperature problem. If the piston expands (or the bore shrinks), it will show scuffing on both sides, and the scuffing will often shift toward the crown where the piston is hottest.

Yep, it's all theory. Theory is all we have; none of us were there to see inside the bore when it happened.
 
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FasGlas said:
To remove the pins you have to heat up the pistons where the pins are and beat them out. A small propane torch works fine. It's common in non oil squirter engines for the pins to get locked in the pistons from carbon and heat. If you had detonation you'd see it on the piston face, in severe cases detonation will burn a hole(s) right through the top of the piston. Scuffing is usually caused by excessive heat, not enough piston clearance or lack of oil film. This may have happened many hours ago. It would be wise to remove all the cylinders and see what you have.
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Yeah, I'm not going to do that. I really want to avoid putting any side load on the big end bearing. They are designed to take thrust, not side loads.

I have ordered a piston pin puller from spruce. If that doesn't work, I will build a tool to use a threaded rod to press the pin out.
 
DanH said:
It's likely that not all readers follow the significance of scuffing on one skirt only, or the mechanics of piston side thrust.

Here's a little morning coffee sketch, with crank rotation as viewed from the pilot's seat. Crankshaft stroke has been exaggerated to make the mechanics more obvious:
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I continue to be amazed by, and grateful for, not only the level of expertise on this site and in this community, but also the level of effort those individuals are willing to expend to share it with the rest of us.

I only hope that I can help expand the collective knowledge here, even if its by asking stupid questions that others can answer.
 
DanH said:
Oil pressure has nothing to do with any of this. The bore walls are splash lube, and there is very little of that during the initial stokes of a start.
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Disagree with you on this point. If there is more oil pressure particularly at low RPM there is going to be more oil escaping around the main and rod bearings, hence more oil to splash. The same goes for tappet bores and if set up with piston oil squirters. This is why some looseness is desired with main and rod bearings so that 1. oil can escape to splash and 2. so oil effectively cools the bearings.

Saw the following article a while back. There are asymmetric pistons available for Chevy LSx engines. http://www.enginelabs.com/engine-te...-je-pistons-asymmetrical-line-for-ls-engines/
 
rocketbob said:
Disagree with you on this point. If there is more oil pressure particularly at low RPM there is going to be more oil escaping around the main and rod bearings, hence more oil to splash. The same goes for tappet bores and if set up with piston oil squirters. This is why some looseness is desired with main and rod bearings so that 1. oil can escape to splash and 2. so oil effectively cools the bearings.

Saw the following article a while back. There are asymmetric pistons available for Chevy LSx engines. http://www.enginelabs.com/engine-te...-je-pistons-asymmetrical-line-for-ls-engines/
Click to expand...

With all the cylinders off is it possible to put in oil nozzles/squirters or any other mods like this?
 
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