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Blow-by
Blow-by is considered the phenomenon where combustion gases flow from the combustion
chamber past the ring pack to the crankcase. The combustion gases flow past the piston ring
at various locations: (a) at the piston ring gap, (b) past the front side of the piston ring at
starved lubrication conditions, (c) or past the backside of the piston ring when the ring is not
in contact with either of the ring-groove walls. The hot blow-by combustion gases cause the
piston and piston rings to overheat. The blow-by disturbs the piston and ring lubrication by
affecting the oil film: combustion gases contaminate the lubricant and cause the oil to entrain
in them. When the combustion gas reaches the crankcase it pollutes the lubrication oil.
Blow-by cannot be totally prevented as long as the rings have gaps and move in their grooves.
This means that some blow-by will always have to be allowed. The blow-by affects directly
or indirectly the engine power (fuel) efficiency: the blow-by consumes some of the combustion
power and increases the friction as a result of less favourable lubrication conditions.
The gap between the piston and liner wall is greater on the anti-thrust side of the piston
than on the thrust side. This requires that the gap between the back-side of the ring and the
ring groove is quite large and thus has a large gas-flow area. Measurements have shown that
the twist of the piston rings affects the amount of blowby past the ring pack. A negative twist
on the second ring can cause instability of the ring, which results in an increase in the blow-by.
A positive twist on the second ring can, in turn, cause high land pressure, which may result in
radial collapse or axial movement of the ring.
Ring wear
The most intensive wear of the piston ring pack and the cylinder liner normally occurs in the
running-in, or break-in, stage of the engine, during which the most predominant surface profile
peaks are worn off by the counter surface, and the surfaces eventually obtain improved
conformity. The abrasive wear of the piston rings decreases when the cylinder liner surface
irregularities become smaller, and the wear of the cylinder liner decreases when the piston
rings become smoother by running in. Consequently, under favourable conditions this
self-stabilising process leads to a decrease in the wear rate of both the piston rings and the
cylinder liner. According to experimental work presented by Henein and co-workers, the wear
rate of a cylinder liner is approximately 12 times higher during the first hour of operation than
during the subsequent two hours. Wear tests with neutron bombarded compression rings in fired
engine tests including a gamma ray spectrometer have shown that the piston ring wear rate
during the start-up period was up to 45 times the steady-state wear rate, and that approximately
84 % of the ring wear occurred during the first approximately 22 minutes of operation .
Ring groove wear
Wear of the parallel surfaces in piston ring grooves, commonly called ring-groove wear,
occurs mainly in the top ring groove. The main reason for the wear is the combined effect
of gas forces and radial motion of the ring, and the wear process is accelerated by poor
lubrication and a high temperature. The reasons for the radial motion of the ring are
the cylinder distortion, the secondary movement of the piston and piston tilt allowed by
the piston/cylinder clearance. Mass forces, friction forces, axial ring movement and ring
rotation increase the ring groove wear. In stationary gas pressure and gas blow-by may cause
radial vibrations in the ring, which accelerates the ring groove wear at the ring-groove contact
areas. As the result of the wear, the lower surface of the ring groove becomes rough and
rounded towards its edge, and the upper surface becomes rough. In addition to the deformation
of the groove arising from wear, the width of the groove increases, and the side clearance
between ring and groove increases. Alternatively to wear, the ring-groove tribosystem
may suffer from ring welding, as a consequence of overheating of the top ring or partial
seizure of the piston owing to poor lubrication.
View Full Version : What do worn piston rings look like?
Stephen Tashiro
What do piston rings look like when they are worn enough to let too much oil leak by, but not worn enough to disintegrate? If I looked down the side of a piston (say, using a borescope) could I see that the rings were bad?
The way I think of piston ring, as the rim wears off, there is less and less pressure forcing the ring against the sides of the cylinder. When the rings start leaking oil, can you actually see some daylight between the edge of the ring and the wall of the cylinder? Or does the leakage start a long time before there is any visual evidence?
Shawn Pixley
If you could see a gap, I doubt that the engine would run at all. The ring sits in a gallery in the piston. The gap in the ring will get larger with ring wear but you wouldn't see a gap around the cylinder. Long before that you would get so much piston slap your engine would be ruined. Your engine would sound like it is about ready to fly apart.
Testing rings is done using a compression tester.
Brett Robson
The rings you could see are the compression rings. The oil control rings are below the compression rings and wouldn't be visible without pulling the pistons.
Here's a picture, just because I happen to have some pistons lying about! The oil control rings are just above the wrist pin.
Mike Henderson
With today's oils, if you maintain your car as prescribed, it's unlikely you'll have significant ring and bore wear. My indication has always been to look at the top of the cylinder bore and see if there's a "lip" at the top - at the location where the the top ring of the piston reaches. If so, your rings may need to be changed, but you may also need to bore out the cylinders. But again, I say that with modern oils, worn out rings are unusual.
I'm old enough to remember when the bearings on the crankshaft would wear and you'd have to change the inserts (a "bottom end" job). That's unheard of now. Next, it was ring and cylinder wear - take out the pistons, hone or bore out the cylinders, and replace the rings and maybe the pistons (a "top end" job). Essentially unheard of now. Lastly, it was the valve guides that wore and oil would seep past the shaft of the intake valve (a valve job was needed about every 50K miles). Today, highly unusual in a well maintained engine.
Modern engines that are well maintained just go on and on.
Mike
Rick Potter
A carpenter friend of mine has a 10 year old Chevy pickup. It has over 300,000 miles on it, and the rings are still the originals. The accessories are going bad, at a regular rate, Alternators, starters, fuel pumps, etc, but the basic engine is sound. He maintains it regularly.
We used to hear a lot about how long Rolls Royce car lasted. I am sure they are really well built, but suspect regular mainenance has a lot to do with it. See above for a test case result.
Back to the question of what worn rings look like using a bore scope. Just like good ones, unless a top one is broken. The best way to check is a compression test and a leakdown test, neither of which is visual.
Rick Potter
Myk Rian
The way I think of piston ring, as the rim wears off, there is less and less pressure forcing the ring against the sides of the cylinder. When the rings start leaking oil, can you actually see some daylight between the edge of the ring and the wall of the cylinder? Or does the leakage start a long time before there is any visual evidence?
You do a compression test with a pressure gauge.
John Coloccia
You can actually still get to the pistons? I'm surprised engines aren't just welded shut these days.
Jeff Monson
The way I think of piston ring, as the rim wears off, there is less and less pressure forcing the ring against the sides of the cylinder. When the rings start leaking oil, can you actually see some daylight between the edge of the ring and the wall of the cylinder? Or does the leakage start a long time before there is any visual evidence?
If you look at the picture that Brett provided, the top 2 rings are your compression rings (responsible for creating compression), the bottom set of rings are the oil control rings (2 rings with a spacer inbetween) these are responsible for oil control. If you have an oil consumption issue, its most likely the oil control rings are stuck, they get full of carbon so they can no longer "float" around the piston. That results in oil getting to the compression rings, then the oil gets burnt instead of getting wiped down the cylinder and back to the oil pan. If you are having oil consumption issues, this can also be caused by the valve train (worn valve guides, or bad valve seals), or possibly a PCV system that is not working correctly either.
Kevin Bourque
You can do a simple compression test to determine if the rings are bad, or you can have a mechanic hook the car up to a diagnostic computer. Using a scope won't really tell you much at all.
Joe Angrisani
....Testing rings is done using a compression tester.
You do a compression test with a pressure gauge.
You can do a simple compression test to determine if the rings are bad.....
No no no. Please return to your shadetree and stop giving mechanical opinions for the sake of posting something.
Read Jeff's response. The compression rings have nothing to do with "letting too much oil leak by", as the OP asked. The oil rings have nothing to do with compression, as Myk and Kevin proclaim. Besides, I don't know a single mechanic who would waste time with a compression test. In all my years wrenching, I've NEVER done a compression test. We only did "leakdown tests", which not only show a problem, but shows whether the loss of compression is due to the rings, the intake valves, the exhaust valves, or the head gasket.
Joe Angrisani
What do piston rings look like when they are worn enough to let too much oil leak by, but not worn enough to disintegrate? If I looked down the side of a piston (say, using a borescope) could I see that the rings were bad?
The way I think of piston ring, as the rim wears off, there is less and less pressure forcing the ring against the sides of the cylinder. When the rings start leaking oil, can you actually see some daylight between the edge of the ring and the wall of the cylinder? Or does the leakage start a long time before there is any visual evidence?
Stephen....
What engine are we talking about? What are the symptoms? How fast is it consuming oil? Big stinky cloud on startup? Cloud only on hard acceleration? No cloud - oil just disappears from dipstick? Any oil on driveway? How's it run? What did the plugs look like when you pulled them?
Give us any and all observations.
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Stephen Tashiro
Stephen....
What engine are we talking about? What are the symptoms? How fast is it consuming oil? Big stinky cloud on startup? Cloud only on hard acceleration? No cloud - oil just disappears from dipstick? Any oil on driveway? How's it run? What did the plugs look like when you pulled them?
Give us any and all observations.
Let me emphasize that I'm interested in the general question of whether piston rings can be inspected visually, not in solving one particular engine problem.
An engine that is handy at the moment is a Toyota 4AFE on a friend's Corolla with about 200,000 miles on it. He hasn't changed the oil in a long time. We drained the oil and I think it would be interesting to look in the engine with a borescope. I want to look through the oil pan drain hole and check that the screen on the oil pump isn't clogged. I think it would be interesting to look at the other parts of the engine from below and (from the spark plug holes) above. The engine doesn't emit a cloud of smoke. The plugs have some dark carbon deposits on them. I'll know more after I look with the scope.
I have done both compression tests and leak down tests on various engines. (My friends engine has compression of about 150 psi in all cylinders, which is adequate. The spec is 191 psi for a new 4AFE and 142 psi minimum.) I'm simply curious about what one can tell from a visual inspection.
I think about things like putting a little oil in the sparkplug hole (as in a "wet" compression test), applying compressed air to the spark plug hole and looking at the sides of the cylinder from below with the scope to see how much oil blows by the rings. However, I'd have to do the same test on an engine known to be good to make sense such results.
Jeff Monson
Pretty tough to get an answer with a scope, or with a piston still in the block, if that is the question you are looking at getting answered. I've looked at plenty of pistons still in a block and all looks ok, when they are removed the answer is usually revealed. In the case of your friends Toyota, lack of oil changes, I'd put my money on stuck oil rings, I have seen plenty of them in my day. Its really an easy fix, I'd also bet there is very little to no cylinder wear.
Joe Angrisani
.....I'm interested in the general question of whether piston rings can be inspected visually....
No, they can't. Neither compression rings or oil rings.
Myk Rian
Let me emphasize that I'm interested in the general question of whether piston rings can be inspected visually, not in solving one particular engine problem.
Only if you remove the pistons from the engine.
Joe Angrisani
Only if you remove the pistons from the engine.
In post #6 you said you test rings with a compression test. Now you say you can test rings with your eyes. Please explain both comments.
Steve Meliza
In post #6 you said you test rings with a compression test. Now you say you can test rings with your eyes. Please explain both comments.
Actually, post #6 says "You do a compression test with a pressure gauge." without any claim that it would test the rings. It is a true statement, though of little help in answering the original question. Feel free to read me the riot act too, I'm a shade tree mechanic as well.
Joe Angrisani
Post #6 was not an open statement. It answered a part of the OP's statement that Myk himself chose to quote. I took it in that context since Myk picked and edited the quote, and his answer appeared after it.
I won't read you the "riot act" (my, so sensitive Steve) for being your own mechanic UNTIL you start proclaiming falsehoods as fact. There's a right way and a wrong way. Three amateurs stating something false does not make it true.
Steve Meliza
Today's professional automotive mechanics won't even try to diagnose a problem unless a computer code is set. That's a fact.
Kevin Bourque
Do a compression test...now I'm off to give legal advice in the Pennsylvania thread!
Jeff Monson
Today's professional automotive mechanics won't even try to diagnose a problem unless a computer code is set. That's a fact.
That's a fact quoted by who? Thats a very offensive statement to someone if the profession.
Steve Meliza
That's a fact quoted by who? Thats a very offensive statement to someone if the profession.
Try being in the position of having paid $200 for a "diagnoses" then being told that they don't know why the engine lacks power and blows smoke because the computer hasn't stored any trouble codes and looking for those codes was the extent of what I got for my money. That's pretty darn offensive to me.
The point of my statement was simply a defense of the shade tree mechanic/lawyers/woodworkers that offer advice with good intention. Sometimes our comments miss the mark for various reasons, but the truth usually floats to the top and the price paid for that help is simply the time it takes to sift through the various suggestions.
Scott T Smith
What do piston rings look like when they are worn enough to let too much oil leak by, but not worn enough to disintegrate? If I looked down the side of a piston (say, using a borescope) could I see that the rings were bad?
The way I think of piston ring, as the rim wears off, there is less and less pressure forcing the ring against the sides of the cylinder. When the rings start leaking oil, can you actually see some daylight between the edge of the ring and the wall of the cylinder? Or does the leakage start a long time before there is any visual evidence?
Stephen, many, many years ago I was a professional engine builder (primarily road race and circle track engines). To answer your question literally, yes - you can see wear visually on "piston rings" and there are some simple test to measure for wear. The wear is primarily visible in terms of reduced thickness of the compression rings (in excessive instances a cross section of the ring is wedge shaped), visible as scoring on the face of the compression rings, and measurable in terms of increased ring gap (measured at the bottom of the cylinder and it indicates surface wear and reduced tension on the compression rings.
Additional causes are scored pistons and cylinders (usually due to the engine being overheated) which results in compression loss. Also, the ring lands of the pistons can become excessively worn, allowing the rings to start pounding up and down in the lands and accelerating the wear.
Wear on oil rings is rarely visible. However oil rings can become stuck in their grooves, resulting in reduced ability to manage the oil film on the cylinders.
Having said that, the term "worn piston rings" is a misnomer. The symptoms that are associated with worn rings - ie reduced compression and increased oil consumption - are typically caused by worn cylinders (tapered from top to bottom), and/or worn or misshapen pistons (caused by excessive piston to cylinder wall clearances and indicated by a barrel shape from top to bottom of the piston skirt). The excessive piston to cylinder wall clearance and/or misshaped piston results in an increased thickness of the oil film on the cylinder walls, which can exceed the oil rings capacity to manage.
Rings can also become stuck in the grooves on the piston due to carbon, which is more frequently indicated by poor compression. A leak down test comparison between piston at TDC and BDC can indicate this condition. A wet versus dry leak down test at TDC will also indicate compression loss due to cylinder wear and/or piston/ring wear.
As others have indicated, rarely is ring "wear" an issue in modern engines. During the past 30 years, technological improvements in lubricants, metallurgy, air/fuel mixture management and engine machining have all resulted in the elimination of, or significant reduction of piston and cylinder wear in modern engines.
Myk Rian
Since there are no problems with the engine, go ahead and use a scope on it.
Let us know what you find.
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