Piston Rings & Blowby – Explained


Hello everyone and welcome. In this video
we’re going to be talking about piston rings and the role they play in an
internal combustion engine. And we’re also going to be talking about blowby
and how that occurs. Getting right into it, piston rings really have
three main functions. So here we have our cylinder wall, here is
our piston with these three piston rings as we have shown. And the three major
roles that these piston rings are going to play is that they’re going to keep
whatever is in this chamber—in your combustion chamber out of your crankcase,
they’re going to keep whatever is in your crankcase out of your combustion
chamber, where you have oil and then they’re also going to help facilitate
heat transfer between your piston and the cylinder wall. So your Pistons are in contact with the
piston rings, those are in contact with the cylinder walls and that’s how heat is going to be transferring from your
pistons to your cylinders. So looking at the piston you can see
that there are three rings. So this top ring is the compression ring
and so that’s helping to seal that combustion chamber you get good
compression and so that you don’t get any of that air and fuel mixture down into
your crankcase which is known as blowby. And so your second ring there is kind of
a mixture of both the top ring and the last ring and that last ring is an oil
scraper ring and so it’s helping to keep the oil out from the combustion chamber
that’s in your crankcase that’s lubricating those cylinder walls. So you can see these little holes right
here underneath that oil ring. And so what those holes are for is they’re
going to actually pass that oil through the piston back into the crankcase so
it’ll scrape it down on the walls and that oil to get scraped on the walls will
pass through these holes and fall back into your crankcase. So as I mentioned this middle
ring is going to be helping to scrape oil on the walls and it’s also going to be
helping to seal the chamber for compression. So how do piston rings allow
for blow by well and this is a bit of an exaggerated look into it, but here we
have our cylinder wall, here we have our piston, and here’s our piston rings. So on our exhaust stroke, when this piston is moving up, you can see these piston rings are going
to be pressed down and there’s a gap behind them between them and the piston
itself, and so your exhaust fumes and gases can get behind that gap and then on
your intake stroke which occurs after your exhaust stroke, These pistons are then going to move up
and so as they move up and hit the top of the piston, those gases can now pass
underneath them and travel down and eventually making their
way into the crankcase. So that’s blowby, where you’re getting
gases from your combustion chamber and getting them into your crankcase. Now, another way that you can have blowby
occur is passing through these piston ring gaps and so you can see… you know,
that looks like a fairly large gap. That’s not actually how large it will be
once it gets compressed within the cylinder as you squeeze this piston down
into the cylinder. That gap is actually going to be quite a bit smaller, it’s going to be
very small as far as the gap there, but it exists nonetheless. That gap
exists so that this ring acts as a spring, so it expands out and as the
metals within the engine expand and contract based on different temperatures,
you can make sure that you have good compression, because this ring will
expand out and meet up with the cylinder walls, so that you have a nice sealed
chamber for combustion. Nonetheless, these gaps will allow for
some of the air and fuel mixture and your combustion gases to pass through
and get down within your crankcase. So what happens if your piston
rings have worn out? Well, you’re going to have two problems
occur because of that. First of all, you’re gonna have a low compression and
as a result of low compression, the amount of power you make and the
efficiency of the engine is going to go down and you’re also going to be burning
oil because some of that oil isn’t going to be scraped off and it’s going to be
making it into your combustion chamber. So less power and burning oil. So it’s
really not a great problem to have. And, unfortunately, going in and replacing
these is going to be expensive, because you pretty much have to
tear down the whole engine. So thank you guys for watching. if you
have any questions or comments, feel free to leave those below.

100 thoughts on “Piston Rings & Blowby – Explained

  • Oil makes a big difference. Cheap oil not changed causes carbon to freeze the oil scraper ring to lock up frozen. I replaced rings on properly maintained synthetic oil vehicle and the oil scrapers were freely moving as if new. Oil makes a big difference.

  • If the oil is scraped off before it gets to the compression ring at the top, doesn't that mean that that compression ring will be completely unlubricated?

  • thank you….i was told i have a blow by then pasco told me they don't see a blow by. can you see a blow by just by looking???

  • the lower piston body is not getting in contact with cylinder case due to rings, then what is the role of lubrication as you mentioned?

  • Why don't they cut that top ring at an angle (let's say 30 degrees) in order to have the whole ring always sealed (/closed) instead of this 90° cut which creates an hole of course…..

  • I'm concerned with general aviation piston engines – my understanding is that low manifold pressure and high RPM combo should be avoided, it causes "piston ring flutter."
    Without sufficient pressure to hold everything in place, the pistons rattle up and down and the rings scrape and scar the cylinder walls.
    Any insight on this?

  • Hello

    ive got a question

    my engine is currently running fine but when i turn the AC on

    there's smoke coming out air from the breather

  • what are the common reasons that piston rings wear out? My car seems to be burning oil..and I seem to take care of it in terms of routine maintenance. I change the OIL/Filter every 5000 kms..and I also do enjine flush to get rid off any dirt. Can you explain why my car is burning oil, when I have done everything according to manual. Every month I have to add 1 litre of oil to the engine block….and I just dont understand why this is happening! Do the rings just wear out with age or as long as the proper maintenance is done then they should last. Thanks. Look forward to your feedback. PS: I have a HONDA ODYSSEY.

  • So if my car's mileage is 170k but i do not have any blow by since oil does not get burnt. Chaning these rings as well as the pistons with some otherr rbuilding parts will keep engine running as a new one? In that case? do you i need to replace my camshaft & crankshaft? or i did not reach this stage yet? Knowing that i seen a lot of debris around the camshafts and valve lifters and the valve lifters need to be changed. Please advise @Engineering Explained.

  • Hi
    Rislone has an addictive that claims to reduce piston ring blowby by increasing the viscosity of oil. Do you think this will work? And do you have any advices if I'm not able to rebuild the engine? thanks !

  • @Engingeering explained if the compression test is 140 PSI for the four cylinders but there is some knock and rattling noises. That means i can replace the moving assembly including pistons, rings, rods, rod bearings without grinding/machining the cylinder walls and the crankshaft might need machining for the new bearings? Please advise…..

  • Question: On an older bike like say my 02 Shadow VT750, would it benefit from an in the oil product like Mystery Oil or Lucas? I dont use additives on my bike except Seafoam in the fuel tank once after an oil change to keep the carb clean. Just don't know of any proven oil additives for a car or bike. I would think no on a bike with a wet clutch due to obviouse reasons. Anyways, thanks.

  • What if you have an engine (Volvo T5) that has the defective oil control ring ? How would you diagnose that problem since it is not the compression rings ?

  • Hello , i wanted to ask a question since you seem to be preety knolegdeable on car engines . I have a problem with my wv sirocco i had lost compresion in two cylinders and the mechanic told me that two pistons were damaged and i need to change all four of them. After removing the piston head the only thing wrong with it was a broken piston ring. Did they scum me ?Thank you for your answer.

  • Can you tell me approx press drop across rings. If press in combustion chamber is 100bar then how much would be between first and second and so on???

  • I love your videos, but in the future please put parts on the table, and point to them with a pen. It's hard to see the details of a part when you're holding it in your hand, and it's moving around. Thanks!

  • Very nice video Dost my YouTube channel Subscribe my all friends please aapka motorcycle mechanic technical channel pelesc plezz all frend please plezz

  • What do you think about 6 thou over standard tolerance range for the top ring gap? i.e should be 0.006-0.012" but I have 0.018". The service limit is 0.024". So my new ring is over the standard range yet equally below the service maximum. Thanks! It is odd because the 2nd ring isn't over the standard range. It is a stock civic, just looking for reliability. Just got everything back from the machine shop sooo yeah.

  • Replacing the piston rings should involve taking apart the block and head of the engine. … not the whole engine

  • Hey nice vid.. I have a question, the piston used in this video, what make piston is that if you don’t mind me asking?

  • I have a 2010 Mazda 6 with 2.5, and the blow-by is horrendous. it's only got 82k miles. I took it to the Mazda dealer and they told me it was normal and not to remove the oil cap while engine is running. Sounds like bs to me. Anybody else experience this, or have any info?

  • Would it make it Smokey and smell like gas? I am having a issue with one of my cars and I think it’s the rings. Number 6 isn’t firing and smoke is coming out the back and a gas smell is heavy.

  • Hey i changed my platina 125 es piston recently. Now my byke milage is less than 20 km per litre. Help me

  • Since they are so expensive to replace…there should be government standards about materials…moly coated for the top ring.

  • Hi we own a late model Pequot 308 GT diesel with 25,000km on the clock which my wife has just damaged slightly in the front. The panel man is a good friend & after dismantling the intercooler he called & advised there is dark oil in the intercooler pipe both bottom & top. This 2017 car is apparently suffering blow by. Two Peugeot service agents have looked at the car and deemed it perfectly ok, should we be concerned?

  • me, I change the segments of pistons diesel engine 2.0 l … but now I notice the smoke that comes out of the oil cap with the motor rhythm, after asking the mechanic he confirm me that it will leave with the time, is it normal? … I drove 250 km with the car, of course having to let it run 8 days in idle for reasons of 8 hours per day, now the draw of the car is a lot better but the story of the smoke disturbs me

  • Hi dear i have a doubt that, smoking due to only piston ring wear out, or some other reasons is there for bluish smoking from exhaust, i consult some technicians they directly saying to rebore engine cylinder…. Plz i need a solution for bluish smoking from exhaust

  • @Engineering Explained After rebuilding my engine with dry compression test of 185 PSI across 4 cylinders. Engine burns oil like crazy, is because rings did not seat yet with 3k miles after the rebuild or something is wrong? No smoke from exhaust and no leaks…Please advise.

  • U saved my day …. has a GTI with high oil consumption. …and wasn't getting what's the real issue for vehicle . Almost 2 weeks of restless days. The piston rings are worn out and oil gets to combustion area and burns off…

    In high speed oil just becomes so thick and in 20km around 2 liter if oil gets burned ….

    But in low RPM …. there's no oil burning …. actually your video made me get the actual problem for the vehicle ….

    Anyway it's expensive to rip apart the engine and reassemble. ….going to do it anyway ……

    Thanks a ton …mate

  • I notice the oil holes in the piston wall under the oil ring were only above the extended skirts on the piston. Could it be that the oil holes allow extra oil to be applied to the piston skirt to provide the extra lubrication it's going to need with all that metal in contact with the cylinder wall??

  • not bad for a kid. you missed a lot of detail thou. some manufactures call out a spacing for the ring gaps.

  • why would my 72cc motorbike engine start smoking after running a minute or so. new cylinder and rings, my valve stems are a little sloppy though. new valve stem seal.

  • I just sold my gasoline car bescause of the same problème you talking about , i was'nt ready to pay an expensive cost to repare it ,

  • if they made the highest heat endurant rubber for the pistons with a steel top they would bounce up and down faster and use aprx. 1/3 the fuel.

  • I put new rings on a piston on my bike. I didn't measure ANYTHING. I was so pleased when I started the bike and it ran. It ran pretty well. I did 500 miles on it, including some quite tricky off-road stuff. It smoked a little. It smoked a lot. It used oil quicker than petrol. But it ran. I decided the oil use was more than your everyday break-in consumption so tore it down again and did some measuring. The rings I fitted were standard. I'd assumed the piston was standard. It wasn't. It was a full 1mm oversized. The piston ring end gaps were like the black hole at the centre of the universe. I'm still a bit baffled the bike ran at all, nevermind scrabbling up rocky mountain tracks and a few laps of a race circuit. '85 XT350. Love it.

  • I have a question. I have a Dodge Ram with a 4.7l that really burns oil, especially at start up. After a few minutes the white smoke stops rolling, but it still burns it, a little black smoke, especially under acceleration, to the tune of 2 quarts per 30-40 miles. It had 200k but was just rebuilt by a supposed mechanic. The compression on all cylinders is at 150 psi. How can I tell if this is a valve guide/seal issue or a ring issue? I was told if the intermediate ring was installed upside down, that it could cause this. If it's a valve issue, can I change the seals without pulling the heads or should I have these heads rebuilt or buy new/rebuilt heads? Thanks! Loved the video!

  • My question is if the rings turn/spin gradually while the engine is running? My theory is if they did not turn that they would seize? I am thinking too that the cross hatch pattern done on a cylinder wall promotes a ramping and turning/spinning on the rings too.

  • I know this is and old video but I have a problem with my single cylinder engine. It makes an intermittent ticking from top of the engine. If I switch off the engine when hot and start up.. The idle is weak and ticking is more noticeable but goes away when i start moving again..also consuming more oil. Would this indicate blowby at the rings?

  • Ok you are amazing. I’m going to be an aircraft mechanic and I love watching your videos because it always answers my questions. The internet doesn’t go this in-depth into the properties of piston rings and the small movements of piston rings and so forth. Thank you so much!!!

  • My bike has started to show signs of worns piston rings. I checked the spark plug and its filled with oil. Power drastically cuts in and out as well and my fuel consumption has been terrible.

  • Thank you man. Really good presentation. Like someone mentioned-thanks for using an actual piston along with your illustrations.

  • can piston ring  blow by break a crank ? trying to keep the truck on the road at least until all my hauling, and wood  hauling is done and or I replace the vehicle. no smoke works by gumming up the piston gap should help for the summer.

  • I was behind a guy in traffic the other day, he had a little Honda hatchback, loud exhaust, probably $4,000 worth of wheels and tires, yet choking everyone around him out with white oil smoke…..

  • That is interesting. I'd never heard of the second compression ring performing oil control. Would seem that there is nowhere for it to drain, unlike the oil control ring which (in your example) not only have drain holes below it (which I would argue are for supplying oil to the piston skirt), but also typically behind it. Would be neat to show folks an example of an uninstalled oil ring, so they can see the three parts that make it up (although some are one piece), and the drain holes in the bottom of the ring land in the piston. The piston ring gap exist for the single reason of allowing for the expansion of the ring when it gets hot! This is a mistake many make when adding boost to a naturally aspirated engine. The rings get hotter, they bottom out, then they break and score the cylinder. Boosted engines necessarily must run larger piston ring gaps. 😉

  • You really got this wrong. The main and only source of blowby is close to peak cylinder pressure, late compression and early expansion. During that time, the top ring is shoved down against its bottom of the groove, and the only source of blowby is through the end gap. Sometimes, under super high rpm, the top ring can get unseated from its groove and the excess blowby can even force the ring collapse inwards, but this is an super old engine thing when the rings were thick and heavy. As your rings and cylinder walls wear out (cylinders will wear towards the top end where the pressure is high, unfortunately) the end gap will increase, letting more high pressure gas escape.

  • Great video as always. Picking up where you stopped the video at….id love to see a video about worn piston rings vs carboned up ring grooves leading to rings no longer able to seal correctly.
    Basically, a comparison as to which is the actual culprit for blow-by.
    Much like how warped rotors is a generic term for uneven rotors, when the rotors are not actually warped.

  • How are there engines with over 200,000 miles that barely burn any oil? Piston rings made out of adamantium or something? Lol

  • I hate it when people say stuff is expensive but don't have a price. For a car, expensive ranges from $100 to $3000.

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