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Cylinder 3 - Dead. As a matter of fact it is about 2k close to piston skirt failure and blowout. There is about 1 mm or 0.039 inch wear on top cylinder wall
Last edited by arsupisemnet; Aug 2, 2021 at 11:42 PM.
An example from machining explaining what is going on then coating on the piston goes bye.
Shiny cutting surface - an Alusil liner cut with clean carbide cutter.
Rough surface - a teared walls by aluminum chips stuck to cutter tip. Basically exposed piston aluminum start tearing Alusil walls
apart.
Getting ready for custom sleeve installation. This time it will be a slide in sleeves with retainer (racing cars way). Will show all the process next week.
I was thinking about boring out and running custom pistons, however you would want to keep compression ratio, otherwise you will be detonating big time. My next project will be M157. For M157 you need wet sleeves, since there is not much of original material left to support liner.
Getting the piston coating right is an issue. The only ones I recommend for pistons in an aluminum bore are Kolbenschmidt and Mahle. Yes, there are others out there but the coatings will most likely fail over time. Either that or run a Nikasil bore. The steel sleeves are probably going to be best for a boosted engine.
Now the most important part of the day!
Why this particular engine failed?
The answer will shock some of you.
Improper assembly of oil rings.
This particular pistons have three part oil rings. Top scraper, middle spring, and bottom scraper. In general it is taken to have these rings gaps to be rotated either 120 degrees between or 180 degrees between spring gap and top and bottom scrapers.
In this particular engine in cylinder 3 all three components of oil ring being aligned together, basically making this oil ring useless. As a result an excessive amount of oil leaking inside of the cylinder, leading to extensive carbon buildup and as a follow a cylinder wall damage.
Cylinder 7 got damaged because it share same crankpin and carbon buildup from cylinder 3 was falling into crankcase and being sprayed on the inner surface of cylinder 7 by spinning crankshaft.
Cylinder 1 had bottom scraper gap aligned with spring gap resulting in less than 100 percent performance ending up in same carbon buildup and excessive wear.
So as an advice if in case you are having issues with premature engine wear-out and you are out of warranty please video engine disassembly process. If in your case you will find out same situation where oil ring improperly assembled you might have a legal cause (with a lawyer of course) to get your car repaired at the expense of car manufacturer.
An example of proper piston rings alignments
Last edited by arsupisemnet; Sep 28, 2021 at 12:58 AM.
Very interesting, looks like you cracked the case.
So the carbon itself is what is scoring the cylinder or is it the carbon causing ring degradation which in turn causes the scoring? I suppose the scoring would then be consistent with where the ring gaps are aligned, most images I've seen appear to have scoring mainly on one side vs. concentric.
Did your GL M278 have the same ring gap alignment setup?
The carbon buildup is a kind of soft abrasive. All it does is accelerate piston iron coating to wear. Once iron coating is gone - so is your cylinder wall.
Wear-of the pistons usually occurs on the top part - it is just a gravity pull.
Never checked my own car rings alignment - there was no need for that. The car engine statistically was at about failure mileage.
This donor engine however died at 40k miles - which is very unusual.
Last edited by arsupisemnet; Sep 28, 2021 at 10:05 AM.
So many questions, you are saying the rotation of the rings were incorrect? Meaning there is a certain way all the rings need to be clocked for it to prevent exhaust entering the crankcase? Would you not have had blow by from oil entering the combustion aswell, second point this carbon build up can be seen but also an oil film is also noticed, is this from the intake or entering via the crankcase? Is there a correlation to oil change intervals? Oil vapors full of cabin entering via the intake causing extra carbon deposits as it enters via the PCV system?
Last edited by Ricardoa1; Sep 29, 2021 at 02:12 PM.
Oil ring does two things
1) top oil scraper scrape excess oil from cylinder wall so less oil goes into combustion chamber
2) bottom oil scraper keep extra oil around piston skirt
If oil ring improperly assembles the scrapers pressure on the cylinder wall if far low from normal.
As a result you have two things
1) Excessive buildup of carbon in chamber and on the piston
2) Less lubrication to piston skirt.
Both results in faster wear of components
Last edited by arsupisemnet; Sep 29, 2021 at 04:27 PM.
Rings typically rotate on the pistons so after 40k one should not expect to see them on the orientation that the manufacturer specified during assembly. So the ring gap would end up where it pleases. Does MB pin these rings to ensure rotation does not happen? I guess Im still a little skeptical, unless ring got pressed in stuck in the piston or another freak failure, does the skirt and bore damages be directly correlated to only the rings…
thanks for taking the time to inspect the engine. I think we are all curious and what to expect from a M278 and it’s longevity. We are looking for patterns to see if properly working and maintained do they last 200k or is that a dream with the current design.
Last edited by Ricardoa1; Sep 29, 2021 at 04:19 PM.
There is no doubt that some of the rings might rotate around. Blow by related concern is to avoid both compressional rings gaps aligned together.
In case of oil ring by 40k miles both scrapers will have more than enough carbon buildup to avoid rotation relative oil ring spring. Whole oil ring might rotate around but not it components relative each other.
In this particular case i am quite sure oil ring being assembled that way.
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