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My question relates to the lubrication of cylinder walls and piston rings. It is my understanding that pistons in cars consists of 3 rings; two compression rings and the oil/scraper ring.

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As far as I've read, the cylinder walls are only lubricated by the jet of oil coming out of the connecting rod holes, and any excess oil is primarily scraped off the walls by the oil ring (consisting of 2 small rings and a center bent shape ring - correct?)

If I'm correct on this, it seems intuitive to me that oil would never reach the compression rings as it is scraped by the oil ring sitting beneath it.

How is the compression ring then lubricated and kept from scoring the walls?

  • During deceleration when cylinders experience low vacuum conditions, some oil is sucked (pushed) past the oil scraper ring into the comp rings. Also as SM stated oil scrapers are not designed to scrape 100% of the oil. – Moab Jun 11 '18 at 12:30
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The answer is that the scraper ring does not remove 100% of the oil, a VERY thin film is left on the surface which provides the lubrication necessary.

Some engines will score the walls, but others, on re-build need "glaze-busting" to remove the baked on layer of oil deposited over time.

  • I see. How is this a reliable method of lubrication considering the vast variety of operating conditions an engine will be put through? E.g bad oil, extreme cold metal temperatures during winter and fierce combustion temperatures. Just how bad is it that the ring would breach the film and score the wall, and how frequent does this happen? – Erik Jun 11 '18 at 12:12
  • Depends on the quality of oil how well it protects the engine under all conditions. – Moab Jun 11 '18 at 12:31
  • @SolarMike what is your opinion on this? Do you have more in-depth knowledge of my previous comment? – Erik Jun 11 '18 at 15:52
  • Given the number of engines around the world that rely on this method of lubrication and that it is not the most common source of failure of an engine then I my answer still stands... – Solar Mike Jun 11 '18 at 17:27
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    Engines and useful documentation existed WAY before the internet... books are still useful... – Solar Mike Jun 11 '18 at 17:49

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