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Do only the top two piston rings, which are thicker and each sits in their own lands, affect the compression and sealing of the combustion chamber, or do the bottom three AKA "side rails" also affect it? I was under the impression that the latter only scrape the oil off the cylinder sleeve and do not play a role in compression.

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In theory, the top two (or sometimes 3) rings are the compression rings, and the lower/bottom ring set is the “oil control” ring. It consists of a top and bottom ring with a “corrugated” separator in between them, allowing oil to collect there until flowing back into the crankcase. It’s job is to scrape excess oil from the cylinder walls and allowing it to drain through slots in the piston ring land (the groove where the rings fit into the piston), back into the crankcase. You can have perfect compression (good top rings) and have massive oil consumption (bad oil control rings), as well as have no oil consumption (good oil control rings) and have low compression (bad top rings). I’m sure there is some compression boost from good oil control rings, but I believe it is negligible. I hope this helps!

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  • In actuality, the top and 2nd rings also help control oil, but at a much less extent then do the oil control rings. I realized this recently when rebuilding an engine and reading the specs on the set of rings I bought for the job. It actually kind of surprised me. Sep 21, 2022 at 18:18
  • Agreed Paulster@. The compression ring's job is also to scrape the oil from the bore walls on the downstroke. Thanks for keeping me straight and accurate!
    – X-tech2
    Sep 26, 2022 at 16:50

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