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Illustrated: Piston Rings
Illustrated: Piston Rings
Piston Ring: An open-ended ring which fits into a groove on the outer diameter of the piston. Its chief function is to form a seal between the piston and cylinder wall. Most automotive pistons have three rings: two for compression sealing; one for oil sealing.
Piston rings are subject to wear as they rub up and down the cylinder bore. To minimize this, they are made of wear resistant materials (cast irons and steels) and coated or treated to enhance the wear resistance. Typically top and oil control rings will be coated with Chromium, or Nitrided_ possibly plasma sprayed or have a PVD (physical vapor deposit) ceramic coating. For enhanced scuff resistance and further improved wear most modern diesel engines will have top rings coated with a modified chromium coating known as CKS, a patented coating from Goetze. The lower oil control ring is designed to leave a film of lubricating oil a few micrometers thick on the bore, as the piston descends.

When fitting new piston rings, the end gap is a crucial measurement. In order that a ring may be fitted to the piston, it is not continuous but is broken at one point on its circumference. The ring gap may be checked by putting the ring into the bore/liner and measuring with a feeler gauge and should be within recommended limits. Too small a gap may be completely taken up under hot running conditions, leading to seizure of the piston. Too large a gap will give unacceptable levels of blow-by gasses or oil consumption. When being measured in a used bore it may indicate excessive bore wear.
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