A piston ring is an open-ended ring that fits into a groove on the outer diameter of a piston. The three main functions of piston rings in internal combustion engines are:
Most automotive pistons have three rings: two for compression sealing (compression rings); one for oil sealing (oil control rings). Typical compression ring designs are rectangular, taper faced or keystone types. Typical springless oil control ring designs are napier or taper faced napier types. Typical spring loaded oil control rings are coil spring loaded 2-piece designs or expander spring backed 3-piece designs.
Piston rings are subject to wear as they rub up and down the cylinder bore. To minimise this, they are made of a very hard material - generally cast iron - and the lower oil control ring is designed to leave a film of lubricating oil a few micrometres thick on the bore, as the piston descends.
When fitting new piston rings, the end gap is the 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. After fitting the rings, then inserting the piston into the cylinder with the aid of a ring compressor, the width of the end gap is measured with a feeler gauge and must be within the required tolerances. Too small a gap may be completely taken up under hot running conditions, leading to seizure of the piston. Too large a gap indicates excessive bore wear and will give unacceptable "blow by" of combustion gases.
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