P I S T O N R IN G S
VEN IF YOUR SHOP DOES not rebuild engines on a routine basis, it’s beneficial to gain an understanding of the role that various engine components serve. In this article, we’ll discuss piston rings, their purpose and tips on ring service. Piston rings, in simple terms, are the components that form the dynamic seal between the piston and cylinder wall. Consider, for example, that the rings must provide both a sealing function and an oil control function, in a range of conditions and temperature extremes — from winter-morning cold starts to prolonged highRPM, high-load operation in hot weather. Also consider that most pistons “move” and “grow” laterally within the cylinder during operation, creating a sometimes unpredictable sealing area and contact surface. It should be obvious that long-term wear of cylinder walls and rings can create ring sealing and blowby issues, even a new OEM engine can experience issues if the rings were not manufactured properly. As a case in point, KIA recently issued a problem with oil rings in some 2020-2021 Soul and 2021 Seltos 2.0L engines. Apparently, the oil ring rails were not heat treated properly, resulting in excessively hardened rails, causing excessive wear on the cylinder walls. This has the potential for excessive oil consumption, exhaust system/ converter damage, and minimizing available oil for the rod bearings. Virtually all current automotive applications feature three rings per piston. The top ring is exclusively a compression ring, meaning its function is to seal the expanding combustion gases above the piston. Without an effective combustion seal, of course, these gases would leak around the sides of the piston, resulting in a power-robbing process known as blow-by. The top ring is sealed against the cylinder wall and the lands of the ring groove by the pressure differential created during the piston’s combustion cycle. As pressure increases above the ring and between the ring’s inside diameter and the piston groove, the ring is forced downward and outward, creating a tight seal over a wide range of engine RPM. The primary job of the top ring is to serve as a compression seal. It also helps to protect the second ring from combustion heat. Most modern OE compression rings feature a premium grade cast iron, ductile iron or steel alloy coated either with a heat-resistant graphite, plasma-molyb-
Fig. 1: Cross-sectional view of ring-to-piston identity and dimensional reference. Note how the oil ring expander causes the upper and lower oil ring rails to cock at slight angles to aid in sealing during up/ down strokes.
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