How to Select Piston Rings

Federal-Mogul Document #1110

Pretend for a moment, that you were an early engine design specialist and were given the assignment of creating a product that would seal the explosive force of internal combustion, control the amount of oil along the engine’s cylinder walls and act as a primary conductor of heat. Now, hold up one of today’s piston rings. Can you honestly say that your solution to those performance criteria would be such a small, light and seemingly fragile component? Of course not, and that’s why the piston ring ranks as one of the true marvels of the internal combustion engine. What’s more, these slivers of cast iron are being called on to withstand significantly greater abuse in today’s applications than ever before. Piston rings, in simple terms, are the components that form the dynamic seal between the piston and the cylinder wall. That’s a thankless job. Consider, for example, that the rings must provide a perfect seal in spite of a wide range of temperature extremes – from winter-morning cold starts to prolonged high-rpm, high-load operation in hot weather. Also consider the most pistons “move” and “grow” laterally within the cylinder during operation, creating an unpredictable sealing area and contact surface.

How Rings Work

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 bottom 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. Most of today’s OE compression rings feature a premium grade cast iron, ductile iron or steel alloy coated either with a heat resistant plasma-molybdenum or chromium facing material. “Plasma-Moly” is a relatively soft and porous ring facing with excellent oil carrying capacity and comparatively fast conformability (break-in) to the cylinder bore surface. Chrome, used today primarily by overseas OEMs and in industrial engines, is a much harder, non-porous material.

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The Second Ring, although commonly referred to as a compression ring, plays a more important role in oil control by scraping excess oil from the cylinder walls. This is a critical function, because oil reaching the combustion chamber can lead to engine-killing detonation and tailpipe pollutants in the form of hydrocarbons. The Third Ring is exclusively an oil control ring that essentially removes oil from the cylinder wall. Most OE oil control rings feature multiple pieces – an expander spacer, and upper and lower scrapers. The expander must maintain original tension over an extended period, providing virtually instantaneous seating and resist corrosion. Selection of the best replacement piston rings needn’t be particularly difficult. Your decisions will be based on your choice of base materials and face coating, plus your application and product availability. Federal-Mogul offers OE ring technology within the replacement parts market. There are some exceptions. Chrome-faced compression rings are a popular choice among some OEMs. While chrome is extremely durable, it does not offer the oil-carrying capabilities of molybdenum, and can be very hard on the cylinder walls. As a result sometimes moly rings are a replacement for OEM chrome-faced rings. Economy ring sets typically feature the same material formulations and designs as premium ring, but do not have ring face coatings. As a result, an economy ring set will likely require replacement in 50,000 to 60,000 miles, compared to the 100,000-plus mile life expectancy of a premium, coated ring. Are premium rings always the best choice? Certainly not, in cases where the vehicle owner expects to keep the vehicle for several years, or the performance of the engine is being enhanced in any way, it doesn’t make sense to cut corners by selecting an economy ring. Not all designs and materials are available in all sizes. Designs used in ring sets are adopted only after extensive testing. We recommend against substitution of designs from those listed in our catalogs.

Selecting Your Rings

The first and most important step in selecting rings is to identify your application and its intended usage. If your application is to be used for common automotive passenger car and light truck usage the cataloged set is a replacement for the O.E.M.cast iron or moly rings and will give dependable service life and performance. Stock replacement rings assure proper fit and function, emission compliance and durability. Only when you move into high performance or industrial uses will you benefit from the product features of high-end rings. High performance applications must consider the operational RPM level, engine cylinder pressure, piston dimensional constraints and anticipated service life. The engine builder will have to make the choice of the primary goal of this engine to help choose between the trade-off of available product features.

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Use of Nitrous boost of 150 HP and up will require the HellFire rings, which are made of heat-treated ductile iron base product. These rings are made to take the extreme heat of Mega Doses of Nitrous Oxide. For the Speed-Pro Ring Sets use the application guide to identify your vehicle make, engine size, year and cylinder bore size. Catalog listings will identify what type of ring sets are available (standard or file fit), the ring set part number, width of each ring, standard or low tension, and available over-sizes. Hellfire Ring Sets are listed by bore diameter and list available over-sizes. These sets include the Speed-Pro reverse twist second ring and a standard tension oil ring for exceptional oil control. Claimer Ring Sets are listed by bore diameter and list available over-sizes. Beveled 1/16” top rings deliver enhanced high RPM sealing. Claimer Series sets utilize a tapered iron second ring, which provides improved oil control under race conditions. Although economically priced, Claimer Series sets still include our famous SS50U oil ring. Plasma-Moly Standard Gap Ring Sets are listed by bore diameter and list available over-sizes. Sets include all of the features of Speed-Pro’s file fit plasma-moly set, without the need for file fitting. The face of the top rings are filled with the latest generation Plasma Moly. Improved bond strength of this applied coating provides excellent resistance to flaking. The controlled porosity of the coating results in improved top ring lubrication. Due to the high melting point of moly, this ring set is highly resistant to scuffing. Recommended for street use, marine use, and for racing where file fitting is not required. Plasma-Moly File Fit Ring Sets are listed by bore diameter and list available over-sizes. These sets deliver superior quality piston ring sets with numerous advantages. The face of the top rings are filled with the latest generation Plasma Moly. Improved bond strength of this applied coating provides excellent resistance to flaking. The controlled porosity of the coating results in improved top ring lubrication. Due to the high melting point of moly, this ring set is highly resistant to scuffing. Recommended for all racing applications, where file fitting the ring end gap will deliver maximum power. Chrome File-Fit Ring Sets are listed by bore diameter and list available over-sizes. The face of the top rings in these sets are electroplated with chromium to provide a surface that is resistant to abrasive wear. The base material and design are the same as used in Speed-Pro Plasma Moly ring sets, only the face coating is changed. Being chrome, the face coating will not flake off, even when exposed to high vibration and detonation. Being harder and less porous than a moly ring, chrome rings provide increased life in engines that are operated in a dusty or dirty environment. Recommended for use when running on dirt tracks and for off-road racing.

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Pro-Series Plasma-Moly File Fit Ring Sets are listed by bore diameter and list available over-sizes. The Pro Series ring sets are designed for serious racing, with several features that maximize horsepower potential. Most Pro Series sets incorporate reduced radial wall thickness rings for lower tension and enhanced sealing. They also use a very low tension, thin profile version of the SS50U, which includes a wire latch mechanism to simplify installation. This ring combination gives dependable sealing and allows maximum horsepower production. A system creating a vacuum in the crankcase is highly recommended. .031 Dykes are listed by bore diameter and list available over-sizes. Pressure Back (referred to as .031 Dykes) top rings feature a 1/16” wide face and a .031 step. These rings are dependent upon gas loading to provide satisfactory sealing. Low R.P.M. sealing is sacrificed for improved sealing in the higher R.P.M. ranges. The reduced weight of Pressure Back rings make them especially desirable in high R.P.M. long stroke engines. Dykes ring'’ radial thickness is .170"”regardless of diameter. Dykes rings are not recommended for street engines. Pressure Back rings feature a Plasma Moly coating, which provides long life and superior bonding of the moly to the base material.

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