Piston rings, in simple terms, form the dynamic seal between the piston and cylinder wall. 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 that most pistons move and grow laterally within the cylinder during operation, creating an unpredictable sealing area and contact surface.
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Virtually all current automotive applications have three rings per piston, commonly called the top ring, the second ring, and the third ring. They each have specific features and functions.
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 leak around the sides of the piston, resulting in a power-robbing process known as blow-by.
During the piston’s combustion cycle, cylinder pressure pushes the top ring against the cylinder wall and the bottom of the ring groove to form a seal. 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.
Positive-twist, barrel-faced top compression rings are the most popular designs for performance applications. The barrel face provides quick ring breakin and uninterrupted cylinder wall contact. Dykes top rings have a step cut on the top side (sort of an L-shape profile) and are designed for drag racing applications (including Top Fuel and Funny Car) to aid in gas-loading. Dykes rings require pistons designed with Dykes ring grooves.
Most modern OEM-compression rings have a premium-grade cast iron, ductile iron, or steel alloy coated with a heat-resistant graphite, plasma-molybdenum, or chromium facing material. Graphite and plasma-moly are relatively soft and porous ring facings that have excellent oil carrying capacity and comparatively fast conformability (or break-in) to the cylinder bore surface. Chrome, used today primarily by overseas OEMs, is a much harder, nonporous material.
The second ring, although commonly referred to as a compression ring, functions primarily as a final oil control (about 80 percent of the second ring’s duty is oil control and about 20 percent for compression control). The two common designs are RBT reverse-twist, taper face and THG hook groove (sometimes called a Napier ring).
The reverse twist, taper-face second ring is very common in performance ring sets. An inside bottom bevel causes torsional twist in the ring. The twist prevents oil migration from creeping up behind the second ring. The taper face scrapes oil from the cylinder wall for oil control. Most RBT second rings are made from gray cast iron, with no special face coating.
THG second rings also have a twist, but with no inside bevel and no taper. Instead, THGs have a hook groove on the bottom OD. THG rings offer slightly better oil control than RBTs. THGs also act as a check valve to relieve excessive combustion pressure built up under the top ring, helping to stabilize the top ring (avoiding or minimizing ring flutter).
Like the top rings, OEM second rings are typically coated with a phosphate, plasma-moly, or chrome facing to protect them from combustion-related heat. These rings usually also have a lowtension, tapered face designed to reduce drag while maintaining the ring’s oil control capabilities. Most major OEM and aftermarket suppliers build a reverse torsional twist into the second ring to create a more effective seal at the piston land, preventing oil migration behind the ring.
The third ring serves as an oil control ring package that essentially removes oil from the cylinder wall during the power stroke. Oil control rings have multiple components, including an expander (spacer) and upper and lower scrapers (rails). The expander must maintain original tension over an extended period, provide virtually instantaneous seating, and resist corrosion. The oil rails in this classic design rest on angled expander pads that deliver lateral and vertical force to seal the top and bottom surfaces of the ring groove.
Oil rings are offered in standardtension (for most street applications and high-output engines susceptible to bore distortion) and low-tension (for high-performance applications) designs. Standard-tension rings apply around 21 pounds of tangential pressure against the cylinder wall. Low-tension designs apply approximately 15 pounds of tension.
Tension and Gap
Ring tension is a key consideration for racing applications because the piston rings can account for nearly 15 percent of an engine’s internal frictional power loss. The theory is that the lower the tension, the higher the power output. The trade-off, in many cases, is ring durability, but that’s a secondary consideration in many high-output applications where rings may be changed on a routine basis.
The best way to start a debate among engine builders is to ask whether an end gap is necessary in the second compression ring. That’s because there’s been considerable talk about the sealing properties of “gapless” rings. The theory behind the gapless configuration is quite simple: By eliminating a secondary escape path for combustion gases that slip past the top ring, you should be able to increase compression.
Most domestic engine manufacturers have concluded that an increase in end gap in the second ring actually improves sealing performance due to what’s known as the “Pressure Balance Theory.” Without a second-ring end gap, gases become trapped between the second and top rings. As the piston moves through its power stroke, the buildup of these gases actually lifts the top ring off its land. This, of course, causes extreme blow-by and power loss. Many dynamic engine tests have proven this to be case, whether OEM or aftermarket examples. As a result, most engine manufacturers use a larger, not smaller, end gap in the second ring.
Piston Ring Trends
The trend in ring design today focuses on reduced friction and high ring conformability. As ring-to-cylinder-wall friction is reduced, horsepower is freed and fuel economy is increased simply due to a reduction in parasitic drag. Ring conformability refers to the rings’ ability to seal within a bore that is not perfectly round (as often occurs when a bore is distorted during heat cycles and the stress of head bolt clamping). Ring sealing affects power output, emissions control, and oil consumption.
As engines continue to be smaller and lighter, block decks are becoming shorter and pistons are being downsized. The result is that rings are thinner; some are 1.2 mm, while 3-mm oil rings are common. Ring locations have gradually moved upward toward the piston dome. This trend is likely to continue for several reasons including performance and emissions concerns. For example, a smaller crevice volume (the circumferential void area between the piston and bore, above the top ring) is beneficial in reducing unburned hydrocarbons.
Also, regarding fuel economy, rings are becoming radially thinner, in an effort to reduce ring-to-wall friction. The thinner dimension also allows the rings to be more conformable (flexible) to the shape of the bore. This allows lighter ring tension, providing the bore geometry is correct.
Materials and Coatings
Although steel remains the current top choice for the top-ring position, iron (coated or uncoated) is preferred for the second ring position. The popularity of the traditional three-piece oil-ring assembly continues, using a stainless steel expander and chrome plated or gas nitrided steel rails. Although uncoated cast-iron top and second rings continue to serve well in the economy ring class, ring material is migrating toward steel for the top rings. For ring faces, manufacturers have developed superior wear coatings such as plasma-applied moly for increased durability.
Using a premium-grade material depends largely on the OEM application. For turbocharged or supercharged applications, you may choose a ductile iron with a plasma-moly coating, or steel rings with plasma-moly or chrome. Generally, most domestic applications use a moly coating, while most imports use chrome. The basic plasma-moly-coated ductile ring still does a good job on racing engines.
Molybdenum-disulfide (moly, for short) offers high scuff resistance. Used in a variety of configurations, it is also somewhat self-healing and absorbs some abrasion. Moly is longer wearing than a bare cast-iron surface, and more heat resistant than chrome. Phosphate and moly coatings enhance break-in and reduce the possibility of ring microwelding. Moly coatings are able to withstand temperatures up to 1,200 degrees F. It is applied to cast iron, ductile iron, or steel rings.
Gas nitriding provides a hardened surface finish to improve wear resistance. This coating is applied to steel or stainless steel rings.
Chrome plating continues to be one of the best wear-resistant coatings available, with a temperature resistance rating of around 800 degrees F.
On bores that have been plateau honed, the best applications are plainfaced and plasma-moly rings.
For standard replacement rings, the gap is already established by the ring manufacturer. In general terms, the smaller the bore, the smaller the gap; the larger the bore, the larger the gap. Top rings generally take a .010-inch minimum gap; the second rings require a .018- to .020-inch minimum gap.
For gap tolerance, particularly on the second ring, a slightly larger gap than the top-ring gap can be used. Increasing the second-ring gap helps to balance pressure between the top and second ring. This also helps to maintain seating of the top ring, thereby eliminating blowby pressure that might otherwise lift the top ring. Although, a small bit of blow-by past the second ring can help oil control, blowing oil back down into the sump.
If you plan to use a gapless ring anywhere on the piston, it should be used at the top ring location only. Although a gapless top ring provides great leakdown readings, the primary benefit is on alcohol engine applications, to prevent excess fuel runoff into the cylinders.
Some racing applications currently use a gapless second ring, such as those by Childs & Albert and Total Seal. Popular designs include use of an interlocking step design at the mating joint or a counterbored iron ring that sandwiches to a thin oil-ring rail, with gaps placed at opposing locations. The reason for using a gapless second ring is that it dramatically increases the anti-leakdown property.
However, a gapless second ring can create two problems a buildup of pressure that can unseat the top ring (creating ring flutter and instability) and the reduced leakdown reading provided by a gapless second ring can mask potential leakdown problems at the top ring.
Current replacement rings utilize unmodified conventional gaps on hypereutectic pistons. However, some experimentation is taking place. The Keith Black Racing hyper pistons specify a larger-than-normal gap for the top ring, with the intent of compensating for ring expansion during high-heat operation.
There is also a growing trend among OEM designs to use a slightly larger second-ring gap, simply to maintain top-ring stability (for pressure balancing). This gap increase at the second ring, in some applications, may be as large as .020 to .030 inch.
Determining Ring Depth
Here is a formula to use as a guideline when selecting top and second ring depth (22 is the SAE constant):
Ring Depth = bore diameter ÷ 22
The same applies to the oil ring. Standard depth on a ring for a 4.0-inch bore previously was about .180 inch. Today, this is about .130 (for all applications, including OEM and racing).
Ring Installation Methods
Winding a ring onto a piston can easily create a helix in the ring, resulting in ring distortion that may not recover. Always use ring pliers to install rings onto pistons. The tapered ring tool is the best to use, especially with today’s thinner, more fragile rings.
As far as synthetic oils and other ultra-slippery materials are concerned, don’t use a full-synthetic assembly lube or oil at the ring-to-cylinder wall in a fresh engine. If the lube is super-slippery, the rings may take much longer to seat, or they may not seat at all. For initial break-in, it’s best to use a petroleum-based engine oil.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks