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Piston Rings Explained

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Piston rings seal compression, reduce blowby and emissions and control oil consumption. The changes that have occurred in ring designs, materials and cylinder bore refinishing techniques in recent years have been more evolutionary than revolutionary. Subtle refinements over the years in rings and cylinder bore refinishing techniques have created a generation of engines that can easily go 150,000 to 200,000 miles or more in most passenger car and light truck applications, and up to a million miles in big heavy-duty diesel trucks before a ring job is needed.

Better ring sealing and longer lasting engines are a bonus for vehicle owners. But eventually all piston rings reach the end of their service life and need ot be replaced. In many instances, by the time an engine needs a new set of rings, the vehicle it powers is virtually worthless so the vehicle and its engine end up in a scrap yard. But in other instances, it may be worthwhile to overhaul or rebuild the engine if the vehicle is worth keeping on the road. The same it true for older collector cars, classic muscle cars, performance cars, sports cars and pickup trucks. The engines is many of these vehicles are worth installing a new set of rings and other parts that can extend or restore the vehicle's life.

So if the piston rings in your engine are worn out, leaking compression and causing your engine to burn oil and/or lose power, what kind of replacement rings should you install?

One major supplier of piston rings said it has added new applications for its plane faced cast iron ring sets to cover engines that were not equipped with such rings from the factory. Why? To offer consumers and engine builders a less expensive alternative to new ductile iron and steel ring sets.

Cast iron rings do not have the durability or service life of ductile iron or steel rings, or rings that are moly faced or have a hard nitride coating. Plain cast iron rings will work in many late model engines, but it is critical the cylinders are properly cleaned after honing. This means scrubbing the bores with hot soapy water to remove all the honing residue and abrasives from the surface of the metal. If this is not done, rapid ring wear will occur when the engine is first started and during the ring seating break-in period. The result will be shortened life of the rings.

The best advice we can offer is to use the same type of piston rings (or better) as those that were original equipment in your vehicle's engine. For most high output late model engines, that means ductile iron or steel rings with some type of hard anti-wear coating such as moly or nitride.

Rings Keep Getting Smaller and Smaller

To reduce friction and increase the conformability of the ring set, auto makers have been using "low friction" rings that are both thinner and narrower. Thinner rings can more easily conform to small irregularities and distortions in the cylinder bore. This improves ring sealing, reduces combustion blowby and compression losses, while also reducing oil consumption.

Rings are also running hotter than ever before. As rings move up higher and higher on the piston to reduce emissions, they are seeing more and more heat. The top ring on many engines today run at close to 600 degrees F, while the second ring is seeing temperatures of 300 degrees F or less. Ordinary cast iron compression rings that worked fine in an old school Chevy 350 V8 can't take the heat in late model Chevy LS and similar applications. Turbocharging and supercharging also increase piston and ring temperatures, so today's top compression rings have to be steel or ductile iron rather than cast iron.

In recent years, the ring sets used for passenger car and light truck engines in the North American market have been mostly moly faced 1.2 mm top rings, 1.5 mm second rings and 3.0 mm three-piece oil rings. Many top rings are now steel while the second compression rings are still cast iron with a reverse-twist taper face. It is a combination that has worked well for over a decade.

Even so, there have been exceptions. The Buick 3800 V6 that was made for many years used a narrow 2.0 mm oil ring. Many second compression rings are also napier faced now. The napier design has a small notch in the bottom face of the ring to improve oil control and sealing as the ring scrapes against the cylinder wall. The napier design is also used with a positive twist to improve its sealing characteristics.

Most Japanese and European import engines are now using using 1.0 mm and smaller top compression rings. The Honda Civic that's sold in the Japanese market has tiny 0.8 mm top compression rings. The Japanese don't use moly facings but prefer gas nitrided rings for added longevity. North American ring manufacturers say nitriding is too expensive and moly works better because it is porous, holds oil and is more scuff resistant. Even so, nitriding remains popular in Japan.

The Japanese have also developed titanium nitride surface treatments for some new production rings. Titanium nitride rings are available for high-end racing engines but they are expensive. However, the rings are super hard and experience almost zero wear.

The Europeans, by comparison, use a mix of ring facings: moly, chrome and nitride. Like the Japanese and domestic OEMs, they, too, are using smaller and smaller rings. But much of the ring development work that's going on in Europe in recent years has been for small diesel engines. Traditionally, the Europeans bought more diesel-powered cars than gasoline-powered cars because of the diesel's higher fuel economy.

Diesel engines run leaner and hotter than gasoline engines, so hard, durable ring facings are needed to provide good longevity. Moly works well in diesels, but engineers have also developed various composite facings that combine ceramics, moly and other ingredients.

One of the big changes that has impacted the longevity of diesel rings in recent years has been changes in emission regulations that require ultra low sulfur fuels and exhaust gas recirculation (EGR) on heavy-duty trucks. The higher temperatures combined with reduced lubricity means some diesel engines that were capable of going a million miles before EGR may need new rings by 500,000 miles or less.

Performance Piston Ring Sets

Performance engine builders are always pushing the envelope and are the most demanding of all ring customers. They want rings that can withstand extreme abuse while also providing the best possible seal. Reducing blowby increases horsepower so a tight seal is absolutely critical to winning races.

A lot of the cutting edge ring and piston technology that was developed for racing has found its way into production engines. The piston and ring sets that are found in many production engines today were considered racing parts less than a decade ago, so it is logical to assume such things as "gapless" rings and exotic coatings may be in OEM engines.

Keith Jones of Total Seal Piston Rings says eliminating the end gaps in the compression rings can improve horsepower by as much as 10 percent depending on the application. "Our gapless rings have been very popular with racers, but we also have conventional rings, too, and offer both types with various coatings."

"We have steel rings down to 0.6 mm size in both gapless and conventional designs. Our 'Diamond Finish' rings are manufactured to within 50 millionths of an inch flatness and parallelism, with a finish that is typically 4 Ra microinches or less. This allows tighter assembly tolerances for better performance."

Jones says Total Seal's most popular face coating material is "C23," which has a coefficient of friction of 0.1 (three times better than moly) and won't flake off like plasma moly. It also works well with hard blocks and nickel silicon carbide-lined cylinders. Total Seal also offers a "C72" titanium coating, "C33" chrome nitride coating and conventional moly coatings as well, plus a "D47" side coating for the top and bottom of its steel Diamond Finished rings to reduce groove friction and microwelding.

The key to choosing a particular ring design and coating, says Jones, is to identify an engine's primary function in life. If an engine is a street/strip application, chances are it will spend 90 percent of its time on the street. For this kind of engine, street rings would work better than an all-out racing ring. Of course, it all depends on the compression ratio and whether the engine has a blower, turbo and/or nitrous oxide (in which case racing rings would be better).

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Vern Schumann of Schumann's Sales & Service, Blue Grass, IA, says ring selection for performance engines depends on three things: compression ratio, the type of fuel (gasoline or alcohol), and horsepower. Schumann says plain cast iron rings should never be used in an engine that burns alcohol because alcohol cuts lubricity. Coated rings are a must with alcohol.

Gas nitrided steel rings manufactured from coil wire are best for turbocharged and blown engines, says Schumann, and especially those that run nitrous oxide for an extra power boost. He says nitriding penetrates into the surface of the metal and alters its chemical makeup. Because of this it can handle thermal shock much better than any add-on facing material and won't flake off under load.

"One of the biggest misconceptions that's out there is that moly faced steel rings are racing rings. Welded moly steel rings work great on the street but won't hold up like nitrided steel top rings," says Schumann. "Nitrided rings are stronger, provide better heat transfer, and won't flake from thermal shock. In five years, I think most racing engines as well as many street performance engines will be running nitrided rings instead of moly."

Schumann explains that the different coil steel wire used in rings provides different tensile strengths. "The coil steel wire we use in our rings has a tensile strength of over 200,000 psi with zero porosity. Other alloys commonly used to make moly rings are typically 50,000 to 55,000 psi. Ductile iron, which we recommend for the second ring if the compression ratio is over 11 to 1 or the engine makes more than 400 hp, is rated at 70,000 to 80,000 psi. But ductile is typically two to eight percent porous, which reduces heat transfer and cooling. Ductile iron must be used with a coating, otherwise it smears the cylinder walls."

Schumann says the biggest mistake any engine builder can make is to use cheap rings with racing pistons. The rings should be steel or ductile iron so they don't fail. Otherwise they are likely to break, and when that happens you can kiss the piston and the motor goodbye.

Cylinder Bore Refinishing

As a rule, engine builders should follow the cylinder bore refinishing guidelines by the ring manufacturer. But like every other aspect of engine building, opinions differ as to what techniques work best in any given situation.

Federal-Mogul says a "plateau finish" is the optimum bore finish for today's moly-faced rings. A plateau bore finish is what all types of rings eventually produce when they are fully seated, so the closer the bore can be prefinished to a plateau-like condition the less the rings and cylinders will wear as the engine breaks in, the better the rings will seal right from the start, and the longer the rings will last.

For moly rings, Federal-Mogul recommends a two-step honing process: first hone with a conventional #280 grit silicon carbide vitrified abrasive, then finish by briefly touching the bores with a #400 grit stone or giving them several strokes with an abrasive nylon honing tool or brush.

If the cylinders are honed with diamond, follow up with finer grit diamond, a fine-grit vitrified abrasive or a brush to finish the bores. Diamond stones are fast and long lived, but they are more aggressive than silicon carbide and create more tear outs and other undesirable residue on the surface. Because of this, a rough diamond honing procedure should always be followed up with another operation afterwards to finish the surface.

Equally important is bore geometry. Gabrielson says engine builders have to be especially careful about oil control on late model engines. He says the block should always be honed with torque plates if the manufacturer recommends doing so to minimize bore distortion that can cause blowby and prevent the rings from sealing properly.

Bores must be straight and round for the rings to seal. Keep the Ra finishes within factory specifications, too, which is typically in the 10 to 15 Ra range on many late model engines.

Number #220 grit silicon carbide honing stones are the best choice for plain cast iron and chrome rings, #280 grit is best for moly-faced rings, and #320 to #400 grit is best for moly rings in a racing application. To finish the bores after honing, use a brush hone or flexible brush in a drill.

The main advantage of finishing the bores with a flexible brush in a drill is that you can run the drill backwards. The honing stones usually run clockwise so if you brush in the opposite direction (counterclockwise) it will do a much better job of deburring the surface. No more than 15 strokes should be necessary to produce a high quality finish.

Crosshatch is important. Some people want 30 degrees and others as much as 45 degrees, although most say 30 degrees is best.

Rottler Manufacturing says most ring manufacturers call for a bore surface finish of 10 to 20 Ra microinches. If the bores are honed with #325 to #400 diamond stones, the finish will usually be in the 22 to 24 Ra range. If the bores are then finished with a brush, they usually come down to about 18 Ra which is what you want.

Some OEMs are using a much coarser grit of diamond to increase valley depth in the bores for better oil retention and ring break-in. Some agricultural diesel engine manufacturers use #140 to #170 diamond stones to hone their cylinders, then finishing with #600 stones to plateau the surface. This leaves a surface finish of 10 to 14 Ra but with 60 to 100 Ra of valley depth to retain oil.

Most racers are also using #600 diamond stones to plateau the cylinders after they have been honed to get a really smooth finish.

Sunnen Products Co., says there are a variety of ways to achieve a plateau finish. You can use conventional abrasives, cork bond stones, a plateau honing tool or a two-step diamond honing process.

Sometimes the cycle time dictates the type of process or stone that's used. If an engine builder wants a fast cycle time, he may use a coarser grit stone to rough hone, then follow up with a finer stone to plateau finish the bore.

Typically, most production engine builders are using #320 or #400 grit diamond or CBN stones today, followed by brushing using a #180 grit PHT tool.

Some race engine builders prefer to increase the "Rvk" numbers (valley depth) in the crosshatch to improve oil retention.

Another issue is how to minimize bore distortion when the engine is running. Torque plates have long been used to simulate the bore distortion that occurs when the cylinder heads are installed on the block. Honing the block with torque plates installed results in rounder holes and better ring sealing. But temperature is also a factor that is hard to duplicate.

K-Line says the type of plateau finishing procedure they recommend depends on the engine and type of honing equipment. "What kind of honing machine are they using or are they honing with a drill? What Ra finish are they trying to achieve, and what kind of finish are they getting before they attempt to plateau the cylinders?"

To achieve a plateau finish, K-Line recommends using a brush: either the rigid style that mounts in the honing head holders or a spaghetti style bristle brush in a hand-hone (K-Line sells both types). It usually takes about 10 to 15 strokes in each cylinder to plateau the finish. The improvement is generally about 10 Ra points on the surface finish.

Winona Van Norman, another aftermarket equipment suppliers, recommends a 15 to 20 Ra finish for moly rings. Anything less than 12 Ra can result in glazed cylinders and the rings may not seat. A #280 grit stone will give you the right finish, but it should be followed with a plateau honing tool that loads into the hone head, not a bottle style brush. The soft honing tool does not exert enough pressure against the surface to change the overall Ra finish but it will do an excellent job of removing all the torn and folded metal you don't want on the surface. It makes a huge difference in ring seating and oil consumption.

Diamonds can produce good results, too, provided they are used in a hone head with at least eight stones and are followed up with a brush for 20 to 30 seconds. A set of #400 grit diamond stones will produce a finish that is similar to #280 vitrified carbide stones.

Analyzing Cylinder Bore Finishes

(from a presentation by Rottler Manufacturing)

For many years, cylinder bore finish has been analyzed by using the roughness average (Ra) parameter as a primary means. This measurement is very effective in determining the "smoothness" of a cylinder wall after it has been finish honed, although it is not enough to fully determine if a cylinder has been finished properly. Monitoring the cylinder finish is very important for many reasons, with the importance being to find such a method that makes rings seat faster and last longer.

One method that can be used to analyze bore finish is fax film. Fax film analysis provides a qualitative determination of torn and folded metal, burnishing, pull outs, and hone crosshatch.

A more detailed method of analysis are the Rk parameters of measurement. The Rk parameters directly analyze the bearing characteristics of a cylinder over a given sampling length. These measurements are graphically illustrated on the Abbott-Firestone Bearing Curve. This type of analysis also provides a qualitative determination of torn and folded metal, burnishing, pull outs, and hone cross hatch angle.

The Ra parameter can have many different forms, while still maintaining the same value. This is simply because Ra is only an average. The same Ra number can represent three very different surface finishes though each has the same average roughness. This being the case, it becomes very important to look at additional parameters when analyzing the surface finish. These include the Rk value, the Rpk and Rpk values, and the Rvk and Rvk values.

The Rk value refers to the bearing area that exists after the rings have seated. Rk is the height of the cylinder wall profile after the highest peaks and lowest valleys have been removed.

Rpk monitors how much material must be removed from the cylinder wall before the rings have seated, providing a good seal (reducing effects such as blow-by). Inconsistent high peaks (Rpk) are filtered out of this equation, and are not as important because they immediately are "knocked off" upon starting the engine.

Rvk refers to the valleys in the cylinder bore finish, filtering out the lowest extremes. This is a very important characteristic, indicating the surface oil retention qualities. Typically, a high Rvk value is very acceptable and indicates that the cylinder bore is effective in holding oil across its surface.

The plateau level is commonly described by the ratio of Rk to Rvk. This is a direct comparison of the bearing surface roughness to the valley depths.

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piston rings More Piston & Ring Articles:

Piston Ring End Gap Recommendations

Steel Piston Rings

Engine Rebuilding Tips

Cylinder Bore Honing Techniques

Engine Compression Testing

Engine Leakdown Testing

Measuring Blowby

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