Whats New In Piston Rings, Thin Is In; Steel Is Real; Oil Control Is No Longer A Tension Headache
Whats New In Piston Rings, Thin Is In; Steel Is Real; Oil Control Is No Longer A Tension Headache
Nov 26 2009, 02:22 PM
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What's New In Piston Rings
Thin Is In; Steel Is Real; Oil Control Is No Longer A Tension Headache: New High-Tech Developments Run Rings Around Old-School Cylinder Sealing.
By Marlan Davis
Photography by Manny Davis, The Manufacturers, Marlan Davis
When I first started working for HOT ROD magazine more than 30 years ago, nearly all American domestic production cars used a thick, 5/64-5/64-3/16-inch piston-ring package. The top, usually plain-iron, ring may or may not have been moly-filled. The oil ring was invariably a standard-tension configuration. Full-on race cars used thin, 1/16-1/16-?-inch piston rings. If available, they may have opted for a ductile-iron moly or chrome top ring with a low-tension oil ring. Just about everyone used a plain-iron second ring, although a few companies were pushing a moly second ring as well. The big debate was whether a daily street driver dared install 1/16 compression rings and low-tension oil rings.
Typically used in Harley drag bikes as well as big-bore drag race engines, this high-end, 5.125-inch bore size, Total Seal premium ring set consists of an advanced-profile, stainless 0.043-inch steel top ring with Total Seal's C-33 chrome-nitride face-coating, a 0.043-inch steel Napier second ring, and a 3.0mm, 11-pound-tension oil ring.
Now, everything has changed: 1/16 rings are considered thick, and even production V-8s use thin metric-sized rings-such as the typical GM LS engine's 1.5-1.5-3mm ring pack (about 0.059-0.059-0.118 inch). Ductile iron is now considered the minimum material for serious performance usage, with many racers and even production cars moving up to steel in the top groove. Second rings have become fancier, and even standard-tension oil rings are now thinner and lighter. The impetus for these changes has been OEM requirements to reduce friction for improved gas mileage and better sealing for emissions reduction. Racers have picked up on the new developments and run with them because reduced friction and improved sealing are worth power. Thinner rings also offer the potential for greater conformity to the cylinder wall. Improved precision, piston and ring manufacturing technology, better engine oils, and superior cylinder wall finishing techniques make it possible for engine builders to go to ever-thinner rings.
Understanding current ring developments first requires a brief overview of piston ring function. A piston ring set must do three things: prevent the air/fuel mixture from escaping the combustion chamber, prevent oil from contaminating the combustion chamber, and transfer heat out from the piston to the cylinder wall and ultimately into the cooling jacket. In most cases this is accomplished by a set of three rings, classified (from top to bottom) as the top compression ring, the second compression ring, and the oil control ring. Modern research shows that the top compression ring actually accomplishes most of the sealing function, while the second compression ring serves more as an additional oil control device than it does as a compression sealer. The current trend is to enhance top ring quality and heat resistance for the best possible sealing while revising second ring configurations to boost their oil scraping ability.
The Top Ring
This is the ring that is subject to the severe heat and pressure of combustion gases. It has a tough job, made even tougher because the top ring is getting thinner. For most high-performance usage, rings made from ductile iron with a moly (molybdenum) face remain the most popular choice. Moly can take the heat in most applications yet is porous enough to hold oil for better lubrication. Your favorite ring manufacturer's traditional part numbers for classic 5/64 or 1/16 performance moly rings probably haven't changed, but the ring itself most likely will be made with higher tolerances using an improved manufacturing process. For example, the moly facing used to start out in wire form and was flame-sprayed onto the ring with an oxyacetylene torch; now it's plasma-sprayed at greater velocity and temperatures resulting in a higher, more uniform moly density on the ring face.
Power-adder apps, including nitrous oxide, superchargers, and turbochargers, may require more sophisticated ring materials. Although plasma-moly is very conformable with good wear characteristics, the high shock loads of 150-plus-horsepower nitrous oxide shots, or more than 10 psi of boost, may cause fractures, chipping, or flaking. "It isn't a matter if [the power-adder] will make an engine detonate," Speed-Pro's Scott Gabrielson says. "It's just how much." For these applications, consider upgraded gas-nitrided ductile iron or steel rings, such as Speed-Pro's Hellfire or Perfect Circle's Firepower series.
For performance and Sportsman-level usage, the high-strength, ductile-iron top compression ring with a plasma-moly facing remains the baseline standard, but more exotic materials are gaining currency as rings get thinner.
The modern, thin, metric rings also need to be made from better materials to maintain adequate strength, prevent flutter, and withstand greater heat. For them, high-carbon steel is usually the baseline material of choice. Steel used to be considerably more expensive than iron, but thanks to huge volume purchases by OEM manufacturers, the price is coming down in much the same way that hydraulic roller cams have become affordable. Many of the pistons in JE's new SRP Pro line use thin rings, but JE says pricing is now about the same as its old 1/16 rings.
According to Speed-Pro, plasma-moly remains the preferred coating for steel rings, although gas-nitriding is starting to supplant it. Somewhat akin to the hardening process typically applied to forged crankshafts, gas-nitriding is a surface treatment that hardens the ring face to make it wear resistant while still remaining compatible with the cylinder-wall and piston surfaces. OEM rings with gas-nitriding are intended to last for up to 200,000 miles.
Dirt track cars have the potential for intake system contamination, and some of these guys still prefer a chrome-faced top ring, although improvements in plasma-moly rings have caused many to switch because the moly ring has about 1,000 degrees more heat resistance compared with old-school chrome rings. Many OEMs are again using chrome-faced rings now made from entirely new technology. In fact, the team at Total Seal says that modern, thin rings with vacuum chamber-deposited chromium-nitride have eliminated all the drawbacks of traditional chrome rings and are price-competitive with high-end moly rings.
Compression rings have become as thin as 0.7 mm in some NASCAR Cup engines. This titanium-coated, stainless steel, 0.7mm, top compression ring is available from Total Seal for about $60/ring. Vacuum pump-driven crankcase evacuation systems are mandatory with rings as thin as these.
Some guys in the blown fuel classes use stainless steel Dykes rings. The L-shaped Dykes or headland ring typically has a 1/16-inch face with an 0.017- or 0.031-inch step in the back of it, offering gas pressurization without the need for gas ports. Dykes rings need a special piston, are hard to seat, and accelerate cylinder-bore wear, so they're preferred for only very specialized applications.
The ultrathin rings for high-end pro racing use, such as NASCAR Cup engines or NHRA Pro Stock drag racers, may have exotic, very expensive tungsten or titanium nitride coatings applied using positive vapor deposition over a steel or even stainless steel ring body. This improves wear characteristics while reducing friction even further. But a Cup engine's three-piece ring set for just one piston costs about $160, so this high-end technology isn't yet practical for real-world applications.
Ring makers continue to experiment with different grades of steel, different heat-treating processes, and new coatings. The goal is to further reduce friction and improve durability without beating up the cylinder wall. At the high end, things are changing almost on a monthly basis, but as Total Seal's Keith Jones puts it, "If I told you what we're working on, I'd have to kill you."
The Second Ring
For more than 40 years, the reverse-bevel, taper-face, plain-cast-iron second ring has been the standard. Heat is not really a problem in the second groove, so there has been no need for superexotic materials or coatings (moly rings are a waste here). Today, most second rings continue to be made from cast iron or (for some high-end applications) ductile iron. However, second ring configuration is evolving: Modern theory holds that the second ring is about 85 to 90 percent oil control and only 5 to 10 percent compression control, so to better manage the oil, there's a definite trend toward the Napier (hooked or claw-shaped) second ring. In fact, most GM LS engines come stock with Napier rings. The Napier ring creates a reservoir for the scraped oil to flow through. "If you undercut the bottom of the ring, it exposes more of the endgap back into the ring groove, which opens up the flow area, providing a reservoir for the scraped oil," says Speed-Pro's Scott Gabrielson. A side benefit is that the Napier allows opening up the second ring gap volume even more, improving inter-ring pressure relief. If available for your application, the Napier can only help, never hurt, overall performance.
Total Seal continues to offer its unique Gapless top compression ring. The multipiece ring uses a support rail that deflects compression gas into the piston groove to aid in pressure-loading the ring throughout the four-stroke cycle. The rail also closes off the conventional gap, which Total Seal claims holds blow-by to less than 2 percent throughout the engine's life cycle.
The Oil Ring
Although some imports and high-end racers have been experimenting with an integrated single-piece oil ring design, the three-piece configuration consisting of an expander sandwiched between top and bottom rails remains the standard. However, tension and mass have been reduced for improved oil control, fuel economy, and horsepower. Perfect Circle's Bill McKnight says that "ring tension accounts for about 40 percent of total engine friction, with the oil rings alone accounting for 50 percent of the ring pack friction." The key to reducing tension is the ring's radial depth (for-and-aft width as it sits in the ring groove): If you maintain the traditional SAE-standard 0.190- inch depth, you still need higher-tension oil rings, but by decreasing radial depth to around 0.140 to 0.150 with a correspondingly machined piston, tension can be reduced because the overall oil ring assembly is more flexible and better conforms to the bore. With a thinner ring, even though overall tension is reduced, the effective unit pressure (cylinder wall loading) is higher. "Narrower rails make more pressure," Jones says.
Regularly driven cars should still use a standard-tension oil ring. A traditional standard-tension ring for a 3/16-inch-od x 0.200-inch-deep oil ring groove in a classic small-block iron-block engine once had about 20 to 22 pounds of tension; today it's about 18 to 19 pounds. Big-blocks were around 23 to 24 pounds; now they're down to 21 to 22 pounds. Old-school low-tension rings have dropped to 12 to 14 pounds from the previous 15 to 18 pounds. So-called standard-tension 3mm x 0.135 to 0.150-inch-deep metric rings designed to replace the old-school 3/16 rings in classic small-blocks have only about 15 to 17 pounds of tension.
The second ring actually serves more as an additional oil control ring than as a compression ring. Traditionally, most second rings are made from cast iron and feature a reverse bevel and taper face, allowing the ring to work as an oil scraper.
Today's late-model engines are designed from the ground up for inherently better oil control, operating with tighter bearing clearances, and lower total oil volume in the engine-so they're naturals for lower-tension rings. Ford Modular V-8 and GM LS engines come stock with only 9- to 10-pound rings. Meanwhile, in the extremes of pro racing, tension ranges from a NASCAR Cup car 1.5- to 2mm-thick oil ring with 2.5 to 4 pounds of tension to a 25-pound Top Fuel oil control ring.
The shape and profile of the expander's drain-back holes are also changing. The trend is toward larger, rounder holes in the expander; old-school expanders had little slits. "If you can see the piston's oil drain-back holes through the expander, the oil has a less restrictive return path," maintains JE Pistons' Randy Gillis.
Finally, there are special oil rings designed for stroker applications where the piston is so short that the oil ring impinges on the piston pinhole. Nowadays the preferred solution is adding an additional special dimpled rail support below the three-piece oil ring.
Rapidly gaining popularity with OEM manufacturers and hot rodders, the hooked or Napier-style second ring scrapes more oil with less friction than traditional second ring configurations. Napier rings work in a conventionally machined ring groove, but the rings tend to be most available for late-model engines in metric sizes.
How Thin is Too Thin?
There's no doubt that thin rings improve power and mileage in a properly built engine, but just how thin a ring you can run in a nonprofessional application is still evolving. Everybody agrees that 1/16 rings are the maximum anyone needs today, but what about those who really want to push the envelope? One consideration is bore size. On large-bore engines, there may be insufficient radial depth to maintain adequate tension under high combustion pressures. For this reason, at present, JE Pistons recommends not going thinner than 1/16-1/16-3/16 on a regularly driven big-block with more than 4.25-inch bore sizes when using conventionally machined pistons. On the other hand, Mahle is converting all its shelf pistons (even those for big-blocks) to the 1.5-1.5-3mm standard; below 3.5-inch bore sizes, Mahle is going to 1.0-1.2-2.5mm packages.
One workaround for running thin rings on a big-bore motor is gas porting. Pistons can be gas-ported via vertical holes in the piston deck or lateral slots in the top of the first ring groove. Gas porting allows combustion pressure to directly enter behind the top ring on the power stroke, pressuring the area behind the top ring for enhanced sealing. The ring retains normal tension for reduced friction through the rest of the four-stroke cycle. Vertical holes are more common for drag cars; oval trackers seem to prefer lateral gas ports, which are more resistant to carbon fouling under long-term use. "Gas porting will increase horsepower on every application, but it does wear rings out faster," Gillis cautions.
Most small-blocks have 4- or 4.125-inch-based bores. In this range, everyone says 1.2mm (0.043-inch) or 1.5mm compression rings with 2.5mm or 3mm oil rings are acceptable in nearly every case. Old-school Chevy small-blocks should probably stay to the high side, and the latest new-gen engines to the lower side. And even serious power-adder apps can go thin if the rings are made from steel and nitride-coated.
The three-piece oil ring remains standard in most applications, but the configuration continues to subtly evolve. The support rails tend to be thinner and more open to aid oil drain-back. This Perfect Circle CP-20 oil ring assembly utilizes a 20-degree ear angle on the expander to enhance side sealing in the piston ring oil groove.
Want to go thinner yet, like the Cup guys? You'll need positive crankcase evacuation induced by a vacuum pump plus a dry-sump lubrication system. Of course, successfully running these thin rings requires a complementary piston and enhanced machining.
To work properly, thin rings must be absolutely flat with no runout. According to Gillis, "Rings seal at the bottom of the piston ring groove as well as the outside perimeter of the ring. As the rings got flatter, we had to make the ring grooves flatter." Only modern, precision CNC-machining can achieve such absolutely flat piston ring grooves. "You don't make pistons on a lathe anymore," Gabrielson says with a chuckle. The plus or minus tolerances have been tightened up to such an extent that some manufacturers now claim to hold tolerances to the millionths of an inch (one microinch or 0.000001).
Piston skirt profile and machining have also changed. Manufacturers have found that the piston skirt cam and barrel shape affect ring seal and stability. Even whether the skirt profile is turned (machined) from the oil ring down or from the bottom up has an effect. We all have our own pet theories.
Speed-Pro's electroplated stainless steel SS-50 oil rings have a sturdy, boxlike construction to eliminate oil ring flutter and deformation in high-rpm engines. These rings do not depend on ring groove contact for tension. The configuration maintains consistent pressure under high-heat conditions, as well as conformity to worn or out-of-round cylinder bores.
Proper oil ring expander installation can be tricky with the new thin rings. The main problem is preventing overlap, where one end of the expander gap nests into the other when loading the piston-and-ring assembly into the cylinder. To prevent this on its narrow ring assemblies, Speed-Pro adds a staked-in-place wire running through the expander gap.
This JE stroker piston is so short that the oil ring groove intersects the pin bore. The solution is an additional support rail with a downward-facing antirotation dimple. It's installed with the dimple aligned with one of the pin-bore holes, as shown here. Most manufacturers consider this now-proven support system reliable enough for long-term street use.
Piston skirt coatings reduce friction, permitting tighter piston-to-wall clearances, which in turn make it easier for rings to do their jobs with reduced radial depth. Ring grooves can also be specially coated or anodized for greater wear resistance. Another common practice is adding an accumulator groove between the first and second ring to help fight inter-ring gas buildup. This helps reduce pressure between the top and second ring, aiding in top ring seal while minimizing ring flutter. You can use an accumulator groove with conventional ring grooves and undercut second grooves or (even better) with a Napier second ring-they are mutually compatible.
Representative of today's high-end piston ring combinations, Perfect Circle set 315-0039.005 for a Ford 4.6L Modular V-8 comes with a 1.5mm plasma-moly steel top ring, a 1.5mm THG hook-groove/Napier second ring, and a CP-20 3.0mm standard-tension oil ring.
One thing's for sure: For a higher-end application, pick the rings before ordering pistons. "Figure out the bore size and compression ratio, then what to use for rings," McKnight says. "Pistons can be made any way. It's the rings. Line them up first."
Just as pistonmakers had to raise the bar to keep up with ring technology, so does your machinist. Obviously, hand-honing is out, but what's in? At least a Sunnen CK-11 honing machine or (better yet) a computerized hone like Sunnen's SV-10 that takes operator variance out of the equation. That finished cylinder bore must be round and straight with no taper. It used to be that the rings would wear in to compensate for any slight bore irregularities. No longer. Today's high-end rings "are so hard they don't wear," McKnight maintains. Modern blocks are also harder, so the final finish has to be right from the get-go. Honing (torque) plates and heating the block to replicate in-use stresses are mandatory these days. Adjust the bolt tightness on the honing plate to replicate the distortion induced by the actual cylinder heads (not necessarily the same torque values used for actual head installation).
Your machine shop should have a PAT gauge (a type of profilometer) to actually measure the final bore's surface roughness. Total Seal says typical values in microinches for general performance apps should be around RPK 8 to 12, RK 20 to 30, and RVK 30 to 50. High-end NASCAR or NHRA Pro Stock racers trade off ultimate longevity for maximum smooth finishes: RPK 3 to 5, RK 10 to 13, and RVK 18 to 22.
Perfect Circle's U-Flex oil ring is for high-end, extreme power, pro racing venues like Pro Stock drags and NASCAR Cup racing. The extreme low-tension design develops just 2.5 to 4 pounds of tension. Offered in 1.5mm or 2mm thicknesses for 4.165- to 4.190-inch small-block bore sizes or 4.700-inch-bore big-block Chevys, each ring goes for about $60.
Is your local machine shop capable of this caliber of work? If it can finish late-model Ford or GM stockers with their thin rings to maintain original emissions compliance and factory tolerances, the answer is probably yes.
Heat Management and Endgaps
To keep the piston from melting down, heat must be transferred out through the rings to the cylinder walls and ultimately into the cooling jacket. Today's engines tend to run hotter than earlier engines, but even if the amount of heat generated by the pistons remains unchanged, thinner rings will see elevated temperatures because there's less overall surface area available for transferring the heat out of the piston (yet another reason why thinner rings tend to be made from better material with high-tech coatings).
The problem can be directly attacked by positively cooling the piston via piston squirters, but for most of us, the tried-and-true solution is still increasing the piston ring endgaps. For baseline moderate performance usage, about 0.003 inch of top-ring endgap for every inch of bore used to be the standard, with more severe-duty applications increasing from there. Today, most sources recommended a baseline endgap of at least 0.004 inch for every inch of bore diameter. For example, the baseline minimum top-ring endgap on a moderate-performance, 4-inch-bore engine should be about 4 0.004 = 0.016 inch. Racing or power-adder applications generally need much larger gaps (see table). If in doubt, this is one instance where more is better than less-a ring endgap that butts from thermal expansion is really bad.
The lines on this compression ring are actually lap marks. Many high-end rings are precision-lapped after manufacture on special machines, a procedure made necessary because piston groove tolerances have tightened up, making flatness imperative.
This high-end NASCAR JE piston with 0.043-inch-0.043-inch-3.0mm ring grooves has a full complement of seal and durability-enhancing tricks, including contact reduction grooves (A), lateral gas ports (B), and an accumulator groove ©.
Machined for 1.5-1.5-3.0mm metric rings, this Speed-Pro piston (PN L2621F) for 346ci LS1 engines has an accumulator groove (A) and an undercut second groove (B). Speed-Pro says the undercut or J-groove allows a conventional-style second ring to scrape oil about as effectively as the Napier ring.
The second ring runs cooler than the top ring. With less heat, its expansion should be lower, which would have you believe this means the second ring can be gapped tighter than the top ring-and in fact, this was standard practice for many years. However, tight second-ring gaps don't account for gas pressure dynamics. Research has found that gas leaking by the top ring can get trapped between the top and second rings if the second ring is gapped too tight. Trapped gas results in an inability to load the top ring when the engine fires, resulting in a power loss. For this reason, most experts recommend creating an escape path by gapping the second ring up to 1.25 times wider than the top ring gap. For example, if the top ring gap is about 0.016 inch, gap the second ring around 0.020 (0.016 1.25 = 0.020).
Piston squirters that direct oil to the pins and pistons for cooling and lubrication purposes are increasingly used in high-end racing and are also found in top-of-the-line late-model production engines like GM's LS9 Corvette V-8...
The Engine as a System
Rings and pistons cannot be considered in isolation; they are part of a dynamic system that's greater than the sum of its individual parts. In an internal combustion engine, everything affects everything else. Keep this in mind when applying some of this new technology to a classic engine design. Successfully running thin rings requires oil and lubrication system revisions. Instead of flooding the engine with oil, direct lubrication only to those areas needed to keep parts alive. Run modern, thin, full-synthetic oil. The lower-viscosity high-tech synthetics have superior temperature resistance and sheer strength, which permit tightening up main bearing, rod bearing, and rod-side clearance. Control windage with a sophisticated oil pan and windage-tray setup. With a now-tight bottom end, you won't have oil flooding the cylinder walls, so a power-robbing high-volume oil pump is no longer required. Marginal engine coolant systems will no longer cut it, either; consider coolant system enhancements. For a nonemissions performance application, shoot for 180-degree F coolant temperatures and 210- to 215-degree oil temps. Get your carb and ignition dialed in right; washing down the cylinder walls with fuel isn't good for ring longevity. Combine all the tricks and details and the result will be more power. A few ponies here and a few there can really total up and make a difference
Basic Ring Configurations
Most conventional ring packages still consist of three rings. The positive twist, barrel-faced top compression ring remains the most popular for performance applications. The barrel face allows almost instantaneous break-in, important for racing engines. It also helps the ring maintain cylinder wall contact if the piston rocks slightly at top dead center or bottom dead center. Used primarily in drag racing, the Dykes top rings feature a step-cut on the top side that helps with gas loading in extreme applications like Top Fuel and Funny Car.
...the '06 Northstar, boosted DOHC Ecotec four-cylinders, and the high-value V-6. New oiling passages are added to the block to feed the bolt-in squirters.
Speed-Pro Piston Ring Endgap Recommendations
Use this table as a baseline guide only and adjust per experience. Every engine is different. If the ring's end surfaces show shiny spots after use, it is evidence of ring-butting. All dimensions are in linear inches.
Modern shelf-stock pistons now have high-tech attributes once reserved for professional racing pistons. The SRP Professional line of lightweight forged side relief (FSR) pistons has high-quality wristpins retained by round-wire retainers and precision CNC-machined ring grooves. They come with a matched set of metric-style JE Pro Seal ring sets.
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