Extreme Plateau Honing: State-of-the-Art Bore Finishing

Research done by Ford Motor Company and the U.S. Department of Energy showed that roughly 45 percent of engine friction comes from the piston-ring-to-cylinder-wall interface. That friction accelerates wear and produces heat, but most important to racers, it decreases power output.

That is where tribology and technology come to the forefront. Tribology is the study of friction, wear, lubrication and the science of interacting surfaces in relative motion. Nowhere in an engine is that study more important and relevant than in the piston-ring-to-bore interface. In fact, engine cylinder bore surfaces are the most engineered surface finishes in the world. Not aerospace or particle accelerators. Internal combustion engines.

An industry leader in piston ring technology for OEM and applications for everything from street performance to Formula 1, Total Seal Piston Rings also has become a leading light in tribology. By researching the best seal from the piston rings while decreasing friction in the engine, Total Seal has expanded its expertise from the science of piston ring development to state-of-the art techniques for proper bore honing to make the most of ring seal.

Bore Honing Innovation

Piston bore honing techniques have evolved over time. Like so many things, the techniques and methods have been passed down from one generation of machinists to the next over the last 100 years or so. For most of the last, say 40 to 50 years, techniques to achieve the smoothest bore surfaces to get the piston rings to seat involved boring cylinders to a certain dimension and then honing to the spec.

Machinists would shape the bores with a machine bit to get close to the final dimension, then hone the bores to the final spec using honing stones of a specific grit and method. These stones are what give you that cool “cross hatching” finish on the inside of a cylinder bore. For decades, “hone to spec” was state of the art, but the quest for more power and longevity led technological advances that began to chip away at that industry practice.

One of those advances came in the form of new tools that could measure the surface of a bore down to one micro, or a millionth of an inch. For reference, 100 micros equals one 10,000th of an inch. The profilometer revealed new data and reinforced some of the things tribologists were learning in the field. A profilometer measures Average Roughness (Ra), Core Roughness (Rk), Reduced Peak Height (Rpk), and Reduced Valley Depth (Rvk).

The important dimensions here are the peak heights and the valley depths. Peak height is the surface with which the piston ring interfaces. The valley depth is essentially a second surface in which the lubricant pools. It’s kind of mind-boggling to think of a cylinder bore as two surfaces, but measured properly at the microscopic level, that’s what we’re dealing with.

Keith Jones, a technical specialist with Total Seal Piston Rings was working with a tractor pulling team — not what we do in NASA, but stick with us here — building diesel engines that produce upward of 5,000 horsepower on ghastly amounts of boost. Jones was telling the teams what he thought the best bore profilometer measurements should be, was building the team special rings, adding coatings, and nothing was working. Engines were failing on the dyno.

Over at Driven Racing Oils, tribologist Lake Speed Jr. was formulating special oils for the team and tear-downs were still indicating cylinder wall lubrication was insufficient. By using a profilometer and experimenting with cylinder honing techniques, Jones and Speed and the tractor pulling team, among others focused on cylinder surface research, found that by reducing peak height (RpK) in the cylinder bores and maximizing valley depth (RvK), they were not only able to get these monster engines to live, but also to enjoy success on track.

“These guys weren’t afraid to spend money. And it’s like, well, let’s crank those numbers up higher. Let’s try to get more oil reservoir. And through much trial and error, we finally landed on it,” Jones said. “We kept cranking the numbers up higher and higher. Next thing you know it’s off the dyno. Next thing you know it’s in the tractor. Next thing you know it’s winning races. And it’s like, wow, we got it. We were on the right path.”

So, honing to spec might have been acceptable for a small-block Chevrolet making 350 horsepower, but when it came to more extreme and more modern applications, the technique’s shortcomings became apparent. But that state of the art was what had been passed down from one generation of machinists to the next.

“I believe that there is a lot of C-minus information that is repeated around the industry because it works. It’s C-minus. It passes the test. C’s get degrees,” said Lake Speed Jr., formerly with Driven Racing Oils, now a tribologist for Total Seal Piston Rings. “So, because it works, it gets repeated. This is how you do things. Because we are fortunate enough at Total Seal to work with Formula 1 teams, NASCAR teams, IndyCar, NHRA, we know the A-plus way of doing it. Now we realize that the A-plus way is usually financially out of reach for most people, but there’s like an A-minus B-plus way of doing things that is actually very cost effective.”

That’s what makes extreme plateau honing so attractive to grassroots racers who are looking for that increase in power and longer engine life. It doesn’t cost much more, if any, because any good machine shop is already doing a hone after boring an engine block anyway. You’re just asking them to change how they do it. There are also different honing stones required for the honing machine. Not long after joining Total Seal, Speed conducted an experiment with one of his father’s old NASCAR engines.

The engine was built in 2004, then dyno-tested at 733 horsepower, and it had been sitting ever since. Speed dusted it off and put it back on the dyno. The only variables were the fuel used and the carburetor. It was the same model carb, but obviously not the very same carburetor used in 2004. That engine produced 730 horsepower on the dyno.

“So basically the same number, two different dynos, 20 years apart, two different fuels, two different carburetors, OK?,” Speed said.

Speed pulled the engine apart, changed the bore finish to a plateau hone, changed the pistons and rings and that same engine made 774 horsepower. Think about that for a minute. The pistons, rings and hone were worth between 40 and 50 horsepower on the test engine, without changing compression, ports or camshaft. Where did that power come from? Simply put, plateau honing allows for the use of thinner rings at the top, middle and oil ring grooves. Thinner rings resulted in less friction and heat. The hone results in better oiling and sealing. The piston change was only necessary to allow for the use of thinner rings.

Back in 2004, that test engine was built with a .043” ductile moly top ring, 1.5 millimeter second ring, and a 3 millimeter oil ring. That was the state-of-the art at the time for NASCAR. Speed said those engines were good for one 500-mile race and they’d be down 5 horsepower by the time the checkers flew. Today, NASCAR engines run 0.6 millimeter top and middle rings and a 2 millimeter oil ring package, and that same short block will run 1,400 to 1,500 race miles and be down nothing on power.

“So it’s triple the life,” Speed said. “Even though you cut the size of the rings in half, you triple the life of the engine.”

Think back to that number from Ford Motor Company and the U.S. Department of Energy at the beginning of the story: 45 percent of engine friction comes from the piston-ring-to-cylinder-wall interface. By reducing the thickness of the rings, you can reduce friction and heat, and increase power. Plateau honing makes thinner rings possible.

“The horsepower is the bonus that comes at the end. The reason for looking at why should I hone my block differently, why should I use different piston rings is all about durability and the longevity of the engine,” Speed said. “That’s the main thing. To do it for horsepower is a bonus.

“You’d like to have more power if you need it, but are you really using it all the time? No, but if your car is overheating, that’s not a good thing,” he added. “If your engine’s wearing out, that’s not a good thing. So we can prevent bad things that can happen just by making very simple changes.”

Speed pointed to “swatches” that were made using a three-dimensional profilometer that uses light to take an optical scan. They’re shown in the photos below. Using the light-enhanced profilometer, 3D modeling software and a 3D printer, they can take a 1 mm by 1 mm scan of an actual cylinder bore and enlarge it to show you what that surface looks like on a 4-inch by 4-inch 3D-printed square. The differences between the surfaces are remarkable.

Take the Lokasil bore surface from a late 1990’s Porsche engine. Lokasil is similar to the Alusil cylinder surface found in the engines in 944 Spec cars. Enlarged to 4 inches square, it’s an alarmingly rough surface with the standard production finish.

Later Porsche engines are manufactured using the SUMEbore process, which is as much a thermal application process as it is a machining process. SUMEbore uses a tool that is a lot like a plasma torch into which iron powder is fed. The result is basically a “spray welded” surface on the inside of the cylinder, which is then machined and honed in a proprietary way.

Oil as a Lubricant and a Gasket

At this point it is important to point out that oil provides more than just lubrication to a cylinder bore and the piston rings. Oil also serves as the gasket to help seal all those microscopic peaks and valleys.

“If you’ve ever leaked down an engine both hot and cold, you’ve probably seen a difference in the readings,” Speed said. “Part of that difference is thermal expansion of the parts, but part of the difference is also the amount of oil on the cylinder wall and piston rings. A dry cylinder wall and piston rings will leak more than one wetted with motor oil.”

Cylinder bores that have those low RpK numbers and high RvK numbers hold more oil, which fosters better lubrication and better sealing.

Iron Cylinders

The three photos above, from top to bottom, show the enlarged surfaces of a iron cylinder bore, which is what most of us amateur racers play with. The top photo shows an old-school bore, with no plateau honing. This engine would have high RpK and high RvK numbers. The middle photo shows a semi-plateau-honed cylinder bore, which would have a lower RpK than the top photo but theoretically the same RvK. The bottom photo shows what a plateau-honed cylinder looks like. Notice the valleys that hold the oil and the flattened top surfaces to provide a better interface for the piston rings, which now can be thinner thanks to decreased friction.

Remember, these images are of a 1 mm by 1 mm scan enlarged and 3D-printed to 4 inches square.

Of course, question that leaps to mind is, “Wouldn’t the rings eventually smooth out the bore surfaces?” Well, yes. The rings are the last step in the honing process, if you will, but that is part of that C-minus thinking that Speed talked about and reflective of materials no longer used in contemporary engines — and you will have incurred a lot of wear in the process.

“Now we’ve got blocks that are two, three times harder than they used to be,” Jones said. “And we’re dealing with very thin rings that are very light tension and we combine that thin, light-tension ring with that really hard block and a screwed up bore finish. Will that ring eventually create the right surface? Will it eventually make it the way it wants it and needs it to be have good ring seal? Yeah, it will do that eventually, but it’ll take forever. If we want it right now, we want to put that thing on the dyno, two hits, three hits, bang, we’re as good as it’s going to get, sealed up tight, right? Making the best power, no blow by, best vacuum in the crank case. And that all comes down to the bore finish.”

If you are having an engine built, it’s worth your time to ask the machine shop about extreme plateau honing. Many of the good shops are aware of it by now, but it is in a grassroots racer’s best interest to get the longest life possible out of a race engine. Total Seal conducted three hands-on honing classes last year, with anywhere from eight to 16 engine shops attending, so word is getting around.

“What I would say is this, and of course I’m wearing a Total Seal shirt, so I understand how this is going to sound,” Speed said. “But what I would say is that you would want to go to your machine shop and say, I want to run the thinner steel piston rings. So it needs to be honed the way Total Seal recommends for these rings. Because the reality is almost every machine shop in the U.S. is our customer at some point in time. They know how to get hold of us, right? And we can show them. That way the end user doesn’t have to become a savant about honing technology to try to explain this to that guy.

“So when we go from the old school ductile moly ring, thick iron rings, to a thinner steel ring, you’re going to increase horsepower and extend engine life and reduce operating temperature,” Speed concluded. “Power goes up, temperature and wear go down. It’s almost like having your cake and eating it, too.”