The Smooth Science of Cylinder Honing
By Larry Carley
The basics of honing cylinder blocks hasn’t changed much in recent years, but what has changed are the type of abrasives being used by many engine builders.
Silicon carbide and aluminum oxide honing stones of various grits have long been used in power honing machines and portable hones to finish cylinder bores. These types of abrasives are popular with engine builders because of their flexibility and low cost.
But in recent years, a growing number of performance engine builders and custom engine builders have started using the same type of honing stones that production engine rebuilders and OEMs use: diamond abrasives.
Conventional vitrified abrasives cut cleanly and do an excellent job of finishing cylinders – provided the right honing procedure is used to achieve a bore finish that meets OEM specs or the ring manufacturer’s requirements. But as the stones work the surface, they experience a lot of wear. In fact, the stones wear almost as much as the metal surface in the bore. Consequently, the honing machine operator has to constantly monitor the honing process and compensate for stone wear to keep the bores round and straight.
Tim Mera of Sunnen Products Co. in St. Louis, MO, says conventional abrasives require a balance between cutting action and stone life. As a rule, harder metals require softer stones. A softer stone requires less honing pressure, produces less heat and causes less bore distortion. So the bond that’s used in conventional abrasives is designed to wear quickly and expose the abrasives for good cutting action.
OEMs and production engine builders, on the other hand, don’t have the luxury of being able to baby-sit their honing equipment. Because of their higher production volumes, OEMs and PERs have to run their honing operations at higher speeds and with less operator supervision – which means diamond honing stones in most cases.
Diamond has long been the material of choice for high speed, high volume honing applications because of its excellent wear characteristics. Stone life depends on the hardness of the abrasive, the hardness of the substrate that holds the abrasives, the hardness of the engine block, honing speed, load and the amount of metal that’s removed. Diamond is the hardest natural substance known, so it can hold a cutting edge much longer than a conventional abrasive. This means the bond that holds the diamonds can also be harder because it doesn’t have to wear away as quickly to expose fresh stones on the surface.
Typically, a set of conventional vitrified honing stones might do up to 30 V8 blocks (240 to 260 cylinder bores) before they’re worn out and have to be replaced. A set of metal bond diamond honing stones, on the other hand, might do as many as 1500 V8 engine blocks (12,000 cylinder bores) before they have to be replaced. That’s a huge difference.
However, diamonds require a sizable up-front investment. A set of stones can cost $600 to $700 – which is a big jump from $15 to $35 for a set of conventional honing stones. Consequently, many small custom engine builders say diamonds are too expensive for their purposes. They also say they can’t afford to buy several sets of diamond stones to cover all the different bore sizes they do.
Even so, when the longer life of diamond stones is compared to that of conventional abrasives, diamonds may be more economical in the long run, even for a small shop (assuming an operator doesn’t overstroke a bore and break a stone!).
Pim van den Bergh of K-Line Industries, Holland, MI, says he sees more and more shops switching to diamond for a variety of reasons. "We were one of the first to offer diamond for honing machines because we saw its many advantages." He says it gives very consistent results with minimal stone wear.
Pros & Cons Of Diamonds
Because diamond is a harder material and wears more slowly than conventional abrasives, it cuts differently and requires more pressure. Diamond tends to plow through a metal surface rather than cut through it. This can generate heat and distortion in the cylinder bore if the wrong type of equipment, pressure settings or lubrication is used in the honing process. When done correctly, though, it can actually improve bore geometry by producing a rounder, straighter hole.
Diamond is also good for rough honing cylinders to oversize because it can remove a lot of metal fast. But finishing requires at least a two-step procedure. Otherwise, the surface will be too rough.
If you’re switching from conventional stones to diamond, you’ll generally have to use a higher grit to achieve the same Ra (roughness average) when finishing a cylinder. For example, if you have been using #220 grit conventional stones to finish cylinders for chrome rings, the equivalent diamond stones might be a #325 grit. If you have been using #280 grit conventional stones to hone for moly rings, the diamond equivalent might be #550 grit stones. The actual numbers will vary somewhat depending on the brand and grade of the stones.
A cylinder bore must have a certain amount of cross hatch and valley depth to retain oil. However, it must also provide a relatively flat surface area to support the piston rings. Ring manufacturers typically specify a surface finish of at least 28 to 35 Ra for chrome rings, and 16 to 25 Ra for moly faced rings. These numbers can be easily obtained with diamond stones and brushing, say those who use this honing technique.
One rebuilder we spoke to says he uses #325 grit diamond stones to end up with an Ra finish in the 20 to 25 range, which he feels is about right for moly rings. For some applications, though, he uses a #500 grit diamond to achieve a smoother finish in the 15 to 20 Ra range.
Something else that’s different when honing with diamond is what diamond does to the bore surface. Diamond tends to leave a lot of torn and folded metal on the surface, causing sort of a smeared appearance that doesn’t make a very good bore finish. Consequently, finishing the cylinder requires a second step to remove the damaged material.
One way to get rid of this material is to plateau the surface with a fine grit conventional abrasive (like a #400 or #600 grit stone). All that’s needed are a few strokes to shave off the tops of the peaks. But, the most popular method for finishing the bores when using diamond stones is to sweep the bores with a flexible brush or a nylon bristle plateau-honing tool. Brushing helps remove the torn and folded debris while improving the overall surface finish.
Chris Jensen of Goodson Tools & Supplies in Winona, MN, says, "there’s a lot of confusion about how to finish cylinder bores when using diamond. Since diamond leaves a lot of folded and torn metal on the surface, the bores need to be brushed to remove the debris. Many different names are given to the same tool and process. Some call it a plateau hone, a soft hone, a whisker hone or an ultra-fine hone. But they all do the same thing: they sweep across the surface to remove jagged peaks, folded and torn material."
Bristle style soft hones consist of mono-filament strands that are extrude-molded with a fine abrasive material embedded in the strands. The filaments can be mounted in different types of holders or brushes that can be used with portable or automatic honing equipment.
When finishing the cylinders with a brush, only light pressure is required. The rpm of the brush should be similar to that which the cylinder was originally honed, and no more than 16 to 18 strokes should be applied (some say 8 to 10 strokes are about right). Too many strokes with a brush may produce too smooth a finish that doesn’t hold oil.
Reversing the direction of rotation while brushing helps to remove the unwanted material on the surface. The end result should be a cylinder that provides immediate ring seal with little if any wear on the cylinder wall or rings when the engine is first started.
Sunnen’s Mera says, "brushing the bore after honing makes a huge improvement in the surface finish, whether diamonds or conventional honing stones were used to hone the bore. You can get the overall Ra down to 8 to 12, with RPK (relative peak height) numbers in the 5 to 15 range, and RVK (relative valley depth) numbers in the 15 to 30 range."
Something else to keep in mind about diamond is that it works best in power honing equipment that has been designed to take maximum advantage of diamond’s honing properties. There are a number of companies that make diamond honing heads for use with various honing machines: Rottler, K-Line, Kwik-Way, Peterson, Winona Van Norman, Sunnen and others. But because of the increased loads, diamond may overtax some older power honing machines and increase the risk of stripped gears. It may be better to buy a new honing machine that has more horsepower and rigidity to handle diamonds.
"Most of our customers who hone with diamonds use a CK21 machine," says Sunnen’s Mera.
As for portable honing equipment, conventional abrasives are the better choice for this type of application. Most of those we spoke with say diamonds require too much pressure for portable honing equipment.
Another difference with diamond is the type of lubricant that’s required. A synthetic water-based lubricant is usually recommended instead of honing oil.
K-Line’s van den Bergh says, "water-based lubricants are easier and cheaper to dispose of than oil-based lubricants because they can be evaporated down to reduce their bulk. On the other hand, they occasionally require make-up water and have to be monitored to prevent bacterial growth.
"The type of lubricant you choose is very important because it can make quite a difference in honing performance. With conventional abrasives, you want a good quality honing oil. A lot of people run into honing problems because they’ve diluted their honing oil or tried to use something else like diesel oil or kerosene," says van den Bergh.
Anthony Usher of Rottler Mfg. in Kent, WA, says the OEMs all use long-lasting superabrasives with metal bonded honing stones. But the equipment and controls they use are very expensive, which makes it difficult to bring the same technology into a typical aftermarket job shop.
"About 12 years ago, we decided to change that. If new engines are originally honed with diamonds, why can’t we develop the same technology? So we set about developing honing equipment, controls and stones that would put the same technology into the hands of a job shop," says Usher.
"Diamonds last a long, long time. Because the stones don’t wear away, you can control the size of the bore more accurately," Usher explains. "This allowed us to build an automatic control system that allows us to size bores exactly the same every time."
Usher says for under $30,000, a job shop can buy a diamond honing machine that substantially reduces running costs and gives better results.
"The HP6A power stroking automatic honing machine is our newest product. It runs with diamond abrasives and has a programmable load control for both rough honing and finish honing. When it is finishing the cylinder, it automatically reduces the load because some cylinders have very thin areas that may distort if the load isn’t changed. The HP6A has a base price of $23,900 and a fully equipped unit goes for $28,000 to $35,000."
Plateau Finish Is Best
Regardless of what type of honing equipment or abrasives are used to finish cylinder bores, more and more shops are finding a plateau finish provides the ultimate finish.
A plateau finish is one that closely resembles a broken-in cylinder bore. When the bore is honed, the surface of the metal will have microscopic peaks and valleys. Peaks don’t provide much ring support, so as soon as the engine is started the piston rings start to scrub up and down and shear off the tallest peaks. As the engine continues to run, the peaks will be gradually shaved down until the cylinder bores are relatively smooth and flat (except for the valleys in the crosshatch that must be there to hold oil).
The normal engine break-in procedure will eventually produce a plateau finish anyway. But until it does, the rings and cylinders will experience unnecessary wear and the engine will experience increased blowby, oil consumption and emissions until the rings have seated – which might take several hundred or even several thousand miles to complete.
A better approach is to precondition the bore surface so the rings don’t have to "hone" the cylinders. A plateau finish will provide maximum compression right from the start, and eliminate most ring seating and sealing problems.
One recipe for achieving a plateau finish is to bore or hone to within .003˝ of final size. Then finish to final dimensions with a #220 or #280 grit conventional abrasive and follow up with half a dozen strokes of a #600 grit stone, cork, or a flexible brush or nylon bristle plateau honing tool.
If diamond stones are used, bore or rough hone to within .005˝ of final size. Then hone the cylinder to final dimensions with #325 to #500 grit diamonds, followed by six to eight strokes with a flexible brush or plateau honing tool. Many experts recommend leaving a little extra metal in the bore for final finishing if diamonds have been used to rough hone the cylinder. This is because rough honing with diamond leaves a very rough finish (over 100 RA depending on the grit of stone used).
Honing Hard Materials
In recent years, Nikasil coatings have provided a challenge for engine builders. Nikasil is a hard coating of nickel and silicon carbide about .0025˝ to .003˝ thick that is applied to cylinder bores to improve wear resistance. Invented by the German firm Mahle, Nikasil was originally developed for the Mercedes Wankel rotary engine. It has been used by BMW and Porsche in some of their engines, and is also used in many chain saw engines, some motorcycle and marine engines, and even many NASCAR Winston Cup engines.
Goodson’s Jensen says PERs have had success honing Nikasil treated cylinders with diamond. But for smaller shops that have only portable honing equipment, you can’t exert enough pressure with diamond to hone Nikasil. The best advice here is to use #220 silicone carbine and just do a couple of strokes to deglaze the cylinder. If a cylinder has to be bored to oversize, cut it out with a boring bar and then hone in the usual manner to achieve the desired dimensions and finish.
Ed Kiebler of Winona Van Norman in Wichita, KS, says new harder coatings on cylinder walls are forcing shops to change to diamond honing and to upgrade their equipment.
"I see a lot of shops who are interested in diamond but who don’t fully realize what’s involved in the diamond honing process. Diamond takes a lot of pressure to cut. Some people use diamond on portable hones, but realistically you can’t get enough pressure to make the diamonds perform well. Having said that, I truly believe the new harder cylinder coating materials are going to force people to go to diamonds," says Kiebler.
"The two-cycle stuff is all Nikasil. Now the outboard engines are going to Nikasil, too. All the NASCAR Winston Cup shops are using Nikasil cylinders. If it’s good for NASCAR, it’s not going to be long before you start seeing it in OEM engines," Kiebler explains. "The time is coming when you’re going to have to use diamonds if you’re going to hone Nikasil cylinders."
Kiebler says all most shops do is slightly roughen Nikasil cylinders. "You don’t really remove much material. The Winston Cup shops are running some of these motors five races before they redo the cylinders. The Nikasil coating really extends ring life and cuts down on ring wear."
Dave Riley of Gehring L.P. in Farmington Hills, MI, a supplier of honing equipment to original equipment manufacturers, says almost all OEM internal combustion gasoline engines in North America today are being rough honed with diamond abrasives.
Riley says the OEM focus is on using water soluble synthetic honing coolants, which means diamond abrasives because vitrified conventional abrasives require honing oil. The other industry trend he sees is that cylinder bores are being respecified to smoother finishes.
"We’re talking 0.15 to 0.3 Ra finishes that are extremely smooth," says Riley. "They’re doing this to further reduce emissions. A lot of this is being driven by ring technology because rings can now survive in conditions that provide much less oil. However, in my opinion these new surface finish specifications are reaching the limits of technology."
One of the things that the OEMs do to achieve high quality bore finishes is to use computer numerically controlled (CNC) honing machines. The cutting speeds of these machines are 50 to 75 percent faster than what was used 10 years ago. Faster cutting speeds allows the abrasives to cut smoother, and finer abrasives can be used for a smoother finish without sacrificing cycle time.
Riley says there’s a dramatic difference in the amount of time the OEMs allow to hone a cylinder versus what a typical aftermarket engine builder or production engine rebuilder spends on the same process. He says OEMs typically spend only about 15 to 20 seconds to hone a bore with automated honing equipment. By comparison, it can take up to several minutes to manually hone a bore using a power honing machine.
"The OEM machines are completely automated and automatically control bore size and shape. They also measure and inspect 100 percent of the bores, and can sort by bore size if they run bore grades," he says.
"As the need to reproduce OEM finishes in the aftermarket grows, so too will the demand for honing equipment that can meet these specifications. This will obviously have an impact on honing costs," Riley explains. "We are developing a low cost, CNC-controlled single spindle honing machine for the aftermarket. The operator would load the block and the machine would automatically hone the bores to OEM tolerances."
Riley says Gehring also offers custom honing services for low volume engine prototype development and performance engines.
Cylinder bore quality plays a huge role in reducing friction and blowby for improved engine performance and durability. Better bore geometry also contributes to better sealing and more usable power. Riley says a lot of performance engine builders are hot honing their blocks to more accurately simulate actual running conditions. They also use torque plates when honing (some with simulated manifolds to further stress the block), and may even bolt a bellhousing to the block to reproduce the stresses and loads the block will experience in a vehicle.
"For OEM production applications, we have developed clamping and other methods to stress the block while it is being honed," says Riley. This is done to further improve bore geometry and sealing.
Aluminum Engines Soon
Riley says another OEM trend is the development of future engines that use various types of bore surface coatings in aluminum blocks. The coatings are sprayed-on powder metal or steel wire alloys that create the surface characteristics of a traditional iron bore.
"Last year, about 15 percent of the prototype engines we saw had some type of coated aluminum bores. This year, the percentage is up to 67 percent. So there has been a dramatic shift toward aluminum blocks with coated bores."
Coated aluminum bores have a number of advantages, one of which is better thermal conductivity between the cylinders and water jacket. Another is less heat distortion for better sealing. The coating provides wear resistance and allows the use of larger bores within a given block size for more total displacement.
Riley says the OEMs are currently acid etching the bores to finish them. But acid is environmentally unfriendly so the OEMs are developing alternative ways to finish coated aluminum bores that do not require acid etching. Diamond honing is used for roughing, but the finishing step is being done with nonmetallic bonded abrasives such as vitrified abrasives, rubber or brushes. The goal is to come up with a process that will work using water-based honing fluid.
How will the aftermarket refinish coated bores in aluminum engines? Riley says the most likely approach will be to hone away the original bore finish, then reapply the surface coating and refinish it back to OEM specifications.
A number of years ago, Gehring developed a unique process called "laser structuring" to enhance engine durability. The process uses a laser to burn small pits into areas of the cylinder bore surface where ring loading and wear are highest. The pits improve oil retention and ring lubrication, and significantly reduces ring and bore wear.
Riley says the new laser structuring process is now being used in Europe on diesel engines. "At 150,000 kilometers, the bores are showing almost no measurable wear (only 1 to 2 microns) and the emissions performance is the same as new," he says.
Riley says the laser structuring process can be used to create almost any kind of pattern imaginable in the bore surface. Typically, a series of dots or dashes 25 to 60 microns deep and 40 microns wide are burned into the top third of the cylinder by the laser after the bore has been semi-finished. A final honing step is then done using fine stones to remove any buildup of material around the pits and to finish the bore.
The laser part of the process takes about 9 to 15 seconds per cylinder and uses a special machine that rotates and lowers the laser beam as it is projected onto the surface of each cylinder.
Riley says the laser structuring process is ideal for hard blocks or those with special surface coatings that make them difficult to finish with conventional honing techniques. "It’s a perfect application for high performance, diesel and aircraft engines," he says.
Remember To Clean The Bores
As we wrap up this article on honing abrasives, one final point to remember is the importance of cleaning the bores after honing. Honing leaves a lot of metallic and abrasive debris in the bores – which must be removed before the engine is assembled. Washing and scrubbing with warm soapy water will remove most of the loose debris. Some engine builders follow up by wiping out the cylinders with automatic transmission fluid. The point is get the cylinders clean so there are no contaminants to damage the rings or to get into the oil.
WHY DIAMONDS ARE SO EXPENSIVE
If you’ve balked at the high cost of diamond honing stones, here’s a brief explanation why they’re so expensive:
Diamond is a special form of carbon that is formed naturally under extreme heat and pressure deep inside the earth. As such, it isn’t very plentiful or easy to find. Subsequently, man-made synthetic diamonds are mostly used for industrial abrasives.
Scientists realized that if they could duplicate the heat and pressure that formed natural diamonds deep in the earth, they could transform ordinary graphite (another form of carbon) into diamond. They estimated it would require temperatures in excess of 6,300 degrees F and pressures of approximately one million pounds per square inch to make the transformation occur. But as the scientists discovered, it wasn’t so easy. Try as they might, they couldn’t get graphite to change its crystal structure and become diamond – until General Electric researchers discovered the secret in 1951.
A catalyst was needed to make the change happen. The catalyst turned out to be a mixture of molten iron, nickel and cobalt. The various proportions of ingredients in the catalyst are still a closely guarded secret, so only a couple of companies in the entire world have the expertise to produce synthetic diamonds. In the U.S., synthetic diamonds are produced at GE’s plant in Worthington, Ohio. Several years ago, we were given a plant tour – but nobody except a trusted few are allowed to see inside the room where the diamonds are actually made.
GE says they can create different types and sizes of synthetic diamond for various industrial purposes by varying the temperature, pressure and type of catalyst. Man-made diamonds typically have a yellowish tinge and are as small as grains of sand. Even so, they’re ideally suited for their intended use as an abrasive. They’re just as hard as natural diamonds and actually perform better because of their custom-tailored shapes and characteristics.