There’s little argument today that a plateau cylinder bore finish provides the best surface for a new set of piston rings. A plateau finish is one that is relatively smooth and has lots of bearing area to support the rings, but also has enough crosshatch depth to retain oil and provide good ring lubrication, too.
A plateau finish essentially duplicates a broken-in cylinder. In the old days, cylinders were honed to size, and the rings did the final finishing of the bore surface. But this required a lot of break-in time and shortened the life of the rings. With today’s thin blocks, low tension piston rings, and coated prelapped rings, the cylinders need to be as close to a broken-in condition as possible when the engine is first fired up, otherwise the engine may use oil and never seal properly.
So what’s the best way to achieve a plateau finish? We asked a number of people in the industry and got a variety of answers. The bottom line is there is no single recipe but a number of different processes that can be used depending on the engine’s requirements and the type of honing equipment that’s being used.
Scott Gabrielson, a ring engineer with Federal-Mogul, says he’s very much in favor of plateau finishing because that’s what a ring does to the bore finish when it breaks it in. "The closer we can get the bore to what the ring likes to run on, the less the ring will wear during break-in and the longer the rings will last."
Gabrielson says plateau finishing should be at least a two-step process: a rough honing step followed by a smooth finishing step.
"We are getting close to recommending one bore finish for all types of rings. All our plasma-moly and chrome rings are lapped at the factory so the rings themselves do not need to be broken-in to get them to seal. We recommend honing the cylinders with a #280 grit stone, then touching them with #400 grit stone or an abrasive nylon brush honing tool to plateau the surface."
What about diamond honing? Gabrielson says diamond stones are fast and last forever. But diamonds 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. This could be a finer grit diamond, a fine grit vitrified abrasive or a brush.
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.
"The bores have to be straight and round," said Gabrielson. "Make sure you keep the Ra finishes within factory specifications, too, which is typically in the 10 to 15 Ra range on many late model engines. Also, use a premium ring set. Don’t go with a cheap ring set.
"The typical ring set today has a 1.2 mm top ring, a 1.5 mm second ring and a 3.0 mm oil ring. Because of shorter deck heights and longer rods, there’s not a lot of room left on many pistons today for the ring set. So, if you’re replacing pistons, try to use ones that accept the widest possible rings to maximize ring life. The wider the rings, the longer they will wear."
Gabrielson says replacement rings should always be the same material or better as the original. Engine builders are still using a lot of cast iron rings in older "economy" blocks, but many newer engines use ductile iron or steel rings because of the higher temperatures and loads they encounter. As far as bore finish is concerned, steel rings are really no different than cast rings. Both types live best with a plateau finish.
John Scott with Perfect Circle/Dana Corp. says everybody wants a simple, one page set of instructions that tells them how to produce a perfect cylinder bore finish – but there isn't one because every engine application is different. Passenger car engines have different requirements than light trucks or heavy-duty trucks.
"What’s more important than bore finish today," says Scott, "is bore geometry. With low tension oil rings, you can get into big trouble fast if you have bore distortion problems."
Scott says Ra numbers for bore finish don't mean that much anymore because most engine builders don't have the right kind of equipment to measure surface finish. "We like to see Ra down around 10 or less, but we also want to see certain valley depth and peak height numbers that indicate a good surface for the rings. So to do it right, you need equipment that can measure all the surface finish perimeters."
Smoother is Better
I’ve always been an advocate of a fine surface finish," said Lyle Haley of Peterson Machine Tool. "When we first got involved with honing brushes some time ago, we did a lot of testing with fax film to see how brushing was changing the surface. We quickly realized brushing was a real advantage because it removed the loose hinged material and burrs left on the surface after honing. It didn’t make any difference how fine a honing stone you used, it still left debris. And if you went too fine, all you did was push the junk into the surface.
"The bottom line is if you don’t take this debris out, the rings will (take the debris out) and that will shorten the life of the rings. Rings today are sort of like the new MLS head gaskets. They want a surface that’s as smooth as the top of my head. You can do the best job of honing in the world, but if you don’t end up with the right surface finish, you’ll never see the ring life you should."
Haley says there’s no excuse for building an engine that burns oil or doesn’t have good compression. To get a good bore finish, you need to use common sense and do the following:
- Use a good bore gauge.
- Be careful to hone straight.
- Try to get bore geometry within .0005˝ or less of being round and straight, ideally within .0002˝ to .0003˝ of specifications.
- Brush the cylinders after honing with a good quality soft brush tool to clean away the torn and folded metal debris. The type of honing stones and finishing procedure you should use depends on the application. Haley says #280 grit stones are a good choice for most applications, but are a little on the coarse side and by themselves won’t give you a plateau finish. "Personally, I’d use a #400 grit stone for at least .0004˝ to .0005˝ of stock removal – but not much more than that or you’ll be down to base metal. I’d then plateau the surface with a soft brush tool to finish the job.”
Haley says thoroughly cleaning the block after it has been honed and plateaued is just as important as the honing process itself. He says the usual method of scrubbing out the cylinders with hot soapy water still leaves some material in the valleys of the crosshatch that can eventually end up in the rings.
After the cylinders have been washed, you should go in with some ATF or protectant and wipe out the cylinders. This will lift any remaining material out of the pores so you end up with a much cleaner surface.
Some of these late model engines with moly-coated pistons have almost no piston-to-bore clearance. The coating keeps the pistons from seizing and reduces piston noise when the engine is cold. Coated pistons don’t seem to need much lubrication, but they do require a smooth finish, good bore geometry and a clean bore surface. "If you don't have that, you’re going to have problems," said Haley.
Diamonds Are Forever
Mart Jeltema in K-Line’s technical services department says the type of plateau finishing procedure he recommends to customers depends on what they’re trying to accomplish and the type of equipment they’re using. What kind of honing machine are they using or are they honing by hand? What Rafinish are they trying to achieve, and what kind of finish are they getting before they attempt to plateau the cylinders?
To plateau the finish after honing, Jeltema recommends using a brush of some sort, either the rigid style that mounts in the honing head holders or a spaghetti style bristle brush in a hand-hone. He says 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.
"A lot of people are using diamond honing heads, even some racers, because diamonds give you better bore geometry. You don’t have to chase the taper like you do with regular vitrified abrasive stones that break down. Once a new set of diamonds are broken-in, they stay true."
Jeltema says K-Line has five different hones for various bore sizes. The most common size is the KL-5075 that goes from 4.000˝ to 4.350˝. But, if a shop is doing a lot of different bore sizes, they’ll need several different hones.
Brushes or Stones?
Andy Rottler of Rottler Manufacturing is another advocate of plateau honing with diamonds. "A lot of people have been using brush tools to plateau the surface, but lately, we’ve seen a trend away from brushing. Some people are now using a #600 grit finishing diamond to plateau the bores. They may still use a brush tool for final cleaning, but not to plateau. Those who are using a two-step diamond process are honing with a #280 to #400 diamond, then plateauing the surface with #600 diamond stones. The initial finish is typically 25 to 38 Ra before finishing. After the second step, the surface finish is brought down to 16 to 22 Ra."
If you don’t have a high cost profilometer to analyze the surface finish, use a rough honing stone that produces a finish within a predictable range or can be measured with an inexpensive profilometer. Then plateau the finish with a fine abrasive to achieve the desired finish.
Rottler says that most people who are using diamonds today for rough honing are also using diamonds to finish the bores. "Racers were the last ones to switch over from vitrified abrasives. But now that they’ve made the change to diamond they’re very satisfied with the process."
Rottler warned that the honing procedures with diamond are not all the same. Because of differences in the type of bonding used in diamond stones, different manufacturers’ products behave and cut differently. "We use a very strong bond that gives long diamond life and excellent consistency in finish. But with our stones, it is necessary to dress the diamonds every 50 engines. It’s a simple thing to do but it is necessary to maintain a good finish.
"Diamonds have become popular because they produce a more accurate finish with less effort. But to take full advantage of them, you need a machine that’s designed to use diamonds. If you don’t have a machine that has control systems in it, you won’t be able to take full advantage of the diamond process. With our machine, it’s a walk-away process. You set up the first cylinder, hit the start button and that’s it. The only time any manual intervention is needed is if the block has some obstructions at the bottom of the cylinders that interfere with the honing overstroke."
The type of coolant used when honing with diamond is also very important. Coolant eliminates the problem of heat build-up in the engine when it is being honed, and it improves the dimensional stability of the bores. Rottler recommends a synthetic coolant, but says it is very important to maintain the mix within certain limits.
If the coolant is too rich, it can gum up the machine. If the coolant is too lean, you may get some rust and not have the proper stone lubrication. Coolant problems can be avoided by using a refractometer (costs $150 to $200) to monitor the strength of the coolant mixture.
Round, Straight...and Hot
Tim Meara of Sunnen Products Co., said 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 2-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 rebuilders are using #320 or #400 grit diamond stones today, followed by brushing using a #180 grit PHT tool."
Meara said Sunnen has recently introduced a new honing head for the CK21 that holds both diamond stones and brushes in the same tool. This allows a user to hone with diamond. The diamond stones then retract and the brushes extend to finish the cylinder without having to change anything.
One change Meara said he’s seen lately among some race engine builders is a desire 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’s hard to duplicate.
Meara said Sunnen has been evaluating and testing a new type of "hot hone" torque plate that allows hot coolant to be circulated through the block while it is being honed. The result, says Meara, is even better bore geometry and less distortion than with regular torque plates.
"You can’t get the same results by just heating up the block and honing it. You have to have the coolant flowing through it to simulate the kind of distortion that takes place inside a running engine." He says Sunnen has done an in-depth study of this process including dyno testing that shows hot honing actually produces an improvement in ring sealing and horsepower.
The latest twist in high tech cylinder resurfacing technology is "laser structuring." The process was developed by Gehring L.P. of Farmington Hills, MI. Gehring has provided plateau honing of prototype engines produced by major OEMs such as Ford, General Motors, Mercedes and many others for many years.
Laser structuring uses a high power laser to burn small grooves or pits into the surface of the cylinders to improve oil retention. The laser structuring, says Dave Riley of Gehring, improves ring sealing, reduces oil consumption up to 40 percent, reduces particle emissions 10 to 30 percent hydrocarbon emissions up to 20 percent, and extends ring life up to 50 percent compared to conventional honing and finishing procedures.
The laser is not used to hone the cylinder or change bore geometry but only to create a unique pattern of oil retaining pockets in the walls of the cylinder. Riley says the 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. This is the area where ring pressures and wear are the highest.
A final honing step is then done using fine stones to remove any build-up 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.
The cost for this procedure, according to Gehring’s Riley, is $500 to $750 per engine, which includes honing and the laser structuring.