Cylinder Bore Honing
When an engine is rebuilt, the cylinders usually need attention. Wear tends to create taper in the upper part of the cylinder that can reduce ring sealing and increase blowby and oil consumption if not removed. The cylinder may also be out of round, scored or have other damage that requires correcting before a new set of rings will seal properly.
The main objective when refinishing the cylinders is to make the walls as straight as possible (no taper), the bores as round as possible (minimal distortion, which is especially important with today's low tension rings), provide the right amount of crosshatch for good oil retention and ring support, and produce a surface finish that meets the requirements of the rings. This is done by boring and/or honing the cylinders in one or several steps with various types of abrasives (vitrified or diamond).
After honing, the cylinders need to be cleaned to ensure removal of residual abrasive and metallic debris that's left in the bores. Washing and scrubbing with warm soapy water will remove most of the unwanted material. But washing alone does not loosen or remove surface "swarf" such as torn or folded metal that can wear rings and delay ring seating. The only way to get rid of this material and smooth the bores is to "polish" the bores after honing with some type of flexible abrasive brush.
Brushing after honing not only helps clean the bores, but can also plateau the surface depending on the characteristics of the abrasive used. Brushing sweeps away the torn and folded metal as well as the sharp, jagged peaks, leaving a much smoother surface. The result is a better bore finish with little extra effort. Another way to plateau the surface is to use very fine #600 grit stones or cork to polish the bores after honing.
One of the advantages of a plateau bore finish is that it preconditions or breaks-in the cylinders. Some say this reduces the time it takes to seat a new set of rings as well as initial ring wear, blowby and oil consumption. Others feel it may actually increase the time required to seat new rings. The engine delivers good compression right away, there's no blue smoke in the exhaust, emissions and oil consumption are reduced, and the rings last longer because they haven't had to wear to conform to the bores.
A plateaued surface also provides increased
bearing area to support the rings while retaining enough depth
in the crosshatch for good oil retention and lubrication. That's
why the original equipment engine manufacturers (OEMs) favor this
type of bore finish and use it in many new engines.
One of the concerns expressed by OEMs who have engine reman programs is that many aftermarket engine rebuilders may not have the know-how or right type of honing equipment to reproduce an OEM type of cylinder bore finish. With emissions testing a fact of life for many motorists in many parts of the country, the worry is that a rebuilt engine with cylinders honed the "usual way" may not pass an emissions test. The challenge here is to develop procedures that allow aftermarket engine rebuilders to duplicate an OEM bore finish.
Ring manufacturers are also concerned that some engine rebuilders may not be using the proper honing procedures or stones for their rings. Too rough a bore finish will produce a lot of scrubbing when the engine is initially fired up. With prelapped rings, this isn't good because it creates unnecessary wear. The challenge here is to use honing procedures that produce the best possible bore finish for a given set of rings.
Most ring manufacturers specify a #220 grit honing abrasive for finishing the bores when using cast iron or chrome rings because the recommended bore finish for these rings is 28 to 35 RA (roughness average in micro inches). A #280 grit stone is generally recommended for moly rings because moly rings like a somewhat smoother finish of 16 to 23 RA. But these recommendations are for conventional vitrified abrasives, not diamond.
Diamond cuts differently from a vitrified stone, so higher numbers are generally required for an equivalent finish. A #325 to #550 grit diamond stone may be required for the final honing step to achieve an RA finish in the desired range. One manufacturer we spoke with said a 500 to 550 grit diamond honing stone will produce a surface finish in the 13 to 15 RA range.
To add to the confusion over which honing stones may be required to produce a certain kind of finish, some vitrified honing stones with identical grit ratings will produce different finishes that may not always agree with the reference charts.
For example, one #220 grit vitrified stone may produce a surface finish of 28 to 35 RA while another may leave a much rougher finish. Differences in actual surface finish can be due to the grading of the abrasive particles, as well as the type and quality of lubricant used during the honing process.
The third challenge is profitability. Cylinder
bore refinishing is a time consuming and expensive step in the
engine rebuilding process. So anything that can be done to reduce
honing costs and streamline the procedure while also improving
the bore finish is a step in the right direction.
Conventional versus diamond
Many shops bore or rough hone cylinders to within .003" of final oversize (.010" to .030" depending on the application), then finish hone the last .003" of the bore with #220 or #280 grit vitrified abrasives. Most shops do not have a profilometer to measure surface finish parameters such as RA, RK (core roughness), RPK (average peak height) and RVK (average valley depth), bearing area and so on, so they rely on stone grits along with the right honing pressure, head speed, stroke rate and lubricant to achieve the desired bore finish.
Consequently, there's no way for most shops to know if the bore finish actually meets the requirements of the ring manufacturer or the OEM - unless a customer complains about excessive ring wear, blowby or oil consumption. But even if you haven't experienced any ring problems, it doesn't necessarily mean the cylinders are as good as they could or should be.
One of the limitations of vitrified abrasives is that they wear rapidly. Depending on the grade of stones and the hardness of the block, a set of vitrified honing stones might do 30 V8 blocks (240-260 cylinder bores) before they're worn out and have to be replaced. And with each cylinder that's honed, the operator or equipment must compensate for stone wear to keep the bores straight. If you fail to compensate, you can end up with taper in the bores.
By comparison, metal bond diamond honing stones wear very little. A set of diamond honing stones might do 300 V8 engine blocks (2,400 cylinder bores) before they have to be replaced. The slower wear rate means the stones tend to cut straighter (less taper) than with vitrified stones.
The slower wear rate of diamond versus vitrified abrasives helps to more than offset the much higher initial cost of diamond stones. Using the above figures, a set of $7 vitrified honing stones cost about $.02 per hole if they do 240-260 cylinder bores. A $700 set of diamond honing stones cost about $.06 per hole if the set does 12,000 cylinder bores. The abrasive cost per hole with diamond may be higher than that for vitrified abrasives, but with diamonds the operator doesn't have to stop and restart while honing so labor costs are less.
For these reasons, many production engine remanufacturers
(PERs) have switched to diamond honing. Diamond lowers their overall
costs, saves labor (fewer stone changes), and gives better overall
bore geometry (straighter with less distortion).
"We've had excellent results with diamond honing," said Tom Wilson of Recon Automotive Remanufacturers, a large PER based in Philadelphia, PA.
"The type of stones we use is dictated by the type of rings that are going into a motor," said Wilson. "We tried various stones before we came up with the best combination. For an RA of 20 to 25 with moly rings, we use #325 grit diamond stones. For a finer finish in the 15 to 20 RA range, we sometimes use #500 grit diamond stones.
"Diamond cuts differently from vitrified abrasives. It rips the metal out and leaves a lot of microscopic fuzz on the surface," said Wilson. "So after honing, we brush the bores with a hand drill eight to 10 strokes. Brushing does a good job of cleaning the debris off the surface and eliminates any break-in period. We've also found that it improves the RA, too, getting it down around 18 or so."
Wilson said Recon uses a water-based synthetic
lubricant with the diamond stones, which he said runs "clear
as water." He said the lubricant is filtered to take out
the dirt, and monitored constantly to prevent any bacteria growth.
Franklin Power Products, Inc.
"As an OEM supplier to Navistar, the only way we can meet their cylinder bore specifications is to follow a three-step diamond honing procedure," said Jim Ormsby of Franklin Power Products, Inc., Franklin, IN.
Ormsby said Franklin Power Products will first rough hone to within .005" of final size with coarse grit #200 diamond stones. Then they final hone to size with fine grit #600 diamonds. The last step is to brush hone the bores eight strokes with a plateau honing tool (PHT).
"We believe we actually get a better finish and maintain closer tolerances than the original OEM bores because we pay close attention to every bore we do," Ormsby said. "OEMs are not set up that way. They turn on a production line and let it go."
Ormsby added it's easy to be consistent with
diamond honing - provided you have the proper equipment.
Trends & recommendations
Several honing equipment suppliers we interviewed said the trend today is towards diamond honing. Andy Rottler of Rottler Manufacturing, Kent, WA, estimates that about 80% of the new honing machines he's selling are equipped with diamond stones or are soon converted from vitrified stones to diamond once operating in the field.
"It used to be PERs were the only ones who would buy diamond honing equipment," said Rottler. "But lately the smaller shops are buying it too. More and more people are switching to diamond because it's less expensive over the long term, costing about one-fifth as much as vitrified abrasives - as long as you don't break a stone. Diamond also gives a more consistent bore finish and better bore geometry. But to maximize the benefits of diamond, you need a rigid hone head.
"Vitrified stones never wear at a consistent rate," Rottler continued. "Wear can vary with the grade of stones and the hardness of the block. It's hard to predict how much metal the stones are actually removing, so you have to stop your equipment, measure the bore size, then restart the machine to finish the cylinder. With diamond, you can set up your equipment, turn it on and walk away. It will (automatically) hone it to the right size."
Rottler said there is no set procedure for honing with diamond. Procedures vary from one application to another, and from one rebuilder to another. Many use a three-step procedure that starts with rough honing with an aggressive grit to within a few thousandths of final size, finish honing with a fine grit (#325 to #550), then brushing to clean and smooth the bores.
"If you don't use a diamond properly, you can end up with a lot of smeared and folded metal," said Rottler. "And if you don't take care of the stones, they can leave a lot of torn metal on the surface."
Rottler said brushing isn't absolutely necessary when honing with diamond provided you use the correct load on the stones. The load factor will vary from one manufacturer's equipment to another, but generally the finish load should be in the 30% to 35% range. For roughing, use maximum load for rapid metal removal.
Another plus with diamond according to Rottler is that a water-based synthetic lubricant eliminates heat as a factor, which reduces bore distortion. "It also doesn't stink like mineral-based honing oils," he said.
Lyle Haley of Peterson Machine Tool, Inc., Shawnee Mission, KS, said the coolant is a critical factor in using diamonds. "If the coolant mixture is off, it can affect the finish of the bores," said Haley. "As a rule, the concentration of a water-based synthetic should be 1-1/2% to 2%. A refractometer should be used to monitor the concentration."
Chuck Downs of Kwik-Way Products, Marion, IA, said a lot of people are looking at diamond to save money and to get better results, not necessarily just to reduce honing time.
"Diamond requires a lot of pressure to break down and cut properly, so to get the most from it you need equipment that's designed for diamond," said Downs. "Some older honing machines may not work well with diamond. Newer honing equipment typically has higher horsepower ratings and more rigid designs to hone with diamond.
"With a lot of pressure and a multi-stone hone head, you can remove .008" to .010" of stock per minute with diamond," explained Downs. "The greater the number of stones in the head, the less the pressure required to achieve a specific rate of stock removal."
Downs said comparing grit sizes between diamond and vitrified abrasives can be misleading. With diamond, the grit rating is actually a concentration of diamond in the stone. The higher the concentration, the finer the rating. A diamond stone with a 35% concentration would be a rough honing stone and would cut similar to an #80 grit vitrified abrasive, he said. A stone with a 65% to 70% diamond would cut similar to a #325 grit vitrified abrasive.
Mark Henson of LDX Genesis, Cedar Rapids, IA, said their new honing machine was designed from the ground up to work with diamond. The machine uses constant head pressure (accomplished electronically with control software) to optimize the cutting action of diamond stones in various engine applications.
"We recommend rough diamond honing with #70 to #90 grit stones to within .002" of final bore size, then finishing to size with #500 to #550 grit diamond stones, followed by 10 seconds of brushing to clean the surface," said Henson. "With this procedure, you can achieve surface finishes in the 13 to 15 RA range without brushing, or 6 to 8 RA with brushing."
Skip Green at Winona Van Norman, a division of D & S Manufacturing Co., Inc., Black River Falls, WI, reported that although they've received a lot of inquiries about diamond honing, most of the aftermarket is still using vitrified abrasives ñ except for PERs. "It takes a honing machine built for heavy-duty use to handle diamonds," Green said.
"Many shops are using a plateau finishing
procedure, but are doing it with vitrified stones and a brush.
The typical shop has to work on such a wide variety of engines
and bore sizes that diamonds are too expansive," said Green.
Ed Kiebler of Sunnen Products Co., St. Louis, MO, said some ring manufacturers have not been in favor of diamond honing because diamonds can leave torn and fragmented metal on the surface.
"Diamond is a dull particle and is bonded so tightly that it takes a lot of pressure to hone a bore," explained Kiebler. "Consequently, it tears up the surface and needs to be followed up with a PHT (plateau honing tool) to clean away the debris. If you're going to use diamond, we recommend a two-step honing procedure that uses a brush to clean the cylinders after honing. We recommend using a brush in a hone head fixture to put a controlled amount of pressure on the brush."
Kiebler said Sunnen recently conducted a series of tests for a ring manufacturer to see which honing procedure produced a bore finish that most closely matched their requirements. The ring manufacturer's bore finish requirements for a one-step honing procedure are 10 to 20 RA, 40 to 60 RK, 10 to 20 RPK and 35 to 65 RVK, or 10 to 20 RA, 30 to 50 RK, 5 to 20 RPK and 50 to 100 RVK for a two-step procedure.
One cylinder was honed using a one-step procedure with #500 grit diamond only (no brushing). The results were RA 13, RK 40, RPK 18 and RVK 17. The results were not considered satisfactory because of the low RVK number (which reduces the cylinder wall's ability to hold and retain oil for proper ring lubrication).
A second cylinder was honed using a two-step procedure: #220 grit diamond followed with a #120 grit plateau honing tool (brush). The results were 20 RA, 44 RK, 16 RPK and 66 RVK. These results were considered well within the ring manufacturer's requirements.
A third cylinder was honed using #400 grit diamond, followed by brushing with a #320 grit plateau honing tool. The results were RA 13, RK 35, RPK 11 and RVK 33. The results were again outside the ring manufacturer's specifications because the RVK valve of 33 was too low.
"Our tests show that you can get the same bore finish with #500 grit diamond as you can with #320 grit vitrified abrasive, provided you follow up with a brush," said Kiebler. "We also think that if you're rough honing with diamond, you should leave a little more metal (.005") for finish honing than with vitrified stones (.003"). Rough honing with a #80 to #100 diamond will leave a surface finish that's over 100 RA. Rough honing with #80 grit vitrified abrasive will leave a surface finish of around 60 RA.
"As for speed, diamond honing can be just as fast as boring a cylinder," offered Kiebler. "With an eight stone head at 450 rpm, 80% to 90% pressure, and 80 strokes per minute, you can achieve a stock removal rate of .020" per minute," he said.