When an engine is rebuilt, the cylinders usuallyneed attention. Wear tends to create taper in the upper part ofthe cylinder that can reduce ring sealing and increase blowbyand oil consumption if not removed. The cylinder may also be outof round, scored or have other damage that requires correctingbefore a new set of rings will seal properly.
The main objective when refinishing the cylindersis to make the walls as straight as possible (no taper), the boresas round as possible (minimal distortion, which is especiallyimportant with today’s low tension rings), provide the right amountof crosshatch for good oil retention and ring support, and producea surface finish that meets the requirements of the rings. Thisis done by boring and/or honing the cylinders in one or severalsteps with various types of abrasives (vitrified or diamond).
After honing, the cylinders need to be cleanedto ensure removal of residual abrasive and metallic debris that’sleft in the bores. Washing and scrubbing with warm soapy waterwill remove most of the unwanted material. But washing alone doesnot loosen or remove surface "swarf" such as torn orfolded metal that can wear rings and delay ring seating. The onlyway 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 cleanthe bores, but can also plateau the surface depending on the characteristicsof the abrasive used. Brushing sweeps away the torn and foldedmetal as well as the sharp, jagged peaks, leaving a much smoothersurface. The result is a better bore finish with little extraeffort. 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 finishis that it preconditions or breaks-in the cylinders. Some saythis reduces the time it takes to seat a new set of rings as wellas initial ring wear, blowby and oil consumption. Others feelit may actually increase the time required to seat new rings.The engine delivers good compression right away, there’s no bluesmoke in the exhaust, emissions and oil consumption are reduced,and the rings last longer because they haven’t had to wear toconform to the bores.
A plateaued surface also provides increasedbearing area to support the rings while retaining enough depthin the crosshatch for good oil retention and lubrication. That’swhy the original equipment engine manufacturers (OEMs) favor thistype of bore finish and use it in many new engines.
One of the concerns expressed by OEMs who haveengine reman programs is that many aftermarket engine rebuildersmay not have the know-how or right type of honing equipment toreproduce an OEM type of cylinder bore finish. With emissionstesting a fact of life for many motorists in many parts of thecountry, the worry is that a rebuilt engine with cylinders honedthe "usual way" may not pass an emissions test. Thechallenge here is to develop procedures that allow aftermarketengine rebuilders to duplicate an OEM bore finish.
Ring manufacturers are also concerned thatsome engine rebuilders may not be using the proper honing proceduresor stones for their rings. Too rough a bore finish will producea lot of scrubbing when the engine is initially fired up. Withprelapped rings, this isn’t good because it creates unnecessarywear. The challenge here is to use honing procedures that producethe best possible bore finish for a given set of rings.
Most ring manufacturers specify a #220 grithoning abrasive for finishing the bores when using cast iron orchrome rings because the recommended bore finish for these ringsis 28 to 35 RA (roughness average in micro inches). A #280 gritstone is generally recommended for moly rings because moly ringslike a somewhat smoother finish of 16 to 23 RA. But these recommendationsare 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 finalhoning step to achieve an RA finish in the desired range. Onemanufacturer we spoke with said a 500 to 550 grit diamond honingstone will produce a surface finish in the 13 to 15 RA range.
To add to the confusion over which honing stonesmay be required to produce a certain kind of finish, some vitrifiedhoning stones with identical grit ratings will produce differentfinishes that may not always agree with the reference charts.
For example, one #220 grit vitrified stonemay produce a surface finish of 28 to 35 RA while another mayleave a much rougher finish. Differences in actual surface finishcan be due to the grading of the abrasive particles, as well asthe type and quality of lubricant used during the honing process.
The third challenge is profitability. Cylinderbore refinishing is a time consuming and expensive step in theengine rebuilding process. So anything that can be done to reducehoning costs and streamline the procedure while also improvingthe bore finish is a step in the right direction.
Conventional versus diamond
Many shops bore or rough hone cylinders towithin .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 shopsdo not have a profilometer to measure surface finish parameterssuch as RA, RK (core roughness), RPK (average peak height) andRVK (average valley depth), bearing area and so on, so they relyon 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 shopsto know if the bore finish actually meets the requirements ofthe ring manufacturer or the OEM – unless a customer complainsabout excessive ring wear, blowby or oil consumption. But evenif you haven’t experienced any ring problems, it doesn’t necessarilymean the cylinders are as good as they could or should be.
One of the limitations of vitrified abrasivesis that they wear rapidly. Depending on the grade of stones andthe hardness of the block, a set of vitrified honing stones mightdo 30 V8 blocks (240-260 cylinder bores) before they’re worn outand have to be replaced. And with each cylinder that’s honed,the operator or equipment must compensate for stone wear to keepthe bores straight. If you fail to compensate, you can end upwith taper in the bores.
By comparison, metal bond diamond honing stoneswear very little. A set of diamond honing stones might do 300V8 engine blocks (2,400 cylinder bores) before they have to bereplaced. The slower wear rate means the stones tend to cut straighter(less taper) than with vitrified stones.
The slower wear rate of diamond versus vitrifiedabrasives helps to more than offset the much higher initial costof diamond stones. Using the above figures, a set of $7 vitrifiedhoning stones cost about $.02 per hole if they do 240-260 cylinderbores. A $700 set of diamond honing stones cost about $.06 perhole if the set does 12,000 cylinder bores. The abrasive costper hole with diamond may be higher than that for vitrified abrasives,but with diamonds the operator doesn’t have to stop and restartwhile honing so labor costs are less.
For these reasons, many production engine remanufacturers(PERs) have switched to diamond honing. Diamond lowers their overallcosts, saves labor (fewer stone changes), and gives better overallbore geometry (straighter with less distortion).
"We’ve had excellent results with diamondhoning," said Tom Wilson of Recon Automotive Remanufacturers,a large PER based in Philadelphia, PA.
"The type of stones we use is dictatedby the type of rings that are going into a motor," said Wilson."We tried various stones before we came up with the bestcombination. For an RA of 20 to 25 with moly rings, we use #325grit diamond stones. For a finer finish in the 15 to 20 RA range,we sometimes use #500 grit diamond stones.
"Diamond cuts differently from vitrifiedabrasives. It rips the metal out and leaves a lot of microscopicfuzz on the surface," said Wilson. "So after honing,we brush the bores with a hand drill eight to 10 strokes. Brushingdoes a good job of cleaning the debris off the surface and eliminatesany 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 syntheticlubricant with the diamond stones, which he said runs "clearas water." He said the lubricant is filtered to take outthe dirt, and monitored constantly to prevent any bacteria growth.
Franklin Power Products, Inc.
"As an OEM supplier to Navistar, the onlyway we can meet their cylinder bore specifications is to followa three-step diamond honing procedure," said Jim Ormsby ofFranklin Power Products, Inc., Franklin, IN.
Ormsby said Franklin Power Products will firstrough 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 eightstrokes with a plateau honing tool (PHT).
"We believe we actually get a better finishand maintain closer tolerances than the original OEM bores becausewe pay close attention to every bore we do," Ormsby said."OEMs are not set up that way. They turn on a productionline and let it go."
Ormsby added it’s easy to be consistent withdiamond honing – provided you have the proper equipment.
Trends & recommendations
Several honing equipment suppliers we interviewedsaid the trend today is towards diamond honing. Andy Rottler ofRottler Manufacturing, Kent, WA, estimates that about 80% of thenew honing machines he’s selling are equipped with diamond stonesor are soon converted from vitrified stones to diamond once operatingin the field.
"It used to be PERs were the only oneswho would buy diamond honing equipment," said Rottler. "Butlately the smaller shops are buying it too. More and more peopleare switching to diamond because it’s less expensive over thelong term, costing about one-fifth as much as vitrified abrasives– as long as you don’t break a stone. Diamond also gives a moreconsistent bore finish and better bore geometry. But to maximizethe benefits of diamond, you need a rigid hone head.
"Vitrified stones never wear at a consistentrate," Rottler continued. "Wear can vary with the gradeof stones and the hardness of the block. It’s hard to predicthow much metal the stones are actually removing, so you have tostop your equipment, measure the bore size, then restart the machineto finish the cylinder. With diamond, you can set up your equipment,turn it on and walk away. It will (automatically) hone it to theright size."
Rottler said there is no set procedure forhoning with diamond. Procedures vary from one application to another,and from one rebuilder to another. Many use a three-step procedurethat starts with rough honing with an aggressive grit to withina 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," saidRottler. "And if you don’t take care of the stones, theycan leave a lot of torn metal on the surface."
Rottler said brushing isn’t absolutely necessarywhen honing with diamond provided you use the correct load onthe stones. The load factor will vary from one manufacturer’sequipment to another, but generally the finish load should bein the 30% to 35% range. For roughing, use maximum load for rapidmetal removal.
Another plus with diamond according to Rottleris that a water-based synthetic lubricant eliminates heat as afactor, which reduces bore distortion. "It also doesn’t stinklike mineral-based honing oils," he said.
Lyle Haley of Peterson Machine Tool, Inc.,Shawnee Mission, KS, said the coolant is a critical factor inusing diamonds. "If the coolant mixture is off, it can affectthe finish of the bores," said Haley. "As a rule, theconcentration 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 andto get better results, not necessarily just to reduce honing time.
"Diamond requires a lot of pressure tobreak down and cut properly, so to get the most from it you needequipment that’s designed for diamond," said Downs. "Someolder honing machines may not work well with diamond. Newer honingequipment typically has higher horsepower ratings and more rigiddesigns to hone with diamond.
"With a lot of pressure and a multi-stonehone head, you can remove .008" to .010" of stock perminute with diamond," explained Downs. "The greaterthe number of stones in the head, the less the pressure requiredto achieve a specific rate of stock removal."
Downs said comparing grit sizes between diamondand vitrified abrasives can be misleading. With diamond, the gritrating is actually a concentration of diamond in the stone. Thehigher the concentration, the finer the rating. A diamond stonewith a 35% concentration would be a rough honing stone and wouldcut similar to an #80 grit vitrified abrasive, he said. A stonewith a 65% to 70% diamond would cut similar to a #325 grit vitrifiedabrasive.
Mark Henson of LDX Genesis, Cedar Rapids, IA,said their new honing machine was designed from the ground upto work with diamond. The machine uses constant head pressure(accomplished electronically with control software) to optimizethe 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 surfacefinishes in the 13 to 15 RA range without brushing, or 6 to 8RA with brushing."
Skip Green at Winona Van Norman, a divisionof D & S Manufacturing Co., Inc., Black River Falls, WI, reportedthat although they’ve received a lot of inquiries about diamondhoning, most of the aftermarket is still using vitrified abrasivesñ except for PERs. "It takes a honing machine builtfor heavy-duty use to handle diamonds," Green said.
"Many shops are using a plateau finishingprocedure, but are doing it with vitrified stones and a brush.The typical shop has to work on such a wide variety of enginesand 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 diamondhoning because diamonds can leave torn and fragmented metal onthe surface.
"Diamond is a dull particle and is bondedso tightly that it takes a lot of pressure to hone a bore,"explained Kiebler. "Consequently, it tears up the surfaceand needs to be followed up with a PHT (plateau honing tool) toclean away the debris. If you’re going to use diamond, we recommenda two-step honing procedure that uses a brush to clean the cylindersafter honing. We recommend using a brush in a hone head fixtureto put a controlled amount of pressure on the brush."
Kiebler said Sunnen recently conducted a seriesof tests for a ring manufacturer to see which honing procedureproduced a bore finish that most closely matched their requirements.The ring manufacturer’s bore finish requirements for a one-stephoning procedure are 10 to 20 RA, 40 to 60 RK, 10 to 20 RPK and35 to 65 RVK, or 10 to 20 RA, 30 to 50 RK, 5 to 20 RPK and 50to 100 RVK for a two-step procedure.
One cylinder was honed using a one-step procedurewith #500 grit diamond only (no brushing). The results were RA13, RK 40, RPK 18 and RVK 17. The results were not consideredsatisfactory because of the low RVK number (which reduces thecylinder wall’s ability to hold and retain oil for proper ringlubrication).
A second cylinder was honed using a two-stepprocedure: #220 grit diamond followed with a #120 grit plateauhoning tool (brush). The results were 20 RA, 44 RK, 16 RPK and66 RVK. These results were considered well within the ring manufacturer’srequirements.
A third cylinder was honed using #400 gritdiamond, followed by brushing with a #320 grit plateau honingtool. The results were RA 13, RK 35, RPK 11 and RVK 33. The resultswere again outside the ring manufacturer’s specifications becausethe RVK valve of 33 was too low.
"Our tests show that you can get the samebore finish with #500 grit diamond as you can with #320 grit vitrifiedabrasive, 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 honingthan with vitrified stones (.003"). Rough honing with a #80to #100 diamond will leave a surface finish that’s over 100 RA.Rough honing with #80 grit vitrified abrasive will leave a surfacefinish of around 60 RA.
"As for speed, diamond honing can be justas fast as boring a cylinder," offered Kiebler. "Withan eight stone head at 450 rpm, 80% to 90% pressure, and 80 strokesper minute, you can achieve a stock removal rate of .020"per minute," he said.