Print

Email

Honing Stones & Equipment

Cylinder honing is one of the key components in building any engine, whether it is a street stock budget remanufactured engine or an 900 horsepower all-out performance motor.

The finish on the cylinder walls is critical for proper piston ring lubrication and sealing. The bore geometry is also important, and must be round, cylindrical and straight for optimum sealing and minimum blowby.

The equipment used to hone cylinders has changed in recent years as the demand for better surface finishes, faster cycle times and lower production costs have driven the technology. Many engine builders have replaced their manual honing equipment with programmable automatic honing machines that have load-sensing controls. These machines provide the precision and repeatability to achieve today's higher quality cylinder bore finishes. When the load-sensor detects a high spot in a cylinder, the controls keep the hone head working in the same location until the high spot is gone. The ability to vary dwell in any part of the cylinder bore results in a rounder, straighter bore with better overall bore geometry.

Some of the newest honing machines with all the bells and whistles even have the capability to cycle the hone head from one cylinder to the next, and then from one bank to the next. Once the operator sets up the equipment and the numbers he wants, the machine does the rest. He doesn't have to baby-sit the equipment while the machine is honing the block.

DIAMONDS ARE FOREVER - WELL, ALMOST
Honing machines have also been upgraded to handle the latest generation of diamond honing stones. Diamond has become the material of choice for both high volume production engine rebuilders (PERs) as well as custom engine builders (CERs) and performance shops. The reason? Diamond honing stones cut faster, last up to 50X longer and leave a much more consistent bore finish than conventional vitrified abrasives such as silicon carbide and aluminum oxide. A set of diamond honing stones may cost up to 20X to 30X times as much as a set of conventional honing stones, but when their much greater longevity is factored in, diamonds cost less in the long run - and their consistency is much better regardless of any cost difference. Advantages such as these have won over more and more converts to diamond honing.

Diamond honing makes the most economic sense when an engine builder is working within the same range of bore dimensions on a series of engines. Because of the high initial cost of diamond stones, a custom engine builder who rebuilds anything and everything that comes in the door may not find it economical to buy diamond stones to fit a wide range of bore sizes. But if the majority of honing work he does is on engines with bores in the four-inch range (plus or minus a quarter inch), he can probably cover most of these applications with a single set of diamond stones.

The best estimates say that somewhere between 50 to 60 percent of all the cylinders that are honed today by aftermarket engine builders are being honed with diamond stones. That's a dramatic shift from a decade ago when diamonds where used almost exclusively by the OEM's in their new engine plants and by only the biggest PERs. Nowadays, almost everybody uses diamonds - even many of the die-hard performance engine builders who don't build a lot of engines in a year's time, but who require the tightest tolerances and highest quality bore finishes for their professional racing customers.

Diamond honing stones are available for most popular honing heads. But to maximize the benefits of diamond honing, the honing machine must be capable of handling higher loads. That means buying new equipment if you are serious about upgrading to diamond.

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. The trade-off is that diamond cuts differently than conventional abrasives 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.

Diamond is also good for rough honing cylinders to oversize because it can remove a lot of metal fast. Consequently, you can use a diamond hone in place of a boring bar. But rough honing takes more pressure and requires more horsepower from the honing machine. Because of this, diamond stones work best in equipment that has been designed to take maximum advantage of diamond's cutting properties. That's why honing machines that have been re-engineered for diamond stones typically have more rigid components and more powerful motors. This doesn't mean you can't use diamond stones in an older hone head or an older honing machine. But if your equipment can't handle the higher loads, you may not achieve the same degree of accuracy and repeatability as you could with equipment that has been specifically designed for diamond honing. Nor can you take advantage of the automation features that are available in the newest generation of honing machines.

BORE FINISH
Because of the way that diamond cuts metal, it tends to leave more torn and folded debris on the surface of the cylinder bore than a conventional abrasive. Consequently, a final finishing step is often required to remove this material and to leave a plateau finish in the bore.

OEMs have long favored a plateau finish in their cylinder bores because it allows the rings to seat almost instantly and extends ring life. By shaving the peaks off the bore surface, the bearing area that supports the piston rings is increased without reducing the valley area that is needed to retain oil for proper ring lubrication.

If the bores are not plateau finished with a final honing operation or finishing step, the rings will do the work instead. The scouring action of the rings will wear down the peaks in the bores, but it will also take a toll on the rings, shortening their ultimate service life. The metal that is worn off by the rings will also end up in the crankcase, can can contribute to wear elsewhere inside the engine. That's why many engine builders today favor the plateau honing process as the final step in finishing a cylinder.

A plateau finish can be achieved a variety of ways by using a two or three-step finishing process with conventional or diamond stones, by polishing the cylinders with a cork stone, or by finishing with a plateau honing tool or brush. Stroking the bores with a flexible abrasive brush (such as Brush Research "Flex-Hone" tool), or a plateau honing tool with abrasive embedded in nylon bristles (such as Sunnen's PHT tool) shears off the sharp peaks and significantly improve the surface finish without changing the bore dimensions.

The proper plateau honing technique can generally get the surface finish down to 8 to 12 microinches (roughness average or Ra), with relative peak height (RPK) numbers in the 5 to 15 range, and relative valley depth (RVK) numbers in the 15 to 30 range. This is well within the ideal range for most ring manufacturers. For stock and street performance engines with moly rings, an average surface finish of 15 to 20 Ra is typically recommended.

WHAT'S NEW IN ABRASIVES
Diamond honing stones as well as the honing machines that are designed to use them continue to evolve to meet the honing challenges posed by today's engines. Special abrasives are needed to hone performance engines that have hard liners, high nickel or silicone alloys, or hard facings such as "Nikasil" (Nikasil is a trademarked electrodeposited oleophilic nickel matrix silicium carbide coating). The surface hardness in a cylinder coated with Nikasil is about 90 HRc, and the thickness of the coating is only about 0.07 mm (.0025" to .003") thick. Consequently, you don't want to remove a lot of material when honing the cylinder. Nikasil retains oil well, so the bores can be honed to a super smooth 4 to 6 microinch finish to minimize friction.

Tim Meara of Sunnen Products Company, said Sunnen is constantly developing new abrasives to meet the specific needs of both PERs and CERs. Meara says by using different concentrations of diamond in the stone and changing the bond that holds it together, the cutting characteristics of the abrasive can be custom-tailored to suit the needs of almost any kind of engine or honing operation.

Dave Cox of Peterson Machine Tool said his company has just developed a brand new line of diamond abrasives to fit popular honing equipment such as "AN" style honing heads. The stones come in two holders with four stones total in the set. Cox said the new diamond stones can be used in place of conventional abrasives to take advantage of diamond's longer tool life, and to achieve more consistent bore finishes.

Winona Van Norman is also introducing a new line of diamond abrasives, according to Britton Harper. The stones are designed to work with existing hone heads, and will be offered in short and long lengths for 2.7 to 4.1 inch bore diameters, and 3.5 to 5.5 inch bore diameters. Harper says the new stones will work well with all applications, including Nikasil and other hard alloys. Grit sizes will include 100, 220 and 400. The stones can be used with conventional honing oils or water-based synthetic coolant, though coolant is recommended for best results.

Mike Miller of Brush Research says his company is currently developing a new line of diamond abrasives for their popular Flex-Hone tool as well as for plateau honing brushes. "Finer surface finishes are more important than ever before with low tension rings. Silicon carbide and aluminum oxide are fine for finishing the cylinders in most engines. But for harder materials such as Nikasil, you really need diamond," said Miller.

WHAT'S NEW IN HONING EQUIPMENT
Though honing has always been done on dedicated honing machines, Bates Technology in Indianapolis, IN has developed honing mandrels with an ISO-tapered tool holder that can be used in multi-purpose Computer controlled numeric (CNC) machining centers. The honing tool can be stowed in the tool carousel like any other tool, and loaded in the spindle when a honing operation is required. This allows one machine to do both milling and honing, eliminating the need to move the engine block from one machine to another. CNC honing lends itself to high volume production applications where cycle times must be kept to a minimum. Most applications of CNC honing are at the OEM level due to the cost of the tooling, but some aftermarket equipment suppliers are also interested in the process.

"Most of the development work that equipment suppliers are doing today is being done on CNC multi-purpose machining centers," said Dave Cox of Peterson Machine Tool. Our current CNC machine can do both boring and resurfacing, but not honing. We plan to add that capability by early 2008."

Ed Kiebler of Rottler Mfg., says everybody wants a diamond honing machine today because of the better bore finishes it can deliver. Rottler's HP6A Diamond Honing Machine was the first such machine to be engineered specifically for diamond honing, and was also the first to have automatic load sensing, which allows a constant load to be programmed into the unit and maintained throughout the entire honing cycle. Automatically controlling the load allows the machine to compensate for thin areas in the block that can cause the cylinder to distort. The end result is better bore geometry and better ring sealing.

The HP6A's list of automatic features include programmable roughing and finishing load sensing, automatic feed-out, automatic controlled stock removal with .0002" (.01mm) resolution, automatic plateau finish program, automatic lower bore short stroke or dwell finish program, and automatic brush finish program. The machine also has infinitely variable stroke speed control and spindle speed control. Kiebler said the HP6A can hone nickel/carbide cylinders and liners with ease.

Sunnen's CK-10 honing machine is no longer in production, having been replaced by the SV-10 cylinder hone several years ago. The SV-10 has two motors, one for the spindle and one for the stroker, and is designed to be used with traditional tools as well as Sunnen's DH-series diamond hone head. The DH-series tool has four slots and can hold up to 16 stones, though it can also be used with just one or two stones per holder.

The automated controls on the SV-10 includes a full-bore profile that shows a real time graphical display of the bore cross section. This allows the hone head to dwell in any area of the cylinder bore that needs additional honing to improve bore geometry. Variable spindle speed and stroke speed provides the flexibility to create any required crosshatch finish.

Meara says a new automated capability that will be offered on Sunnen's SV-10 includes the ability to index the hone head from one cylinder bank to the next. The automated controls can cycle the hone from one cylinder to the next now, so adding the capability to reposition the hone head from one cylinder bank to the next on a V6 or V8 engine will save additional time and operator input.

Sunnen also has a SV-200 vertical honing machine (which replaces the previous CK-21 machine). The SV-200 is designed for medium to high volume production engine rebuilders, and can be fully automated for minimal operator input. A new SV-310 servo-driven ball stroke honing machine is also available for automated production.

Winona Van Norman's current honing machine is the PS2V, which can accommodate engine blocks up to 42 inches in length (big enough to handle many diesel blocks). The hone head handles bore sizes from 2.7 to 5.5 inches in diameter, and from 1.5 up to 7.0 inches with optional tooling. The infinitely variable hone speed and stroke rate allow the machine to reproduce virtually any crosshatch finish. The PS2V uses mechanical drive systems for the power stroke and hone head, which do not slow down or slip as the load increases. Variable speed motors deliver power to the moving components through gear reduction boxes.

Solutions For Common Honing Problems (With Conventional Abrasives)
Courtesy of Goodson Tools & Supplies

PROBLEM: Hone is not cutting
If stones are loading, you need to apply more honing oil or lubricant to properly flush stones. Filtered oil or lubricant is preferred.

Make sure that your guide blocks are not too tight. Honing stones should contact the cylinder first. You should put the hone in the cylinder and then expand. The guide blocks should have about .010" to .030" clearance.

Try narrowing your stones, this will put more pressure on the abrasive. Use an old file or dressing stone to narrow the face of the hone stone. Along the entire length of the stone, take approximately 1/3 off the leading edge. This produces more effective pressure on the stone without changing the pressure setting of the hone.

PROBLEM: Hone is chattering
To eliminate chatter, try varying the stroke rate as you hone. Try some of the tips listed for "hone is not cutting" as these will also help overcome chatter.

PROBLEM: Stones and guides are wearing unevenly
This is most commonly the result of light honing pressure. To remedy this problem, increase pressure and vary stroke rate momentarily.

PROBLEM: Taper in cylinders with limited access
Taper will develop while honing a cylinder that doesn't permit equal overstroking at both ends. There is a tendency for the hone to dwell at the open end of the cylinder and remove too much material. Dwelling at the web end is not a solution because it removes too much material from the center and causes a barrel-shaped cylinder. Goodson offers special honing stones that prevent taper in these cylinders. They have shorter stones that exert more pressure at the bottom and are specially designed for use in short or blind holes.

PROBLEM: Bore distortion
Use torque plates when honing lightweight blocks. When the torque plate is bolted to the block, it will simulate the distortion caused by the cylinder head bolts. The end result will be rounder bores in the assembled engine with minimal distortion and blowby.