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
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
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.
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
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
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,"
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
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
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
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
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.