OK, Ra is not a good indicator of actual surface
roughness so what is? As it turns out, an ISO standard
of Rk surface measurement parameters fills the bill
nicely. Rk attempts to segregate peaks, core roughness
and valleys. So, for the first time, you can see how
rough the bearing area of a surface is without much
mathematical interference from valleys (Rvk). All
together, they identify surfaces both good and bad. I
won't get into every parameter of Rk but we will look
at Rpk (peaks), Rk (core roughness. This is the area
of surface that will ultimately bear load.), Rvk
(valleys) and Vo (potential volume of oil retained in
the surface). Armed with values for these four
measurements, you can come very close to understanding
the most critical elements of a honed cylinder wall
surface.
Most of us have heard about plateau honing.
Essentially, this is a surface that is purposely honed
to greatly reduce peaks while leaving the deeper
valleys for oil retention. If done correctly, the
peaks are eliminated and core roughness is prepared
for maximum load bearing capability. This type of
surface demands pre-lapped ring faces because there
are no peaks left with which to carry out this ring
lapping task. Now think about what was just said. If
you leave a peaky surface and this surface is required
to finish lapping ring faces (hey, we need that light
tight seal if we are to maximize our compression and
suction efficiencies, yes?), where do you think all
that ring and cylinder wall material goes? More about
that later. Common sense dictates that we do not want
an uncontrolled machining process, like using peaks to
lap ring faces, to take place in our race engines (or
our passenger car engines either). We should strive to
finish hone a cylinder wall to remove all peaks and
micro burrs. What is left behind resembles a run-in
engine. How is a plateau finish accomplished? Well,
there is more than one way to do this but an
acceptable method has you hone about .003" stock with
a 80-100 grit stone, .0015" stock with a 280-320 grit
stone, .0005" with a 400-600 grit stone and finish
this off with a 320 grit abrasive filled nylon
filament brush. The 80-100 grit abrasive removes the
bulk of stock. It also leaves sufficiently deep
valleys that you will likely not remove entirely with
upstream honing operations. This is ok. The 280-320
grit abrasive prepares the core roughness of the
surface and regulates the amount and depth of valleys.
The 400-600 grit abrasive prepares the pleatu or load
bearing attributes of the surface. Finally, the 320
grit brush removes all surface defects left behind the
honing operations. Now, here is where you can get
creative. You can use the 280-320 grit abrasives to
alter the Rvk measurement to leave more or less valley
depending on what you are trying to accomplish. If you
have a diesel engine, I would build into the surface
considerably more Rvk (valley) that I would for a
gasoline burning engine. Cylinder pressures are way
higher for diesel engines and they suffer cold start
scuff if there is insufficient oil retention on the
surface. Consequently, I would also want to see higher
Vo (volume of oil retention) numbers coming off my
profileometer. I did use this type of surface on a
gasoline burner a few times. The surface looks like
trash to the naked eye but boy, did that engine make
power. By leaving a greater surface area of the
cylinder in valley, I was able to reduce surface area
available to the ring, thereby greatly reducing
friction in the highest friction-producing areas of
the engine. Just be prepared for oil consumption. If
this engine was to be used for endurance, lowering Rvk
and Vo would be vital. You want all the benefits of
reduced peak count but greater control of oil
consumption. It does you no good to build a killer
engine that consumes more oil that is required to
finish a race. The Indy 500 is such a race and the
teams are not allowed to add oil once the race starts.
Now you should begin to see why it is so important to
select rings, pistons and cylinder wall surfaces that
match the venue in which you race. One size does not
fit all.
Well, as it turns out, there is another family of
surfaces that is not a pleatu. It is called peak hone
and is further broken down into smooth peak and
conventional peak. This type of surface is pretty
straight forward with exception that Rvk and Vo are
dramatically reduced from what is considered a normal
plateau. In fact, it is quite possible to bring the Vo
down so low that there is virtually no oil retention
available to rings or pistons. Only a good deal of
hot-run data can point you to that line where to
little Vo is dangerous to the longevity of the engine.
While we are talking about smooth peak surfaces, it is
my opportunity to say a few derogatory remarks about
polishing cylinder walls. At first blush, this would
seem a good thing. But think about what you are doing.
Remember all those surface defects generated in a
normal honing process? You know, the ones you removed
with brush honing? Well, that defect layer is full of
other unwanted debris that comes from spent abrasive
crystals, unknown particles in the honing fluid, and
whatever you are using to polish with. You may end up
with a mirror finish but it will be one of the most
destructive finishes you could have produced for your
rings. Within a few rotations of the crankshaft, this
unstable polished layer is removed. Particles are
released from the surface and imbeded into piston
skirts and ring faces. Continued running of the engine
increases the damage caused by these abrasive
particles until rings fail completely. If you are
lucky and the engine doesn't destroy itself, all that
premature wear of sliding seal components go directly
into your oil supply and is pumped to every nook and
cranny of the engine. Just think about all those
abrasive bits in your soft bearing material,
constantly working on crank and cam journals. It ain't
a pretty sight.
I have probably rambled on way too long to be of any
benefit to you but please approach cylinder wall
surface with as much respect investigation as you do
with other critical areas of the engine. IT IS NOT A
BLACK ART! Surface finish is understandable,
definable, measurable, controllable and easily subject
to analysis. Choose wisely and you will have trouble
free and almost break-in free race engines with
increased reliability and performance.
John Goodman
P.S. Please forgive my spelling and syntax. I am doing
this on the fly and and within an email window. I can
only see a few sentences at a time. I hope some of it
made sense.
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