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Cylinder Bore Surface Finishes
by Larry Carley copyright 2022 AA1Car.comAdapted from an article written by Larry Carley for Engine Builder magazine
Cylinder bore refinishing is an integral part of rebuilding an engine. After tens of thousands of miles, the cylinder bores can become worn, tapered or scratched. Cylinder wear can be accelerated by poor air filtration (such as a missing or damaged air filter element, leaks between the air filter element and its housing, or air leaks downstream of the air filter). Any of these conditions can allow dust and abrasive particles to enter the cylinders where they will cause wear on the piston rings and cylinders.
Poor oil maintenance can also accelerate piston ring and cylinder wear. Not changing the oil often enough or lack of proper filtration can allow abrasive particles to attach the rings and cylinders from underneath the pistons.
Cylinders may also be damaged by broken piston rings or by wrist pin retaining clips or wrist pins have have come loose.
When the engine is disassembled, the condition of the cylinders must be inspected to determine the extent they are worn. A cylinder bore gauge can be used for this purpose. Measurements can be taken at various locations from the top to the bottom of the cylinder. Wear will always be greatest at the top of the cylinder where cylinder pressures and temperatures are highest. This is called taper wear. If taper exceeds specifications, the cylinders need to be bored to oversize to restore straightness.
The best cylinder bore finish is one that provides a good sealing surface for the piston rings, minimizes blowby while also retaining oil for proper ring lubrication. The cylinder bore finish will depend on the type of piston rings used, the type of honing stones used, and the boring and honing techniques used to finish the cylinder bores.
A good cylinder bore finish allows piston rings to seat quickly so the engine doesn't burn oil. For most applications, that may mean a plateau finish. The surface finish must also meet the piston ring manufacturer's specifications, and have the proper crosshatch so the cylinder walls will retain oil and provide adequate lubrication for the rings. The finish must also be relative free of torn and folded metal (swarf) as well as abrasive residue.
Good bore geometry is also essential. The cylinder bore in the engine block should be as round as possible with little or no taper or variation vertically. Bore distortion caused by deflections in the casting or improper boring or honing techniques will have an adverse effect on ring sealing and blowby.
Vehicle manufacturers and piston ring manufacturers have developed very specific surface finish and bore geometry specifications for their engines and rings. The recommendations vary somewhat depending on the engine application, type of rings and honing procedure used, but generally specify a finish that meets all of the criteria we've just described.
To the naked eye, a freshly honed cylinder bore looks pretty rough in comparison to a used cylinder bore. Honing leaves a scratched surface that should show a strong crosshatch pattern. A used cylinder, on the other hand, will have a smooth polished appearance with much less crosshatch visible depending on how much the cylinder is worn.
At a microscopic level, the profile of a freshly honed cylinder wall reveals many little peaks and valleys. The valleys are cut out of the metal by the abrasives during the honing process, and the peaks represent the highest point on the surface that will make contact with the rings. Large, sharp peaks will not last long once the engine is started because the tops of the peaks will be gradually sheared off by the rings as the rings break in. As the tallest peaks are knocked off, the mountains become flattened creating a "plateau" effect. This increases the bearing area for the rings and makes it easier for the rings to glide over the surface on a film of oil that is retained in the valley.
Once the rings have seated wear virtually ceases because the rings are now supported by a thin film of oil and do not make physical contact with the cylinder wall.
The ideal cylinder bore surface, therefore, should essentially duplicate this condition.
To minimize the formation of sharp peaks on the surface, the cylinders must be finish honed with stones that have a relatively fine grit size. The finer the grit size, the smoother the finish.
The average roughness of the surface is called "RA" and is typically specified in microinches (1 microinch is one millionth of an inch, or 0.000001 in.) To measure RA, you need an electronic instrument called a profilometer that drags a diamond tipped stylus across the surface to measure the size and distribution of peaks and valleys.
Most OEMs and ring manufacturers specify a surface finish of 15 to 25 RA for moly faced rings, which can be achieved by finish honing with #280 grit stones. Cast iron and chrome rings can tolerate a somewhat rougher surface finish (20 to 35 RA) so coarser #220 grit stones can be used to produce this type of finish.
Unfortunately, RA alone does not reveal much about the actual profile of the surface. A bore finish with tall peaks and deep valleys can have the same average roughness number as one with short peaks and shallow valleys. More numbers are needed to accurately analyze the surface:
* RPK is the peak height.
* RVK is the depth of the valleys.
* RK is the average core roughness depth based on the RPK and RVK measurements. A surface with a low RK value will have long life characteristics.
* RMAX is the highest peak-to-valley measurement taken from five samples.
* RZ is the mean highest peak-to-valley measurement taken from five samples.
When all these numbers are taken together, it creates a more complete picture of what the surface actually looks like. Some profilometers can take this information and plot a graph that shows how much bearing area is on the surface. This is called the "Abbott-Firestone Curve." The curve plots profile height on the vertical axis and percent of surface contact on the horizontal scale. The flatter the curve and the greater the area enclosed by the curve, the better the surface finish.
An easier way to tell whether or not the surface finish has all the "right" numbers is to compare the various "R" numbers to the OEM and ring manufacturers specifications. The numbers will tell you if the surface has the proper depth of crosshatch, enough bearing area to properly support the rings, and is smooth enough to minimize ring wear during the seating process.
According to one honing equipment manufacturer's guidelines, the RMAX and RZ numbers should be about 10 times the RA number for a properly honed finish. If the RMAX or RZ numbers are less than one seventh the RA number, the surface is glazed and won't retain oil. If RMAX or RZ is more than 12 times the RA number, the surface has too many deep scratches.
What are "good" numbers for a properly honed cylinder bore? Sunnen offers the following guidelines:
* RA should be 12 to 24
* RPK 6 to 24
* RVK 20 to 80
* RK 28 to 48
According to Perfect Circle Piston Rings, the numbers will vary depending on the honing procedure used. Perfect Circle's finish specifications for automotive engines honed with a single stage process are:
* RA 10 to 20
* RPK 10 to 20
* RVK 30 to 60
* RK 25 to 50
Another useful tool for evaluating surface finish is "fax film." Though few rebuilders use this technique, most OEMs as well as some PERs find it is extremely helpful for identifying certain kinds of finish problems. After the cylinders have been honed and washed, a small piece of thin plastic film is placed on a bore surface with a solvent that softens the film. This allows the film to take an impression of the bore surface. The film is then removed and examined under a microscope at 100X magnification to check for excessive torn or folded metal, burnishing or glazing, embedded particles and debris. It also makes it easy to accurately measure the exact angle of the crosshatch.
Most OEMs and ring manufacturers say the angle of the scratches in the crosshatch pattern should be about 45 degrees to each other, or about 22 to 32 degrees to the horizontal deck surface. The crosshatch angle should be the same throughout the length of the cylinder and not flatten out at either end.
If the crosshatch angle is too steep, the rings can pump oil or experience excessive rotation with will accelerate wear in the rings and piston lands. If the crosshatch angle is too shallow, it can have a ratcheting effect as the rings pass over the valleys preventing the rings from receiving proper lubrication.
A proper crosshatch will also have enough valleys to retain oil, but not too much oil. The secret here is getting the right amount of retained oil volume (called "Vo"). If the crosshatch scratches are too deep or there are too many valleys (not enough peaks and bearing area), the engine will use oil. The greater the retained oil volume (Vo), the higher the oil consumption. This can be caused by finish honing with stones that are too coarse (#150 or less). On the other hand, if the crosshatch scratches are too shallow or there is too much plateau on the bore surface, the volume of retained oil may not be enough to keep the rings lubricated causing accelerated ring and cylinder wear. This can be caused by finish honing with stones that are too fine (#400 or greater).
A plateau cylinder bore finish is a popular one because it combines all the "good" numbers: low peak height (RPK), plenty of bearing area (Abbott-Firestone curve), and adequate valley crosshatch (RVK) for good oil retention and ring lubrication.
To achieve a plateau finish, one of two methods can be used: a two-step honing procedure or a one-step honing procedure followed by a brief brushing process.
With the two-step plateau honing procedure, the cylinder is honed with coarse to medium stones followed by a very fine abrasive for a few strokes to remove only the tops of the peaks. For example, the cylinder would first be honed with #150, #220 or #280 grit stones followed by a few strokes with #400 grit stones. The trick here is to shave only the peaks off the cylinder walls. You don't want to remove too much metal with the finer stones or you will be down to the base metal and loose all of the valley depth in the crosshatch.
The second way to produce a plateau finish is to hone the cylinders with #220 or #280 grit abrasive (or #400 or #500 grit diamond abrasive), followed by a few strokes with a "Plateau Honing Tool" or "Flex-Hone" tool.
A plateau honing tool or brush has silicon carbide abrasive embedded in nylon bristles. The brushes are designed for mounting in a hone head so pressure and feedrate can be controlled. A plateau effect is created as the brush sweeps across the surface to remove sharp peaks and other debris. The amount of metal removed is minimal because the tool is only applied for a few strokes in each cylinder.
The Flex-Hone tool (manufactured by Brush Research Mfg. Co. Inc.) has numerous round abrasive balls mounted on wire bristles. The tool is typically mounted in a hand drill and manually stroked up and down in each cylinder bore a few times to plateau the surface. This style of brush has a more aggressive cutting action than the nylon bristle tool, but is only used to remove just a small amount of material to plateau the surface.
Opinions differ as to which type of brush gives the best plateau effect and why, but both types are used successfully by a wide variety of engine rebuilders to "precondition" cylinders and improve the overall bore finish.
The advantages of a plateau finish are:
* Significantly shorter break-in time.
* Reduced blowby for cleaner emissions.
* Reduced oil consumption in new engine.
* Less ring and bore wear for improved engine longevity.
According to research by Perfect Circle, a plateaued bore surface will stabilize after about two hours of running. In other words, the rings will seat very quickly and experience almost no further wear. A more conventional surface finish, by comparison, may take anywhere from three to 12 hours to seat in depending on the grade of stones used. A bore finish honed with #280 grit stones will seat faster than one finished with #220 stones.
When cylinders are bored to oversize, they are generally bored or rough honed to within about .0025 to .003 inch of final size to allow for finish honing. The finish honing step removes the fractured and torn metal and produces the kind of surface finish and crosshatch we've been talking about.
Whether vitrified abrasives or metal bond diamond or CBN (cubic boron nitride) stones are used to finish hone the cylinders makes no difference as long as the final surface geometry conforms to the desired numbers. Most OEMs and production engine rebuilders use diamond to hone cylinders because it improves consistency and reduces honing costs.
A set of diamond stones will typically do 20,000 to 100,000 cylinder bores compared to maybe 50 to 80 cylinders for a set of vitrified stones. Diamond stones are much more expensive than vitrified abrasives, but over the long run actually cost less especially when labor savings are factored in (provided you don't damage or break a stone!). Because diamond wears very little, the honing process takes less baby sitting and yields more consistent results.
The honing characteristics of diamond honing stones have also improved in recent years due to changes that have been made in the matrix that supports the diamond. Some stones now use a titanium surface treatment that slows the erosion of the supporting matrix. This improves the cutting action of the stone, which in turn reduces the tendency of diamond to "plow" through the metal. The result is less torn and folded debris on the surface and a better overall surface finish.
Is there a downside to using diamond honing stones? Because diamond stones wears so slowly, the stones do not conform very quickly to changes in bore diameters. So if a shop is honing a variety of different bore sizes with the same hone head, they may not get as good a finish as they'd get honing the same sized bores over and over again. Also, some diamond hone heads can only handle a limited range in bore sizes so you may have to shop for a head with a wider range in bore sizes or buy a second hone head.
The most common honing procedure with diamond stones is to use #400 or #500 grit diamond stones, which may be followed by a nylon abrasive brush to plateau the finish. The #500 stones typically leave a finish in the 18 to 22 RA range, while the #400 stones leave a 24 to 30 RA finish which requires brushing to bring the RA down to about 20.
As important as surface finish is for proper ring seating and lubrication, bore geometry is probably even more important on today's engines. Bore distortion is common in the upper cylinder area because of the forces created by the head bolts when they are tightened down. Changes in coolant temperature and circulation within the block can also cause bore distortion as can normal and abnormal combustion pressures.
To get as round a hole as possible, many engines with thinwall castings should be honed with a torque plate and head gasket bolted to the block. The torque plate simulates the loads placed on the block when the head is installed, allowing the bore to be honed to truer dimensions.
Bore distortion can be described by levels of "order." A first order bore is one that is perfectly round with no distortion in any direction. A second order bore is one with an oval distortion, typically caused by machining errors or heat transfer. Rings can usually tolerate some second order distortion by conforming to the bore. But the lower the ring tension, the less able the rings are to conform to bore distortion. A third order distortion results in a triangular shaped hole, and is usually caused by a combination of second and fourth order distortions. A fourth order distortion is a bore with a cloverleaf or squared shape. This type of distortion is caused by the location of the head bolts.
The amount of distortion can vary from almost nothing up to a couple thousandths of an inch! With today's tight piston-to-wall clearances, even .0005 inch of bore distortion may be too much on some applications. So the rounder the bore the better. Some performance engine builders don't want to see less than plus or minus 8 microns (0.000003 inches) of bore distortion! Perfect Circle says a plateau finish should only be used if bore distortion is less than 0.01 mm (.0004 in.).
The effort to produce an ideal bore finish, crosshatch and near perfect geometry can be undone if the cylinders are not thoroughly cleaned after they have been honed. Scrubbing with hot, soapy water is still one of the best ways to remove honing debris that can cause ring problems if it remains in the cylinders. Some rebuilders tell us they even do a second cleaning step that involves wiping out each cylinder with ATF or WD-40 oil to remove anything that might have been missed by the soapy water.
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