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Cylinder Bore Refinishing
Nobody wants an engine that burns oil, especially if it's an engine you just rebuilt. Getting the rings to seat and seal properly requires a ring-friendly finish on the cylinder bores.
By Larry Carley
By ring friendly, we mean a finish that provides good support for the rings, retains oil and does not require a lengthy break-in period. For most engines today, a ring-friendly finish means a plateau finish.
A plateau bore finish is what all types of rings eventually produce when they are fully seated, so the closer the bore can be prefinished to a plateau-like condition the less the rings and cylinders will wear as the engine breaks in, the better the rings will seal right from the start, and the longer the rings will last.
For moly rings, a two-step honing process can be used to achieve a plateau finish. First, hone with a conventional #280 grit silicon carbide vitrified abrasive. Then finish by briefly touching the bores with a #400 grit stone or giving them several strokes with an abrasive nylon honing tool or brush.
If the cylinders are honed with diamond stones, you can follow up with a finer grit diamond, a fine grit vitrified abrasive or a brush to finish the bores. Diamond stones are fast and long lived, but they are more aggressive than silicon carbide and create more tear outs and other undesirable residue on the surface. Because of this, a rough diamond honing procedure should always be followed up with another operation afterwards to finish the surface.
By comparison, traditional honing procedures require no finishing step. Typically most ring manufacturers recommend using #220 grit silicon carbide honing stones if the engine will be assembled with plain cast iron or chrome rings, #280 grit stones for moly-faced rings, and #320 to #400 grit stones for moly rings if the engine is being built for racing or performance. Even so, the cylinders can still be plateaued to some extent by finishing them with some type of flexible brush.
One of the advantages of using a flexible brush in a drill is that you can run the drill backwards. Honing stones usually run clockwise so if you brush in the opposite direction (counterclockwise) it will do a nice job of deburring the surface. No more than 15 strokes should be necessary to produce a high quality finish.
Getting The Numbers Down
The recommended surface finishes for late model engines vary somewhat from one application to another, but many are now in the 15 to 20 microinch range Ra (roughness average).
If bores are honed with #325 to #400 diamond stones, the finish will typically be in the 22 to 24 Ra range - which is too rough. That's why the bores need to be finished with a brush or finer grit stones. Doing so will usually bring the finish down to the desired range of 20 or less.
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 won't 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 valleys.
According to one piston ring manufacturer, once the rings have seated, wear virtually ceases because the rings are now supported by that thin oil film and no longer make physical contact with the cylinder wall.
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.
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.
The ideal cylinder bore surface, therefore, should essentially duplicate this condition. By using the right honing procedure, you can create a surface finish that allows the rings to seat quickly with minimal wear and retains oil so the rings receive proper lubrication.
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.
All About 'Rs'
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.
As we said earlier, many OEMs and ring manufacturers specify a surface finish of 15 to 20 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 doesn't 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.
A plateau cylinder bore finish is a good 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.
According to one honing equipment manufacturer's guidelines, a "good" bore finish should have Rmax and Rz numbers that are about 10 times the Ra number. 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.
Here are some "good" bore finish numbers to aim for from boring equipment manufacturers:
- Ra 12 to 24
- Rpk 6 to 24
- Rvk 20 to 80
- Rk 28 to 48
By comparison, here are some recommended numbers from piston ring suppliers for automotive engines:
- Ra 10 to 20
- Rpk 10 to 20
- Rvk 30 to 60
- Rk 25 to 50
Some performance engine builders run somewhat higher "Rvk" (valley depth) numbers in the crosshatch to improve oil retention in high revving engines.
Another useful tool for evaluating surface finish is "fax film." Though few custom engine builders 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.
When cylinders are bored to oversize, they are generally bored or rough honed to within about .0025˝ to .003˝ 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.
The honing characteristics of diamond honing stones have 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. Consequently, many of the engines that are being built today are being honed with diamond stones.
Typically, most production engine rebuilders are using #320 or #400 grit diamond stones today, followed by brushing using a #180 grit plateau honing tool. The #400 stones leave a 24 to 30 Ra finish which after brushing comes down to 20 Ra or less.
Replacement stones are available in many formulations to fit all of the popular models of equipment.
Good bore geometry is also essential for proper ring sealing. You have to be especially careful about oil control on late model engines because an oil burner can foul the oxygen sensor and catalytic converter. When at operating temperature, each bore should be as round as possible with little or no taper or variation vertically. Bore distortion prevents the rings from conforming to the cylinder walls and allows oil to get past the rings as well as compression (blowby). Bore distortion occurs mostly in the upper cylinder area when the head bolts 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, engines with thinwall castings should be honed with a heavy steel 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. Many performance engine builders even circulate hot coolant through the block while it is being honed to simulate actual operating conditions.
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˝ 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 (.000003˝) of bore distortion! And a plateau finish should only be used if bore distortion is less than 0.01 mm (.0004˝).
Most OEMs and ring manufacturers say the angle of the scratches in the crosshatch pattern should be about 45° to each other, or about 22° to 32° 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, which 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).
Varying the spindle speed and stroking speed of your honing machine will vary the crosshatch.
An article on cylinder finishes wouldn't be complete with saying a few words about cleanliness. All your efforts to produce an ideal bore finish, crosshatch and near perfect geometry can be undone if the cylinders are not thoroughly cleaned after they've 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 engine builders tell us they even do a second cleaning step that involves wiping out each cylinder with ATF or spray lubricating oil to remove anything that might have been missed by the soapy water.
As a final quality check, you can vacuum test the cylinders to make sure the rings are giving the proper seal after the engine has been assembled.
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Great article, you might find it interesting to know that Gregg Spears of Spears Racing in Manteca Ca. had almost identical thoughts on the subject, lol.
by: moparmanjames 6/1/2010