Cylinder bore refinishing is an integral part of rebuilding an engine. After tens of thousands of chilometri, 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. Which Cylinder Bore Finish Is Best? 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. Engine Cylinder Bore Science 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. Engine Cylinder Bore Finish By The Numbers 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 Using Fax Film To Analyze Cylinder Bore Finish 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. Understanding Cylinder Bore 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). The Holy Grail: Plateau Cylinder Bore Finishes 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 proced…

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