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 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:
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