A close look at the surface of a bored cylinder wall will reveal a number of microscopic peaks and valleys. Plateau honing is a process that improves cylinder wall surface finish by gently removing tiny peaks of torn or folded material and increasing bearing area. A plateau finish is desirable because it allows lubricant to collect in the remaining valleys, improving lubrication control and retention. It also extends the life of components such as piston rings and seals because there are no peaks of material for them to remove during initial break-in.
One way to generate a plateau finish is to use ball-style honing tools such as the Flex-Hone tools produced by Brush Research Manufacturing (Los Angeles, California). These tools have a number of round, abrasive stones or “globules” tethered to a shank by nylon filament. The globules effectively float in the cylinder, self-centering and self-aligning themselves to the bore and also self-compensating for wear. When a globule encounters a peak of material, it removes the peak through low-pressure abrasion. Thus, the surface Rp value (reduced peak height) decreases while the Rvk (reduced valley depth) increases, resulting in more effective lubricant retention.
Ball-style hones are suitable for holes and bores found in components such as valve bodies, brake cylinders, master cylinders, firearm barrels and even musical instruments. The Flex-Hone tools are offered in hone bores ranging in diameter from 4 to 914 mm (0.16 to 36 inches). They are available in eight abrasive types and 11 grit sizes ranging from 20 to 800 grit.
The company suggests using the shortest shank possible for each application. The abrasive globules should be well-coated with lubricant and rotating prior to bore entry. In addition, rotation should continue until the tool is fully removed from the bore. Appropriate tool rotational speed ranges between 60 and 1,200 rpm. Small-diameter tools require higher speeds. A continuous stroke rate of 120 to 180 inches per minute should be maintained, and average honing time per cylinder is typically 15 to 20 seconds.