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Piston Rings and Surface Finish

Horsepower is always paramount in the minds of customers who salivate like a Pavlovian dog when the terms "high performance" and "engine" are used in the same sentence. Unquestionably, horsepower is heady stuff, capable of not only moving a vehicle to obscene speeds, but also of propelling its owner to a position of prominence in the eyes of performance enthusiasts and the local constabulary.
Large quantities of it are what a customer deserves if he or she has ventured into the world of ported heads, high lift cams and "trick" machine work. But if the end product doesn't offer reasonable durability, a once happy customer will quickly develop the menacing facial expression and snarly tone of an attorney who has to abide by a code of ethics.
As it pertains to high performance engines, durability doesn't mean 100,000 miles with nothing more than an occasional oil change. Even emotionally impaired customers (of which there seems to be an inordinately high number) will usually acknowledge that a high performance engine isn't expected to enjoy the longevity of a hum-drum standard powerplant. Aside from the fact that stress levels rise with horsepower, vehicles with high performance engines are typically driven aggressively, which also takes its toll on engine life.
But expectations are that power levels and oil consumption will remain relatively stable until an engine, having served its master well, accumulates sufficient mileage to warrant another rebuild. Depending on customer and usage, that could mean anything from tens of thousands of road miles to a few hundred laps on a race track. In either case, proper cylinder wall preparation and judicious ring selection are the arbiters that ultimately establish an engine builder as either a hero or a goat.

Rings and surface finish

Before a set of rings comes face-to-face with the cylinder walls it will be up against for a lifetime, the block that's to serve as an internal combustion condominium is typically mounted on a boring bar, with a deck plate attached, and each cylinder is usually machined to within .004"-.005" of the desired finished dimension. After the boring bar has carved its last bits of metal from the cylinders, a honing operation is required to massage the cylinder walls until a ring-friendly finish is achieved.
For maximum friendliness, cylinder wall finish specifications are selected according to ring material. While opinions vary somewhat as to optimum finishes, a straightforward procedure invariably provides the best results. As Garry Grimes of Grimes Automotive Machine in Alpharetta, GA, notes, "The biggest concern when honing should be cylinder wall movement. Finish doesn't matter a whole lot if the bores are out-of-round, so the use of torque plates during both boring and honing is an absolute necessity. A lot of shops that dabble in performance work think torque plates are a waste of time, but eventually they learn that if you intend to stay in this business, you have to do things right."
One aspect of block machining that is often overlooked is heat induced distortion. As a hone goes about its business removing cylinder wall material, it can generate a considerable amount of heat. To minimize heat-induced bore distortion, Fritz Kayl of Katech, Inc., Clinton Township, MI, notes that his company's procedures call for staggered honing so that two adjacent cylinders are never honed one right after the other.
Since the purpose of honing is to provide a suitable finish for the rings, specific stones are required. With the almost universal use of moly top rings in high performance engines, doing things right usually means a multi-step honing process for those leaving .010'' for final honing, wherein the cylinder walls are first honed with 70- to 100-grit, then with 280-grit, and finally with 400-grit stones.
All honing is done in a manner that leaves the walls with an appropriate cross-hatch pattern. Katech has been building race engines for General Motors for more years than Kayl cares to admit, and dependability has always been a top priority. Consequently, Katech has established specific requirements for its multi-step honing process.
Blocks are initially bored on a CNC vertical mill to a dimension that's .010" less than the finished diameter. Each cylinder is then rough honed at a reading of 40 on the hone load meter, to within .005" of final bore size using 70-grit stones. The next pass, also made with a load meter reading of 40, is with 280-grit stones. Finally, four light passes (with a reading of 20 on the load meter) are made with 400-grit stones (For all steps, the hone is set for 45 strokes per minute and 155 rpm). Surface finish is then checked with a Hommel T500 tester and a printout is filed in the engine build book.
Many machinists feel as though the cylinder walls in a block destined for high performance use should be honed only with conventional stones. In fact, one engine builder went so far as to say, "You can't print what I have to say about diamond stones." Others have an entirely different opinion.
Barry Gowen, manager of the machine shop operations of Summit Racing Equipment, Tallmadge, OH, states, "We bore all our blocks with a CNC mill to within .003" of finished size. Then we do a single honing operation to final dimension using metal-bonded diamond stones. Following that, we use a soft hone as a finishing step. Our process is idiot-proof and the diamond stones give us straight, round bores with no taper every time. And once you get the stones broken in, they'll last (far longer than conventional abrasives). I know there's a lot of controversy about diamond stones, but we have absolutely no ring problems and cylinder leak-down is never over 5%."
Gowen, who used to sell machine shop equipment, also points out that it took some time to perfect the diamond honing process. He pointed out that one of the keys is the use of coolant rather than honing oil. But, hone speed, abrasive and type of hone head are also important. As is the case with building engines, different shops prefer different processes and procedures. However, whatever the methodology employed, successful shops have carefully documented and perfected the right combination of ingredients.
Employing abrasive stones of various grits results in a plateau-type finish where the tops are knocked off of the relatively deep grooves left by the coarser stones. The effect is similar to that of filing the threads on a nut. While the removal of thread material isn't advisable if secure fastening is the objective, it does serve as an excellent means of illustration if you're "visualizationally challenged."
If you had the time and inclination to file the thread's tips down to within a few thousandths of an inch of the root diameter, and then applied motor oil to the surface, you'd find that the oil would collect in the small grooves that remained, thereby providing surface lubrication. Voila! Having completed the experiment, you would have experienced first hand, the benefit of plateau honing.
Having achieved a text-book plateau-honed surface, you might think that you had reached machining nirvana. But according to proponents of two processes known as "soft honing," and ball-brush honing, there is yet work to be done. Whenever an iron surface is machined with a stone of any type, the surface is literally torn. When an engine is fired for the first time, the rings scrape much of the torn material from the surface, resulting in minute vertical scratches in the cylinder walls.
"Soft honing," which is actually an abrasive cleaning process, was developed to produce a smooth surface, devoid of easily dislodged metal particles. The "soft hone" itself looks very much like a bore-cleaning brush; some models may be mounted in an electric drill. But rather than bristles, these hones are composed of silicone carbide filament strips which are tough enough to scrub metal from the surface. In essence, the process removes the jagged peaks, folded material and abrasives left by previous operations.
Having cleaned out the undesireable metal, "soft honing" leaves a more stable load-bearing surface on the cylinder wall, which hastens ring seating and eliminates cylinder wall scarring. Many performance shops swear by "soft honing," others remain unconvinced.
In some instances, resistance to the process may stem more from pride than from logic. Experienced machinists often pride themselves on their cylinder wall preparation skills and are reluctant to admit that their product can be improved. However, "soft honing" equipment is relatively inexpensive, and it serves as excellent insurance that cylinder wall surfaces have been optimized. If moly rings are to be installed, optimized means the relatively smooth surface left by 400-grit stones.
Another means of abrasively cleaning cylinder bores and achieving a plateaued cylinder wall free of loose, torn and folded metal is with a ball-brush hone, also known as a Flex-Hone®. Like "soft honing," proponents of using a ball-brush type hone feel that it removes much of the folded metal that remains following normal honing. Ball-brush hones are also used to clean-up and deglaze cylinder walls when an engine is freshened.
The suppliers of ball-brush type hones and "soft hone" brushes continue to debate the benefits and final surface finish characteristics achieved when using one brush compared to the other. However, we have interviewed more than a few engine rebuilders who use both types of brushes. In each case, they are more than satisfied with the final surface finish achieved with their chosen procedure. Again, success seems to boil down to documented processes and procedures that are consistently implemented from one engine rebuild to the next.

Chrome rings

Chrome rings, on the other hand, require a coarser finish, usually achieved with 280-grit stones, before they agree to seal properly. Prior to the advent of moly, chrome-plated top rings enjoyed widespread use in high performance engines. According to Joe Moriarty of TSE Racing Engines, Greenland, NH, "About the only applications in which chrome rings are still used are those in which abrasion is a major concern.
"Dirt track engines running without air filters are a prime example. Chrome is much more resistant than moly to dirt and other abrasives on the cylinder walls. That's one of the reasons you'll find chrome rings in diesel engines; the carbon deposits are very abrasive. But chrome doesn't have a lot of self-lubricity so it needs a relatively coarse cylinder wall finish so some oil remains on the surface."
Ring width is another consideration that must be "tuned" to a particular application. Since the days when flathead Fords reigned supreme, 5/64" thick top and second rings, and a 3/16" oil ring have been the standard in production engines. Many high performance engines are also fitted with a 5/64", 5/64", 3/16" ring package, although thinner 1/16" or 1.5 mm top and second rings and either a 3/16", 1/8" or 4 mm oil ring are far more common in true race engines.
Extremely thin top rings ó .043" ó have been the hot lick in many drag race engines for quite some time, and may also be found in a surprising number of endurance engines, e.g., Winston Cup. Although "thin" rings are shrouded in the same type of mystery and intrigue that surrounds a government investigation of toilet seats, the parameters for their use are relatively straightforward.
In relative terms, "thick rings" (5/64") have the highest heat dissipating capability and are heaviest. "Thin rings," by virtue of their lesser width and mass, are less able to dissipate heat, but also more stable at higher engine speeds. In round numbers, a 5/64" ring is suitable for applications in which engine speed doesn't exceed 6000 rpm.
The primary incentive for minimizing ring width is to reduce mass and thereby eliminate a phenomenon known as ring flutter. When a heavy ring is cycled at high speed, it tends to flutter around, beat up the ring lands and lose contact with the cylinder wall. When that occurs, ring seal joins suicide knobs and wide white walls as a thing of the past.
In drag racing, where engine speeds of 7500 to 10,000 rpm have been commonplace for years, use of .043" and 1/8" top rings has long been standard practice. But now that many oval track engines are seeing life above 8000 rpm, rings are growing thinner. As an example, some Winston Cup engine builders are now using 1.2 mm (.047") and 1.0 mm (.039") top rings, 1.5 mm second rings and 3.5 or 4 mm oil rings.
Far more common in the universe of metric rings is a 1.5 mm top ring which is only .004" thinner than 1/16". The switch to metric specifications, although not universal, is certainly headed in that direction. Certainly, ring seal would be questionable if metric rings were fitted to pistons with ring lands machined to a fraction of an inch, but the trend in newly designed pistons machined for domestic V8 engines is metrification of ring grooves.
Much of the impetus to switch to metric rings has been to take advantage of new ring materials. Returning for a moment to older technology, one means of improving ring seal is to increase the loading of the top ring against the cylinder wall. In drag racing, this is commonly accomplished through the use of gas ports ñ small holes drilled through the top of the piston into the top ring groove.
Combustion pressures are thereby allowed to reach the back side of the ring, forcing it into firm contact with the cylinder wall. The extremely rapid rate with which rings and cylinder walls in gas-ported pistons wear demonstrates the extreme effectiveness of this process. It also makes it impractical for endurance racing.
With recent improvements in ring materials and block preparation, gas porting, even in hard core drag race engines, isn't as prevalent as it once was. Dykes-style rings are another "old technology" method of improving ring seal in race engines. Use of .031" and .017" Dykes rings, which are "L"-shaped (to allow gas pressure to load them against the cylinder walls) is pretty well confined to blown fuel and alcohol drag race engines.
Another approach to improving ring seal is to eliminate end gaps. At least two companies offer rings, designed to be installed in the second ring groove, that have no open gaps. One design employs two-piece construction with an oil ring-type rail combined with a conventional cast or ductile iron ring. The end gaps of the rail and ring are positioned 180° apart, so there is no open end gap. Another design utilizes overlapping steps at each end of the ring, which again, leaves no open gap. Gapless rings are widely used in a variety of high performance and racing applications, (including Indianapolis) especially in engines fueled by alcohol and nitromethane, where oil dilution is a significant problem.
However, some engine builders prefer to stick with standard type rings. As with most other aspects of high performance engine building, each successful engine builder has a somewhat different opinion about cylinder wall preparation and ring selection. But the fact remains that some engine builders are enjoying success with components and techniques that others flatly state will not work. Clearly, the formula for success involves a good bit of experimentation coupled with an equal amount of common sense.