High Performance Piston Rings and Cylinder Finishes
By Jerry McLain
Today's performance ring sets are thinner, lighter and more conformable than ever before. Thinner, low-tension rings reduce friction for more usable horsepower. Less weight reduces ring groove pound out. Narrower rings also allow tighter tolerances and less blowby. All very good things when you're building a performance engine. But they also require rounder, straighter cylinder bores than ever before.
Though many racers are running 1 mm compression rings, some ring manufacturers are selling compression rings as narrow as 0.6 mm. At the recent Performance Racing Industry show in Orlando, FL, one manufacturer showed us a one-piece 1.5 mm oil ring. The company said the super narrow oil ring is being used in some NASCAR NEXTEL Cup Series engines and is not yet available to the general public. The reason why the rings are being limited to NEXTEL Cup racing is because they are tricky to install and require a zero clearance end gap. These rings are not a true "gapless" design because the ends of the ring just butt up flat against each other.
Another reason for the smaller rings in many performance engines is that pistons are getting shorter. Longer connecting rods with shorter pistons change the combustion dynamics and provide better angularity during the power stroke. Shorter pistons also weigh less, which means the engine can rev higher. But when the piston is shorter, the rings have to move up higher. This means they have to be narrower, stronger and more heat resistant because the top ring is closer to the combustion chamber.
Cast iron piston rings are still a popular choice for dirt track claimer motors as well as many street performance and other racing applications. But cast iron is not in the same league as ductile iron or steel rings when it comes to strength. Ductile iron rings have roughly twice the tensile strength of grey cast iron, and three times the fatigue strength. Steel rings, by comparison, have almost four times the tensile strength and fatigue strength of grey cast iron.
So what does this mean inside an engine? It means ductile iron and steel rings can survive in racing environments that may be too demanding for grey cast iron rings. Stronger rings reduce the risk of ring breakage under severe loads. Steel rings also show less side wear and ring groove pound out.
For high boost turbocharged and supercharged engines, and engines using large doses of nitrous oxide to add horsepower, ductile iron or steel top rings are probably a must. Many racers prefer to use nitrided rings made from steel wire because the rings can handle high loads and thermal shock better than other materials. Nitriding penetrates into the metal and won't flake off like other surface coatings.
Plain uncoated grey cast iron rings are inexpensive and are popular for "budget" motors. But plain cast iron rings should never be used in an engine that burns alcohol because alcohol cuts lubricity. Some type of coated rings must be used with alcohol.
Moly is still the most popular facing material for many piston rings because of its excellent wear and scuff resistance. Another material is tungsten carbide (for hard liners), and chrome. Chrome-faced rings are still a good choice for abrasive environments like dirt tracks, and work best with cast iron blocks and cylinder liners. Chrome rings don't work well with chrome plated bores or hard faced cylinder bore liners such as those coated with nickel/carbide coatings.
The practice of coating cylinders with nickel and silicone carbide (the best known name is Nikasil, which is a registered trademark of Mahle Gesellschaft) got its start in Formula 1 racing, and is now used in many different levels of racing from circle track to drag racing to motorcycles.
The hard ceramic facing inside the liner is a mixture of nickel and silicon carbide that is only about 0.07 mm (.0025" to .003") thick. Small particles of silicon carbide less than 4 microns in size are dispersed throughout the nickel matrix. The result is a very hard and wear resistant surface that reduces friction and allows the engine to develop more horsepower. The surface has a hardness of about 90 HRc.
The liners are also dimensionally stable and experience less bore distortion than ordinary cylinders, which reduces blowby and leak down (some claim less than 1 percent after extended use). But to seal properly, these liners require two things: moly or tungsten carbide faced rings, and a very smooth bore finish.
The surface of a liner with this coating has microscopic pores that do an excellent job of retaining oil for the rings. Consequently, the bore can be finished to a super smooth finish of 4 to 6 Ra or less to reduce friction even more. Such low numbers would be too smooth for grey cast iron and would likely starve the rings for proper lubrication. Chrome plated bores or liners, by comparison, can also provide good lubrication while reducing friction and wear, but chrome is more vulnerable to dirt scoring and there may be some risk of flaking.
Regardless of what kind of rings or liners are used in a performance motor, rings usually seat best and last the longest when the cylinder bores are given a plateau finish. A plateau finish essentially duplicates a "broken-in" bore finish, so there is less scrubbing and wear on the rings when the engine is assembled. What's more, if the surface is finished correctly it will provide plenty of flat, smooth bearing surfaces to support the rings while also retaining oil in the crosshatch valleys to lubricate the rings.
The only exception to this is in motors where there is a lot of bore distortion. If the bores go out of round when the head bolts are torqued down, the rings may not seat as well allowing increased blowby and oil consumption. Thinner rings that can conform to the bore will work better in these kinds of applications, but it's also a good idea to use torque plates when honing when honing the bores to simulate the distortion that occurs when the cylinder heads are installed. The other option is to go with a slightly rougher "peaked" finish to seat the rings.
Most ring manufacturers recommend using a two- or three-step honing procedure to achieve a plateau finish. First, rough hone to within .003" of final bore size to leave enough undisturbed metal for finish honing. For plain cast iron or chrome rings in a stock, street performance or dirt track motor, hone with #220 grit silicon carbide stones (or #280 to #400 diamond stones) to within .0005" of final size. Then finish the bores with a few strokes using an abrasive nylon bristle plateau honing tool, cork stones or a flexible abrasive brush.
For moly faced rings in a street performance, drag or circle track motor, hone with a conventional #280 grit silicon carbide vitrified abrasive, then finish by briefly honing to final size with a #400 grit vitrified stone or #600 grit diamond stone (or higher), plateau honing tool, cork stones or a brush.
For stock and street performance engines with moly rings, an average surface finish of 15 to 20 Ra is typically recommended. For higher classes of racing, you can go a little smoother, provided you don't glaze the cylinders.
For moly or nitrided rings in a performance motor, hone with #320 or #400 vitrified stones, and finish with #600 stones, cork stones, a plateau honing tool or brush.
If the cylinders are rough honedwith diamond, they can be finish honed with a finer grit diamond, a fine-grit vitrified abrasive or a plateau honing tool or brush. Because diamond is a harder material and wears more slowly than conventional abrasives, it cuts differently and may require more honing pressure. But many newer diamond stones now use a more friable bond that stays sharp and doesn't load up, allowing the stones to cut smoother and leave a rounder, smoother bore finish.
When using diamond-honing stones instead of vitrified abrasives, you generally have to use a higher number grit to achieve the same Ra (roughness average) surface finish. For example, if you have been using #220 grit conventional stones to finish cylinders for plain cast iron or chrome rings, the equivalent diamond stones might be a #280 to #325 grit. If you have been using #280 grit conventional stones to hone for moly rings, the diamond equivalent might be #400 to #550 grit stones. The actual numbers will vary somewhat depending on the brand and grade of the stones.
Bristle style soft hones (plateau honing tools) have mono-filament strands that are extrude molded with a fine abrasive material embedded in the strands. The filaments are mounted in different types of holders for use with portable or automatic honing equipment. Another type of brush uses molded abrasive balls that are mounted on flexible metal shafts so the balls can easily conform to the surface. Brushing helps sweep away torn and folded metal on the surface while removing many of the sharp peaks to make the surface smoother.
When finishing the cylinders with a brush, only light pressure is required. The rpm of the brush should be similar to that which the cylinder was originally honed, and no more than 16 to 18 strokes should be applied (some say 8 to 10 strokes is about right). Too many strokes with a brush may produce too smooth a finish in a cast iron cylinder that won't retain oil. Reversing the direction of rotation while brushing helps to remove the unwanted material on the surface. The end result should be a cylinder that provides immediate ring seal with little if any wear on the cylinder wall or rings when the engine is first started.
With the right plateau honing techniques, you should be able to get the surface down to an average roughness of 8 to 12 Ra or less, with RPK (relative peak height) numbers in the 5 to 15 range, and RVK (relative valley depth) numbers in the 15 to 30 range. These numbers are meaningless unless you have a surface profilometer that can measure them (which a growing number of performance shops now have).
Crosshatch is also important because the amount, depth and angle of the crosshatch in the cylinder bores determines how much lubrication the rings receive and the rate of ring rotation.
Excessive shallow crosshatch angles can hinder or slow down the necessary ring rotation that allows the rings to dissipate heat. It can also leave too much oil on the cylinder wall allowing the rings to skate over the surface and the engine to use oil. Too steep of a crosshatch angle may not provide enough oil retention and can result in dry starts and premature ring wear. A steep crosshatch angle can also create excessive ring rotation that accelerates ring and piston groove wear.
Ring manufacturers typically recommend a crosshatch angle of 22° to 32° as measured from horizontal and uniform in both directions.
Bore geometry is especially important in performance engines because of the higher cylinder pressures they generate and the higher rpms at which they operate. Torque plate honing is a must with all performance engines to compensate for the bore distortion that occurs when the heads are installed.
Typically, cylinder bores tend to squash in and deform the most in areas that are next to the head bolts. Depending on how many head bolts are around the cylinder (four, five or six), the bore will experience fourth, fifth or sixth orders of distortion. Oblong distortion may also occur from side loading during the honing process.
The cylinders should be round and straight to a tolerance of .0005" (0.13 mm) or less (ideally, .0002" to .0003").
Bore distortions are bad at high rpm because it can prevent the rings from conforming to the surface, allowing more blowby and oil consumption. If the cylinders are not straight, the rings can bounce away from the surface and lose their seal with the same results.
The amount of bore distortion that occurs depends on the block, the location of the cylinders, and the design of the heads and how much loading is on the head bolts. The higher the bolt loads and the less rigid the block, the more distortion that occurs in the bores.
At the PRI show, C-K Engineering Inc. of Baldwin, MO showed a new Cylinder Bore Geometry Gauge that can measure and display bore distortion graphically on a computer. The drawing that results is three-dimensional and color coded to show the areas of greatest distortion. The gauge system is claimed to be accurate down to 1.0 micron (.000040")!
Hot honing got its start back in the late 1950s and early 1960s when legendary engine builders such as Smokey Yunick and Bill Jenkins experimented with a hot honing process to achieve rounder cylinder bores. The idea was to hone the engine hot (at 200° F) to simulate the bore distortion that occurs when the engine is running. Over the years, various racers have experimented with the process and today a majority of NASCAR engines are hot honed.
Hot honing holds the most promise for endurance engines that run at high rpm for long races (like NASCAR and off-road racing). But it provides less of a benefit for drag racing and street engines. Even so, some Pro Stock drag racers are now hot honing their blocks.
In terms of friction reduction, hot honing is claimed to offer a 1 to 2 percent improvement, which is good for maybe 5- or 6-hp in a 600-hp engine. Tests have shown that some bores can distort as much as .0035" at 220° F compared to room temperature. Less bore distortion when the engine is hot means better sealing and less blowby. The numbers are not huge, but in tightly regulated racing classes every advantage helps.
Several companies now offer equipment that allows cylinder blocks to be honed hot. The equipment typically has a pump and heating unit to circulate hot coolant through the block. The process doesn't totally duplicate a running engine because there is no way (yet) to make the top of the cylinder hotter to simulate the heat effects of combustion. Even so, hot honing will produce rounder, straighter bores in a running engine than honing at room temperature. Just keep in mind that the cylinders will distort when they cool down - but will return to a round condition when the engine fires up and runs.
Ring End Gaps
The old school philosophy of engine building said the end gaps on second compression rings could be tighter because the number two ring is not exposed to as much heat as the top ring. The new school of engine building says it's better to open up the second ring gap 20 to 30 percent so pressure doesn't buildup between the rings and cause the top ring to lose its seal at high rpm. The result is better compression, better piston cooling and reduced oil consumption.
Any pressure that builds up between the rings will blow down into the crankcase, keeping oil out from between the rings. This trick works best on engines that are running a dry oil sump and pull a vacuum in the crankcase.
Some performance pistons also have "accumulator grooves" machined into the piston land between the first and second ring grooves. The added space traps blowby gasses and helps prevent the top ring from unseating and fluttering.
For naturally aspirated engines, a top ring end gap of .004" per inch of bore diameter is often recommended for a stock or moderate performance engine. For a 4-inch bore, that translates into a top ring end gap of .016" to .018". But this will vary depending on the power output of the engine.
For drag or oval track racing, the recommended end gap is somewhat larger (.0045" per inch of bore diameter). With four-inch bores, that would be an end gap of .018" to .020".
For a nitrous oxide street performance engine, the recommended end gap is .005" per inch of bore diameter (.020" to .022" for an engine with four-inch bores). For a nitrous oxide drag engine, the recommended end gap for the top ring is .007" per inch of bore diameter (.028" to .030" with four-inch bores).
With a turbocharged or supercharged racing engine, the top ring gap should be .006" per inch of bore diameter (.024" to .026" with a four-inch bore).
The recommended end gaps for second compression rings are also the same, with slightly larger gaps if you want to minimize pressure buildup between the rings.
The recommended ring end gap for most oil rings (except the new super narrow one-piece rings) regardless of engine application is typically .015".
Another trick to improve ring sealing at high rpm is to run pistons that have gas ports behind the top ring. Combustion pressure blows through the port to help seal the ring from behind and underneath. Some use vertical gas ports with holes drilled from the top of the piston to the top ring groove just behind the ring. Others use lateral gas ports that are drilled through the bottom side of the top land and extend to the back wall of the ring groove. Gas ports work best at high rpm (above 7,000 rpm) and are not recommended for street engines.
Getting rid of the end gap altogether can also improve sealing, cooling and horsepower. Gapless rings eliminate the gap between the ends of the ring by overlapping slightly. Gapless rings are available in popular sizes with various wear-resistant face and side coatings. Some engine builders who have switched to "gapless" top or second compression rings say they've gained three to five percent more horsepower with no other changes. Gapless rings are said to allow less than 1 cubic feet per minute (CFM) of blowby and on alcohol-fueled engines, a gapless top ring or second ring helps keep alcohol out of the crankcase.