Engagement Rings For Your Pistons

Tech Editor's Note: Rings areround, square, tapered, flat, slanted, and above all, precision made.They are required to seal approximately 1,800 psi of combustion pressurethat is trying to slip by the piston. All of this has to be done whilemoving at a speed of about 3,500 feet per minute. That is a distance ofabout two-thirds of a mile. While this is happening, throw in a boatloadof heat as the ring tries to shed any lubricant that comes its way. Itis also trying to keep the lubricant out of the combustion chamber. Theaforementioned describes the piston ring's environment and jobdescription.

There is no one piston ring design or piston ring material that can dothis job in different engines at different power levels. A piston ringset that may work well in your Street Stock engine making 350 hp at6,500 rpm would be out of place in a modified engine making 650 hp at7,800 rpm. The Street Stock engine may not have the extensive andexpensive block machine work and cylinder treatment of the Modifiedengine. Rpm, oil control, compression, and horsepower all have effectson the required ring material and design.

Horsepower in an engine is directly related to several facets in ringdesign. The top ring's primary function is to seal the combustionpressures. However, it must do this without pushing so hard on thecylinder wall as to cause excessive wear. Low-tension rings areavailable that can turn reduced friction into power. These samelow-tension rings might not seal properly or live very long unless somerather heroic measures are taken to make sure the cylinder walls areround and true when the engine is running.

At this point, I'm going to let Marlan Davis explain piston rings.Marlan is the senior technical editor at Hot Rod, a sister magazine toStock Car Racing. The following is from an article he wrote for CarCraft magazine, another Primedia publication. I have read it, and I mustagree with what he has written. --Sleepy Gomez

Piston rings perform a number of important functions. They seal the gapbetween the piston and cylinder wall to prevent combustion gases fromblowing into the crankcase. They stabilize the piston as it travels upand down in the bore. They help cool the piston by transferring heatinto the engine block, and they scrape oil off the cylinder walls.That's a tall order, and in recent years the theory on how to make ringsbest carry out these tasks has undergone revision.

Old-school thinking followed a brute force approach: Make everything asrigid as possible to force the rings into contact with the walls. Today,the trend in current production and racing engines is toward a moreflexible ring package that better conforms to the cylinder wall. Backduring the musclecar days, most production engines used a5/64-5/64-3/16-inch package. The 1/16-1/16-3/16-inch packages were forall-out racing. These days, Detroit automakers and many racers aregravitating toward even thinner "metric" rings. Standard-tension oilrings have been replaced by low-tension rings. Many of the new ringpackages feature reduced radial wall thickness. Besides decreasingfriction, this makes for a more stable package--assuming the pistonrings, piston profile, and cylinder wall finish take advantage of theseimprovements. In the custom piston world, most build-to-order pistonscan be ordered for reduced radial thickness rings; otherwise, spacerstock can be used to convert conventional pistons.

Ring groove design is far more important than it may appear at firstglance. Properly designed ring grooves have a small degree of verticaluplift, which compensates for uneven temperature growth as the pistonreaches operating temperature. Ring groove smoothness is likewiseextremely important; any waviness or roughness causes poor ring seal andcan lead to microwelding--a destructive situation where, under extremepressure, the rings momentarily attach themselves to high spots on thering groove. There also should be a small radius where the vertical andhorizontal portion of the ring grooves meet. Pistons without this radiusare more prone to groove distortion and ring land breakage.

Thinking on piston ring gaps has also changed. In the old days, secondring gap specs were tighter than those for top rings because they didn'tsee as much heat. But this didn't account for inter-ring gas pressurebuildup between the top and second rings. If the pressure between theserings equals or exceeds the pressure above the top ring, it can causethe top ring to lift off the bottom of the piston ring groove and losecontact with the sealing surfaces. It also inhibits the ring's abilityto transfer heat from the piston. To keep inter-ring pressure frombecoming a problem, the current trend is to create an easy escape pathfor the built-up pressure by gapping the second ring larger than the topring. Another benefit is that because gas pressure is now directeddownward toward the sump, any oil that has collected in the ring packareas will go with it.

Of course, normal ring wear causes the gaps to open up, allowing morecombustion gases to escape. At least one ring manufacturer--TotalSeal--offers gapless rings. Traditionally, these gapless rings went inthe second groove along with a conventional top ring, but ringtechnology refinements and the new thinking on ring sealing has ledTotal Seal to revise this installation scheme and introduce a new lineof gapless top rings that achieve significantly less blow-by underreal-world running conditions.

The ultimate in ring seal is drilling the pistons for gas ports.Compression rings normally need about 0.002 to 0.004-inch (vertical)ring-to-groove side clearance to allow cylinder pressure to get behindthe ring and force it to seal against the groove and cylinder wall. Gasports apply combustion pressure directly to the back of the ring,allowing the virtual elimination of side clearance. Since the ring isrestrained by the groove itself, there's less opportunity for high-rpmring flutter. Very thin, narrow, and lightweight 0.043-inch-thick ringsare needed to reap gas-porting's full benefits. Gas ports work best withshort piston-compression heights (under 1.200 inches) on engines running7,000 rpm or higher. The major drawback is that all this positivepressure greatly shortens ring life, so it's not recommended for streetuse.

No matter the specific thickness and configuration, mosthigh-performance and racing engines now use moly-faced rings in the topgroove. Plasma-sprayed moly over a ductile-iron base material is thepreferred choice, but steel is becoming more popular because it's atleast as strong and easier to machine.

Chrome-plated rings still have a place in off-road or dirt-trackapplications. Just as high-end pistons are now machined to closetolerances, many racers now custom-prep (remachine, if you will) pistonrings to higher tolerances to reap the full benefits of the newhigh-tech pistons. For example, precision ring grooves allow a reducedback clearance if ring thickness tolerances are likewise more tightlycontrolled. Custom-prepped precision-gappable rings are also offered byseveral aftermarket ring makers such as Total Seal.

Matching rings and piston design only reaffirms what we've beenstressing for years: When it comes to engines, there's no magic bulletor individual component. Everything has to be considered as part of atotal package. Lightweight, close-tolerance pistons demandhigher-quality rings. But to work, they require a higher level ofcylinder wall preparation. Reducing friction by running low- tension oilrings may mandate trick oil pans with windage trays, crank scrappers,adjustment of bottom-end bearing clearances, lightweight synthetic oils,and even positive crankcase evacuation pumps. Nevertheless, custompiston makers assert that the right high-end piston and ring combinationcan be worth up to 30 hp on a 1.5hp/ci engine if the rest of thecombination is optimized to take full advantage of them.