The Science Behind Piston Ring Grooves

October 12, 2017 / by Mike Magda

Piston ring grooves serve a larger purpose than simply supporting the rings. They impact combustion sealing, oil control, friction, and many more engine attributes. 

Wiseco is a world leader in piston technology because it focuses on the minute details of cylinder sealing science. Among those details, the type of piston rings used in racing and high performance engines is always a hot topic. But the ring grooves in pistons also play a major role in sealing combustion pressure and controlling oil and blow by.

That’s one area where Wiseco extends the extra engineering effort to ensure maximum performance from its pistons. While various types of piston rings are more suitable to specific applications, the ring grooves themselves are often overlooked in the pursuit of optimum cylinder sealing.

The strongest piston in the world is rendered useless without ring grooves designed for the application at hand.

Many engine builders go to great lengths to file precision end gaps, but ring clearance in the groove frequently goes unchecked and the quality of the ring groove sealing surface is rarely considered. Clearance and ring groove quality are frequently assumed. Wiseco goes to great lengths to make sure you get this right, even if you don’t check it.

Decades ago racers like Bill “Grumpy” Jenkins used to purchase unfinished pistons and cut their own ring grooves exactly where they wanted them and to the finish and tolerance they desired. Today, that’s no longer necessary as manufacturers such as Wiseco pay considerable attention to ring groove design.

The interface between the ring face and the cylinder wall is still widely discussed among racers and engine builders, but combustion sealing in the ring groove is equally important. For best results the ring groove must offer the ideal clearance and freedom of movement for the ring and it must provide a hard, flat sealing surface for the ring to bear against under high cylinder pressure.

Ring Groove Nomenclature:

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Axial Clearance
The vertical clearance remaining in the piston ring groove after the ring is installed. Depending on the application, the side clearance is typically .001-.003-inch. With today’s thin rings, some racers run side axial clearance as tight as 0.0004 - 0.0005-inch, using gas ports to supplement ring pressure.

Radial Back Clearance  
Ring groove space behind ring when the ring face is flush with piston ring land. A back clearance of .008 - .012 inch is typical to racing and high performance pistons.

Radial Wall Thickness
Ring dimension from the front face touching the cylinder wall to the back or inside face of the ring. “D-wall” is the standard automotive thickness (SAE standard) and it’s calculated by bore diameter divided by 22. For example: 4.125” bore divided by 22 is .187” radial wall thickness.

Axial Height
The dimension from one side face to the other or ring thickness, usually expressed in fractional sizes like 1/16”, decimal sizes like .043” or metric sizes like 1.5mm. Smaller ring thickness means less mass and less inertia to overcome when the ring reverses direction at TDC and BDC. The smaller ring also generates less friction but there is a relationship between ring dimensions, sealing and oil control.

 Measuring Ring Groove Clearances

A feeler gauge is the most common method for checking ring clearance in the groove. Typical clearance ranges from .001 to .005-inch depending on the application.

Axial clearance is typically checked by inserting a feeler gauge between the ring and the top of the ring groove. A slight drag will indicate the proper clearance. Start with the minimum of .001-inch and work up. Don’t measure between the ring and the bottom of the ring groove as you might damage the sealing surface. Some builders measure the groove itself with a stack of feeler gauges and then compare it to measuring the ring thickness with a micrometer. The first method is preferred and it is good practice to check it at several places around the perimeter of the piston.

Back clearance is checked by inserting the edge of a ring into a groove and pressing it all the way to the back of the groove to make certain it does not protrude past the face of the ring land. You can determine the back clearance by measuring the depth of the ring groove with the depth scale on a set of dial calipers and then comparing it to the measured radial thickness. If the ring groove is too thin to measure with a caliper depth scale, a machinist’s scale is another option. Another way is to use modeling clay or soft wax; press it into the groove then remove it and measure with a caliper.

Most pistons use drilled oil reliefs at the back of the oil ring groove. Some have slotted holes that open up when the groove is cut. They are equally effective but some designers feel they allow more flex in the ring land.

Ring Groove Quality

Ring grooves must be perfectly perpendicular to the cylinder wall so cylinder pressure can press the ring down against the land and out against the cylinder wall for optimum sealing. During the combustion event, cylinder pressure follows a path down the crevice volume and into the ring groove above the ring. It then fills the ring back spacing to press the ring out against the cylinder wall. Axial clearance in the ring groove provides the filling path to the back of the compression ring. So The ring is pressurized against the cylinder wall and the bottom of the ring groove.

The back-space dimension is critical because it controls the ring’s response time. Too much back spacing makes the ring slow to respond, but there must be some clearance so the ring can move and conform during all dynamic conditions. The main purpose for axial clearance is to allow the ring to spin. The cross-hatch in the cylinder walls induces rotation of the rings. Vertical and horizontal gas ports in pistons are also an accepted way of routing cylinder pressure to the back of the ring.

Microwelding

Microwelding is the abnormal transference or welding of ring land material to the piston ring due to excessive heat. It prevents the ring from moving properly and if severe, it can seize the ring in the groove.

One of the most critical problems accompanying severe racing and high performance piston applications is the potential for "microwelding." Microwelding damages the piston ring or ring groove sealing surface when localized friction welding causes the transference of material from the ring land to the ring side, most often the bottom ring side.

High temperature and excessive movement are the root causes. It frequently accompanies high temperature endurance and/or supercharged applications where ring placement is too high on the piston and too close to elevated combustion temperatures, or in endurance applications where severe conditions are ongoing. Microwelding upsets ring sealing quality and can even jam the ring in the groove. It should be noted that ring grooves (especially the top groove) do not expand evenly since material thickness is not uniform due to dome profiles, dish profiles and valve reliefs.

Ring Groove Placement

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Positioning of the ring pack is application specific and is dictated by the piston’s compression height, the size and depth of the valve notches, and the dimensions of the ring package. Many OEM engines have the top ring  .300-.400 inches down from the engine deck surface. Endurance applications place the top ring .125 to .150 inches down. In heavily modified normally aspirate race engines, the top ring location can vary from .060 to .100 inches down from the deck. 

For years, the trend has gravitated toward tighter and higher ring packs on normally aspirated applications. It promotes stability by spreading the contact points between the rings and the skirt and reduces the crevice volume which resists detonation and promotes a more consistent burn, making the cylinder more active. Racers in the Super Stock classes always locate the top ring as high as possible to ensure stability of the ring pack and to promote a more active and thorough combustion event. It also lets them use shorter and lighter pistons.

Boosted applications (right) have the entire ring pack moved well down the piston to move it away from higher temperatures. Rings are placed higher on a max-effort, naturally aspirated piston (left) to minimize crevice volume, accommodate longer rod lengths, and spread the contact points to stabilize the piston on rock over. This is often done on Super Stock and modified drag racing pistons.

Forced induction and nitrous applications that are subjected to extreme thermal and pressure shock loads typically require moving the top ring down from the piston top to about 0.300in. In many cases this is also dictated by valve size and positioning, valve pocket requirements, the radial width of the top ring and the piston pin location.

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Sometimes the ring pack is moved down well over .400-inch to accommodate these concerns. Intake valve pocket specs generally control top ring position because the intake valve is always larger with more proximity to the edge of the piston crown. Thinner rings and smaller ring grooves offer more leeway for optimum ring placement because they require less space, but they are at risk in sever applications.

Moderate nitrous applications in the 250 HP range will work with the ring pack approximately .250-inch down from the crown. Above that, more is always better as nitrous air fuel ratios are always chaotic and unpredictable. In that case, .450-inch or more is not unreasonable.

Gas Ports and Ring Flutter

Combustion pressure aids in sealing the ring to the cylinder wall. Cylinder pressure enters the ring groove from the crevice volume, traveling across the top of the ring and down behind it to force it out against the cylinder wall. The pressure above the ring seals it against the bottom of the groove.

Ultra-thin, low tension rings need combustion pressure provided by gas porting to achieve optimum sealing. Oval track and road racing pistons use horizontal gas ports at the top of the ring groove to resist carbon buildup while shorter duration drag racing engine use more effective vertical gas ports.

Gas ports deliver direct cylinder pressure behind the ring to seal the ring against the bottom surface of the ring land and to force it outward against the cylinder wall. The diameter and number of gas ports is largely based on application and piston diameter. The gas pressure must be evenly applied to the ring to promote a good seal and to prevent detrimental ring flutter. 

Vertical gas ports (Right) intersect the very back of the ring groove to exert direct pressure against the back of the ring. They are use in drag racing applications where carbon buildup is not a factor due to frequent rebuilds. Horizontal gas ports (left) around the top of the ring provide an additional path for combustion pressure to reach the back of the ring groove. They are used in endurance applications because they don’t carbon up as much as vertical ports.

Contact Reduction Grooves

Contact reduction grooves reduce friction by minimizing piston material in contact with the cylinder wall above the top ring. They also disrupt pressure spikes caused by detonation.

 These grooves are machined into the top ring land above the top ring to minimize contact drag when the piston rocks over upon reversal. They add minimal volume to the crevice volume and they also help resist detonation by disrupting flame travel into the crevice volume where pressure spikes might unseat the ring.

Accumulator Groove

The accumulator groove provides additional volume below the top ring so pressure doesn’t build up and attempt to unseat the top ring.

The accumulator groove is machined into the piston between the top compression ring and the second (scraper) ring. Its purpose is to provide additional relief space for pressure escaping past the top ring to build up before it attempts to pass the second ring. It supports top ring sealing by relieving pressure and it helps reduce ring flutter due to pressure changes. Accumulator grooves have proved most effective and they are a common feature on many, if not most, high performance and racing pistons.

The quality and placement of the ring grooves on your pistons is just as important as your cam specs. Proper ring groove placement and ultimate sealing quality are the keys to more power and durability under any severe duty applications. Hence it is important that you use the rings specified by your piston manufacturer or be prepared to share your ring pack information if you are providing your own rings.

A spec sheet is included with all pistons and provides procise measurements for ring end gaps and ring groove clearances.

Topics: featured, PISTONS 101, ENGINE TECH, Tech

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Written by Mike Magda

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