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Single-Ring vs. Two-Ring 2-Stroke Pistons: Which One is Better?


Kevin from Wiseco

Single and dual compression ring two-stroke pistons have been in service for decades, and since their inception, many have wondered if there are advantages to one or the other. If you have been involved with dirt bikes, jet skis, or snowmobiles long enough, you’ve probably noticed different manufacturers have chosen to use one or two compression ring piston designs for their engines. Furthermore, you may have noticed some aftermarket piston companies offer single ring pistons that replace dual ring pistons and vice versa. So, as a consumer, what do these design differences mean, and which one should you choose?

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Wiseco has been manufacturing two-stroke pistons since 1941. In fact, the company started with two-stroke racing pistons being built in Clyde Wiseman’s garage. There’s no replacement for experience, so we want to take this opportunity to shed some light on the advantages and disadvantages of single and dual compression ring two-stroke pistons designs.

Compression Ring Function

We’ll start with a quick review of what a compression ring is designed to do. First and foremost, the compression ring provides a seal that allows the piston to compress the air/fuel mixture as the piston travels upward, then during the combustion event itself, it seals the rapidly expanding hot gases that form during the combustion event. The effectiveness of the compression ring seal, in part, has a significant effect on the power and efficiency of the engine. Should the compression ring lose its ability to seal, the amount of trapped air/fuel mass that will be retained during the compression stroke will be significantly reduced, resulting in less power. Similarly, during the combustion event, a compromised ring seal will allow more gases to leak past the ring, often referred to as blow-by, resulting in reduced power.

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The compression ring, or rings, seals compression so the piston can compress the air/fuel mixture. This plays a critical role in performance, as an improper seal will cause a very poor running condition or not allow the engine to run at all.

Heat Transfer

The piston rings play a vital role in transferring heat from the combustion process to the engine's liquid or air cooling systems. During combustion, the piston crown absorbs a portion of the extreme temperatures it is exposed to. If left unregulated, the piston would become so hot that it would melt. Thankfully, the piston rings transfer heat from the piston by connecting the piston to cooler parts of the engine such as the cylinder liner. From the liner, the heat finds its way to the water jacket or to the cooling fins on an air-cooled engine. Engine designers optimize the size, shape, position, and the number of rings to influence how the piston and rings transfer heat.

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In addition to sealing compression, the piston rings play an important role in transferring heat from the piston crown and through the cylinder wall to be dissipated by the cooling system. Otherwise, the piston material would not survive the extreme heat.

Conformability

The piston ring’s conformability refers to how well it adheres to the shape of the cylinder bore. The conformability of the ring will have a direct effect on how well it seals the mixture and combustion gases as well as transferring heat to the cylinder liner. Factors that influence a ring’s conformability are shape and thickness. In particular, thicker rings will be less conformable than thinner rings because ring thickness has a significant influence on ring stiffness.

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Thicker rings are generally less conformable, and therefore may not seal compression as effectively. However, too thin of a ring will not transfer heat well enough. It's important to develop a balanced ring that performs both tasks effectively.

Wiseco’s Research & Development Manager comments, “Racing applications tend to favor single rings for a lower friction penalty. Also, thinner single rings have better conformability to the cylinder and are less susceptible to flutter at high RPM. Even when specified with lower tension, thinner rings can still have good unit pressure which promotes sealing without a high friction penalty.”

Single Versus Two-Ring Applications

While many have speculated that certain types of two-stroke powered vehicles—whether it be ATVs, dirt bikes, jet skis, or snowmobiles—need one or two ring pistons, it isn’t so much the specific vehicle application that drives the selection, but more the intended use for the vehicle. The big differentiator is whether the vehicle’s intended use is for racing or not. Two-stroke engines designed and developed for racing typically utilize single ring pistons. When designers optimize an engine for racing and select a single compression ring design, several advantages and disadvantages arise when compared to a two-compression ring design. Let’s take a look at the pros and cons of each.

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A single-ring design is common among Wiseco pistons that are designed for racing and high-performance engines, such as the Racer Elite piston.

Single Ring Pros:

●       Lowest friction design translating to increased power

●       Lowest weight design contributing to fast revving

Single Ring Cons:

●       Potentially less longevity due to heat dispersion

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Two-rings designs are popular among riders that prefer added performance durability at the expense of a little performance. However, some big-bore two-stroke applications benefit more overall from a two-ring design.

Two Ring Pros:

  • Improved heat transfer due to the addition of the second ring
  • Engine performance durability due to 2nd ring’s ability to seal if first ring’s seal becomes compromised

Two Ring Cons: 

  • Increased friction and weight
  • More susceptible to ring flutter at high RPM

“Since a good portion of the piston heat is transferred from the piston to the ring and then to the cooler cylinder wall, one advantage of a 2-ring system is that the second ring would provide a second heat transfer path,” adds Dave Fussner on the topic of two-ring designs.

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The last point worth mentioning is that racing piston ring applications are optimized for excellent ring control at high RPM. A condition called “flutter” occurs when a ring becomes unseated from the piston’s ring groove. Flutter occurs around top dead center as the piston transitions from upward motion to downward motion, in part, because the inertia of the ring, which is a function of the ring’s mass, overcomes the gas pressure, pushing the ring against the bottom of the ring groove. When this happens, the ring’s sealing ability is compromised, and engine performance degrades both in terms of performance and durability. Engine designers combat flutter by optimizing the ring's weight so that the ring’s inertia forces cannot induce flutter within the intended RPM range. This is why in many single ring applications the rings are relatively thin.

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Single-ring designs are less susceptible to flutter because there is less ring mass changing direction as the piston begins its return from TDC.

These are the general and major factors that drive single ring and two-ring designs in two-stroke pistons. The intended use of the vehicle usually drives ring selection, not the vehicle type itself. If you are considering a switch from a single ring to a two ring piston, ultimately, how you intend to use your machine should dictate whether the decision  is sensible or not.

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Piston rings seal is very much aided by compression and combustion pressures...

The inherent spring tension style of lateral force that a ring exerts on a cylinder wall doesn't stand a chance of sealing combustion pressures on it's own... The ring MUST use combustion pressure to create an effective seal against both the bottom of the ring land AND to force the ring out against the cylinder wall..

The ring and ring channel have to work in unison to allow for gas pressure to get behind the ring to force it out against the cylinder wall... This is what allows the use of lower and lower tension rings... Low tension rings will have less parasitic drag when they don't need to seal tightly, and then have a tight seal when using gas pressure to enhance the downward and outward forces... 

If a secondary ring is the one doing serious pressure retention of any kind, the performance is compromised and the piston's useful hours are on serious decline...

Appreciable pressure captured BELOW the top ring will unseat the top ring from the bottom of the ring groove... Which will then allow for hot gasses to ensconce the top ring... Heat transfer is lost and carbon build up on the bottom of the ring land will make blow bye even worse... And then you have weight and friction of 2 rings, yet only the seal (maybe) of the second ring...

In my experience, if the second ring is the one doing the sealing, you basically don't have a performance engine anymore...

Single ring for me please...

 

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The proceeding view point is loosely based on vague recall and seat of the pants experience; no math or science to back it up. 

I prefer the 2 ring piston, if for no other reason, the longevity between top ends.  My 06 CR250R (single ring) would make about 25-30 hours, where as both of my 13 & 14 250SX's would make about 80 hours before needed top end.  Some of this may be due to technology improvements, or simply 2 rings will mask wear for a bit more time.  

As far as the extra ring drag causing HP loss, I have no numbers to dispute such, but I'm guessing the numbers are negligible, or at least at the amateur lever.  Maybe the extra ring seals just a bit better against blow-by, and therefore having a cleaner base for the next incoming air-fuel charge.

I raced dirt cars for years before venturing into the world of Mx, and we used and had excellent results with Total Seal rings.  I contacted Total Seal about making a total seal ring for a 2-stroke.  They said they would look into it if I would send them the cylinder and piston, but I never followed through.

I reckon I'm done babbling/meandering, thanks for listening. 

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In the 50,s it was not seldom that a 2 stroke piston had 3 rings.

That was at a time when cast iron cylinders and lousy Piston and ring material needed big over bores.To stabelise the piston against the walls , 3 rings were better then less.

Today,with Nicasil plating , phantastik oil,s we can manage with one single ring for a very long time.

This little air cooled 125 SCREAMER runs on NICASIL-Single RING and 100-1 (Opti2) at least one full season ( before it may need a new ring.)

Zunny 003.jpg

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On 12/7/2018 at 2:23 PM, Antoine Mion said:

Interesting post. 

What if I want my 2 rings piston become a single ring one? I guess I would have to keep the top ring.

Yes, you would have to keep the top ring. But it's always recommended to only run them as they were designed. It would only be recommended to buy a single-ring piston specifically if that's what you're looking for.

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OK, thanks ! Of course, it would only be in case I cannot find a single ring one. For example, I have a 125 TSR Suzuki, for this one, you only find 2 rings pistons. I want to try some scavenging changes and the 2nd ring gets into my way.

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