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Proper engine break-in is equally as important as a proper rebuild. Here, we'll go over a checklist to make your build will last, as well as a step-by-step break-in process. Putting in the time and money to rebuild your motorcycle engine is both a critical job and a prideful accomplishment. The feeling of an engine failure right after a rebuild is a sinking one, and will most likely stir up a mixture of frustration and disappointment. We want to help as many people as we can avoid that feeling. So, we've put together a review checklist for your rebuild, followed by a general engine break-in procedure, because your motorcycle should bring joy and fun to your life, not take tufts of hair out of your head. We'll start with a quick review on the motorcycle top end rebuild. Be sure these critical steps and precautions have been taken. If you find any concerning discrepancies, it's worth it to pull back apart and double check. Be sure that you have proper piston to cylinder clearance. Recently, a cylinder was bored with requested .0035” clearance. This machine shop has been in the area for over 30 years. When complete, it looked like it was tighter. He slipped the piston through the cylinder a few times and said, "It's okay." He was asked to check again, which he refused, and said that it was correct, and that he was too busy. Back in the Brew Bikes shop, it was double-checked, and clearance was .0015”. Yes, way too tight. Don’t just take someone’s word that clearance is correct, always double check it! Always double check your piston-to-wall clearance. Was the honing of the cylinder properly done? Honing is required to be done after boring, and if the cylinder was not bored, it still is needed to deglaze the cylinder for proper ring break-in. Different honing tools are better used for different applications, with common tools being brush hones and flex hones. Safe grits and hone materials depend on the cylinder finish, so check your manual or with the cylinder shop for a recommendation. Be sure that the crosshatch is at 45 degrees. The proper crosshatch will retain the proper amount of lubricating oil while allowing the rings and piston to break-in. Too little of crosshatch or too much will not allow the rings to break-in correctly and never get the proper sealing they were designed for. Read our full guide to cylinder prep. After proper honing and deglazing, your cylinder wall should have a consistent, 45 degree crosshatch. If the bike is a 2 stroke don’t forget to chamfer the ports. If it has a bridge in the exhaust port, most pistons require this area to be relieved. READ the piston specs, and if you don’t understand, be sure to reach out to Wiseco for specifications. Read our guide to relieving the exhaust bridge in 2-stroke cylinders. A critical step in 2-stroke cylinder prep is port edge relief and exhaust bridge relief. This will help ensure smooth piston and ring operation, and combat accelerated ring wear. Be certain that the ring gap is within specification. Don’t assume it is correct, check it. Always double check your ring end gap. With your compression ring in the cylinder, measure the end gap with a feeler gauge to ensure it's within the spec included in your piston instructions. Proper cleaning of the cylinder. Before you start cleaning make sure the gasket areas are clean with no residue of gasket or sealers. First, use a cleaning solvent with a brush and then again with a rag. This is not enough, and you will need to clean with dish soap and water. Using a clean rag you will be amazed on how much grit from the honing is still in the cylinder. Be sure to clean the piston also. Thoroughly cleaning your cylinder for a rebuild is critical. Be sure all old gasket material is removed, and use a 2-step cleaning process of solvent with a brush and rag, followed by soap and water. When the cylinder is clean and dry, you should be able to wipe the cylinder wall with a clean rag and not see any honing material residue. Then before assembly, use plenty of assembly lube on the cylinder and the piston. Don’t forget to lube the piston pin and bearing along with the rings. Assembly lube on the piston, rings, cylinder, pin, and bearing is important for proper break-in. Many rings have a topside for proper sealing. Double check this and be sure the proper ring is on the proper landing on the piston. Again, read the instructions that came with the piston. Piston ring markings vary, but the marking should always face up when installed on the piston. The gaskets and quality play an important part of engine rebuilding. If a gasket is thicker than the original, it could result in a loss of power. Worse yet, a gasket thinner than the original will result in less deck height (piston to head clearance). This reduced clearance may result the piston to come in contact of the head causing permanent damage. After placing the gaskets, be sure while assembling the piston in the cylinder that the ring gaps are in proper placement. Check your engine manual for proper placement of the piston gaps. Then, install the head. Many motorcycle manufacturers have a desired head nut tightening sequence. Refer to their procedures while doing this. Most companies give the head nut torque rating with the washers, nuts and studs being clean and dry. That means if you use oil or a thread locking compound the studs will be over-stressed due to the over-tightening of the head nuts. Engines have been damaged by this. Now you know, follow what the engine manufacturer recommends! Regardless of the type of motorcycle engine you're working on, there should be a tightening sequence and torque spec for the head nuts. Pay close attention to the specs in the manual, as these are critical to prevent damage and for proper operation. Use the proper engine oil and fill to the proper level. The fuel you use should be fresh and of the proper octane. If your engine is a 2 stroke, mix to the proper fuel/oil ratio. For just about any 2-stroke, whether vintage or a newer, a 32:1 fuel/oil mixture is very common, but check your manual for the recommended ratio. Not only is it important for piston lubrication, but also for the crank bearings and seals. After all this work has been done, and you feel confident with the rebuild, what else can go wrong? PROPER ENGINE BREAK-IN! So many mistakes can happen while breaking in the piston and rings, resulting in rings never properly sealing or/and piston galling. Many builders have their own procedures, but most all do heat cycling for breaking in engines. Before we get into it, please note that this is just one of many methods that work well for engine break-in. Many people have many different effective methods, this is just one example that has worked well for us. Use this break-in procedure as a guideline for your next fresh top end: It's important to ask yourself if the rebuilt engine is still using the same carburetor, air cleaner, exhaust system, cam, compression, or if a 2-stroke, the same port work configuration? Any changes can result in air/fuel mixtures to be either too rich or too lean, resulting in engine damage. If your engine is fuel injected and in good working order, the ECU and O2 sensor should keep the air/fuel mixture correct. If you have access to an air/fuel meter, or if a 2-stroke, an EGT (Exhaust Gas Temperature) gauge, check the air/fuel mixture. Even with these tools, spark plug readings are still recommended. Spark plug readings are a sure-fire way of knowing if your engine is running too lean or too rich. We'll get into more detail in a later article, but generally the plug will look white when it's too lean, and dark brown or black and wet when too rich. At first start up, keep the engine just above idle and give it a few revs up and down. This power on and power off RPM breaks in the piston and rings evenly on the intake and exhaust sides. If air cooled, once the engine builds up heat where it becomes too hot to touch, shut the engine off. If water-cooled, once the engine coolant starts rising in temperature, shut the engine off. This initial warm up takes just a couple minutes. Now wait a few minutes until the engine is slightly warm to the touch, repeat #2, letting the engine get slightly hotter. Be sure to keep the engine RPMs above normal idle and keep the RPMs going up and down slowly. Let it cool again till it is slightly warm to the touch. This time, start and run longer until the engine gets near operating temperature. If air cooled, be sure you have a fan pushing air from the front. You now can rev the RPMs up a little higher, being sure not to hold it at a sustained RPM, but revving it up and down. Let the engine cool completely. Check all fluid levels to be sure there is no loss of engine lubricant, or, if water-cooled, engine coolant. After engine is cool, do a plug reading to be sure it is not running lean. Because the engine has run a few heat cycles, the gaskets may have compressed. It is VERY IMPORTANT to be sure engine is totally cooled down, and then check the torque of the cylinder head nuts. Most times the cycling head nuts will need some re-tightening. DON’T over-tighten; just tighten to manufacturers’ specification as you did when assembling the engine. Next, warm up the engine for a couple minutes as you did in the other procedures. Ride the bike, revving the engine up to normal riding RPM. Be sure NOT to keep the RPM too low and don’t lug the engine. These low RPM’s actually puts much more stress on the engine parts. If this is a dirt bike, running on a track is best due to the up and down RPMs the engine will experience. Don’t be afraid to run it normally. If this is a road bike, a curvy road is best due to the RPMs going up and down, this is a must! Don’t lug the engine and don’t go on an open highway that keeps the engine at a sustained RPM. This first initial ride will only be about 5 minutes. Let the engine cool till you can touch the engine. Follow the same procedure as above, but this time running for 10 minutes. This will be your last break-in run. Follow the above procedure and run for 15 minutes. Now is the time to let the engine totally cool down again. Check the fluids as you did before after the engine has completely cooled down, and do another spark plug reading. It is now time to do another check of the cylinder head nuts for proper torque. Sometimes no additional tightening is needed, but don’t be alarmed if you need to, because this is normal Check all your fluids once more after the engine cools, inclduing coolant and oil level. At this time, the rings and piston should be broken in. Go out and ride it. The first few times, just be sure not to get the engine overheated, but your ride times are not restricted. It never hurts to do another spark plug reading and double-check the head nuts after your first long ride. Enjoy your rides, and be safe!
Rebuilding a top end is a task most two-stroke owners will run into at one point or another. Here, we go over critical steps and key tips to installing a new piston and ring(s) in your two-stroke. Periodically, if you own a two-stroke, there will come a point where you need to rebuild the top end of your engine. Hopefully, this won’t come as a surprise to you and will be part of your planned maintenance schedule versus experiencing an unplanned engine failure. While two-stroke engines are relatively simple mechanical devices, rebuilding them requires knowledge of how they work, attention to detail, and a systematic approach. We’re going to cover numerous tips pertinent to two-stroke top end rebuilds. These tips will be discussed chronologically and will encompass all phases of the build from pre-rebuild prep, to disassembly, through post build. The tips we’re going to share shouldn’t be considered inclusive of everything that has to be done, but are tips that focus on things that are either often overlooked or incredibly important. Let’s get started! Pre-Teardown Diagnosis - Before tearing the engine apart, are there any signs that a specific problem exists? If so, are there any diagnostic tests such as compression or crankcase leak down that are worth performing? Before tearing your engine down, asses the specific problem with you're engine if you're rebuilding due to a running problem. Clean Machine - Take time to thoroughly clean the machine before opening up the engine, especially if you will be servicing the top end without removing the engine from the machine. Service Manual - Performing engine maintenance without an OEM factory service manual is not recommended. Make sure you have a manual for your machine prior to starting work. The manual is the only place you’ll find service limits, torque specs, and other key data. Disassembly Limit Contaminants - Once the cylinder has been removed wrap a clean, lint-free rag around the top of the crankcase. Dirt is one of the leading causes of engine wear, and limiting the opportunity for dirt to enter the crankcase is very important. Keep a lint-free rag at the top of the crankcase at all times while it is open and exposed to potential contaminants. Piston Removal - Easy piston circlip removal can be accomplished by using a pick and needle nose pliers. Insert the pick into the dimple in the piston and behind the circlip. Then use it as a lever and pry the circlip out partially. Once out partially, grab the circlip with needle nose pliers. During this process, be careful not to scratch or mar the wrist pin bore as this will make removing the wrist pin much more difficult. Use tools as needed to aid in circlip removal, but be careful not to mar the pin bore so the wrist pin can be easily removed. The ease of pin removal will be largely dependent on the engine design and condition of the bore. If the pin can be removed by hand, great, if not, light tapping while supporting the rod is permissible. Otherwise, a pin puller should be utilized which can be bought or made. In its simplest form, this can consist of an appropriately sized bolt, nut, and socket. Once the wrist pin has been removed, the piston can be removed from the rod. Hopefully, the wrist pin can be removed by hand once the circlip is out. If not, an appropriately sized socket with some light tapping from the opposite end can help break it loose. Power Valve Disassembly - Prior to taking the power valve system apart, spend some time reviewing the procedure in your service manual. For additional insight into how the components interact, review the exploded views in the service manual and look at part microfiches, which can be found online. Online microfiches can be very helpful to double-check reassembly of the power valve. They can be found on many motorcycle dealer websites. When removing the power valve system, consider laying the components out on a clean rag in an orientation that correlates to how they are installed in the engine. This is a relatively simple thing to do that will help you remember how they are installed later. When it comes to cleaning the components, clean them one at a time or in small batches so that they don’t get mixed up. Lay out all the parts of your power valve assembly as you disassemble it. This will help you keep everything organized, and make sure you get it back together correctly. Inspection Reed Valve - Don’t forget to check the condition of the reed valve petals, cage, and any stopper plates. Most service manuals will detail the acceptable clearance between the petal tips and cage as well as the stopper plate height. Ensure any rubber coatings on the reed cage are in good condition. Inspect all reed valves components thoroughly before reassembling the top end. Any parts showing signs of excessive wear or damage should be replaced. Intake Manifold - Check the intake manifold for cracks. Cracks are more common on older engines, and propagation all the way through the manifold can lead to air leaks. Exhaust Flange - Check the condition of the exhaust flange and ensure that it is not excessively worn. An excessively worn flange will make exhaust gas sealing difficult, hamper performance, and leak the infamous spooge. Power Valve Components - Take a moment to review the condition of all the power valve components. Significant wear can occur over time and lead to performance losses. Rod Small End - Check the small end rod bore for surface defects such as pitting, scratches, and marring. Any severe defects in the bore will necessitate rod replacement. The rod small end is a critical point of inspection. Any damage to the inside surface could affect the small end bearing, leading to a chain of top end problems and potential failure. Sourcing New Components When freshening up the top end in your two-stroke, it’s important to reassemble with quality components. A deglazed and honed or bored and replated cylinder is a critical component to ensuring reliable performance from your new top end. Your local cylinder shop should be able to handle the bore and replate when necessary, and a simple deglazing can be accomplished with a Scotch-Brite pad. Be sure to retain the 45-degree honing mark angle. There are a lot of choices for new pistons from the aftermarket out there, but many people choose to stick to OEM. However, when ordering from the OEM, every individual part must be ordered separately, including the piston, ring, pin, clips, gaskets, etc. Dealing with all these part numbers and chancing forgetting a component can be a pain, and get expensive. ProX two-stroke pistons are manufactured by OEM suppliers, and come with the piston, pin, ring(s), and circlips all under one part number. ProX two-stroke pistons are manufactured by the same OEM-suppliers to exact OE specs. They are available in A, B, C, and D sizing for most applications. ProX pistons come with the piston, ring(s), pin, and clips all in one box. Complete top-end gasket kits can even be ordered under one part number. ProX pistons provide an OEM-replacement option with less hassle and less strain on your wallet. Find ProX pistons for your bike here. Even though ProX pistons are made by OE suppliers, the quality control difference is evident. On the left is a ProX piston for a Honda CR250, and on the right is a brand new piston out of the box from Honda. Which would you choose? Measurements The number of measurements that should be taken throughout the top end rebuild will be discretionary. At ProX, we strive for excellence and err on the side of caution when it comes to engine building, so our builds consist of numerous measurements and inspections prior to reassembly. For us, this ensures a high level of confidence and safeguards against external oversights. We recommend the same to anyone building an engine. Below is a list of measurements that we routinely make when rebuilding a two-stroke top end: Piston ring end gaps Piston-to-cylinder clearance Rod small end diameter Out of these measurements, confirming or adjusting the ring end gaps is by far the most important, followed closely by ensuring the cylinder bore is within spec with respect to diameter, straightness, and roundness. Understandably, some measurements may be difficult for the average home builder to execute, usually due to not having the right equipment, however, a competent shop should be able to assist. Ring end gaps can be checked by installing the ring in the bore without the piston, and using a feeler gauge to find the measurement. Correct ring end gap is listed in the installation instructions that come with a new ProX piston. ProX rings often do not need to be filed as they are pre-gapped, but it's always a good idea to make sure your end gap is within the provided spec. Piston-to-cylinder is another measurement that should be checked before final assembly. For this, use a bore guage and a set of calipers to measure the bore size. Next, grab a set of micrometers and measure the piston. ProX pistons should be measured perpendicular to the wrist pin, a quarter of the way up the piston skirt from the bottom. Subtract your piston size measurement from your bore size, and you have your piston-to-cylinder clearance. ProX pistons come with a chart on the instruction sheet that shows the range your clearance should be in. Measuring piston-to-cylinder clearance is a smart precaution to help ensure you won't run into any unexpected issues with your new top end. A final measurement we recommend taking is the rod small end diameter. This is important because sometimes these can get worn out and create free play for the small end bearing, resulting in damage to the bearing and most likely the entire top end. It can be done using the same method as the bore diameter. Compare your measurement to the acceptable range in your owner's manual. Making sure the diameter of the small end of the rod is within spec is often overlooked, but can prevent a serious top end failure. Prep Work Cylinder Cleaning - Once the cylinder has been deglazed or has come back from replating, it should be cleaned one final time. There is almost always leftover honing grit that will need to be removed. To effectively clean the cylinder, use warm soapy water and a bristle brush, followed by automatic transmission fluid or a similar cleaning solution and a brush or lint-free rag. To check the cleanliness of the cylinder, rub a cotton swab around the bore and look for contaminants. Clean the bore until no contaminants are visible on the cotton swab. Any honing grit that remains in the cylinder will facilitate premature wear of the piston rings. A clean, de-glazed, and properly honed cylinder is key to piston and ring function and longevity. Power Valve Function - Cylinders that have been exchanged or replated should have the power valve system reinstalled ahead of final installation. Often times, excess plating can inhibit power valve movement. To correct this, the excess plating must be carefully removed. On cylinders utilizing blade style power valves, the blade position with respect to the cylinder bore should be checked to ensure the blade does not protrude into the bore. Assemble the power valve before installing the new piston and reinstalling the cylinder. Be sure to check that the power valve is moving as it should, and not protruding into the bore. Piston - It is usually easiest to prepare the new piston as much as possible by installing one of the circlips and the ring pack ahead of joining it to the connecting rod. Unless your service manual dictates which circlip must be installed first, choose the easiest installation orientation. Typically, your dominant hand and preferred work orientation will dictate which side you choose to install the circlip on. It's easier to install one clip and the piston ring(s) before fixing the new piston to the connecting rod. Reference your service manual to determine the correct orientation of the circlip. Usually, the open end of the circlip should be oriented to the 12 or 6 o’clock position. Temporarily install the wrist pin and use it as a backstop so that the circlip is forced to move into its groove. Installing the circlip should be done by hand to limit the chance of deformation. Orient the circlip to the desired position, then push the open ends of the circlip into position first. Be careful not to scratch or mar the wrist pin bore in the process! Once installed, use a pick or screwdriver to confirm the circlip is fully seated and does not rotate. Any circlips that can be rotated must be replaced because they have been compromised and deformed during installation. Make sure to note the orientation of each clip after installation. Some manuals may recommend specific positions depending on the piston, but always be sure the gap is not lined up with or near the dimple(s). Rings - The compression ring(s) will be directional, and the top of the ring is typically denoted by markings near the end gaps. Apply a thin coat of oil to the ring, then carefully work the ring into position, making sure to line up the ring end gaps with the locating pin in each ring groove. Install the ring(s) with the marking(s) facing up, and make sure the ring end gap is lined up with the locating pin in the ring groove. Installation Piston - On the top of the piston, an arrow will be imprinted, which typically denotes the exhaust side of the piston. Consult your service manual to confirm the proper orientation of the arrow and piston. Apply a light amount of assembly lube to the small end bearing and wrist pin bore on the piston, then install the bearing. Align the piston with the small end of the rod, and slide the wrist pin into place. Once again, use the wrist pin as a backstop, then install the remaining circlip into position. Use a pick or screwdriver to confirm it is fully seated and does not rotate. Don't forget to apply some assembly lube to the ring and piston skirts before assembly! Cylinder to Piston - In most applications, a ring compressor is not required to compress the rings and install the piston into the cylinder. Lightly oil the cylinder bore with assembly lube or engine oil, then lube the piston skirt and ring faces. Prior to installing the piston and rings, confirm one final time that the piston ring ends are oriented correctly to their respective locating pins. Once the new piston is installed on the connecting rod, apply some assembly lube to the cylinder wall, and carefully slide the cylinder over the piston. Squeeze the ring with your hand as you slide the cylinder on, simultaneously making sure the ring end gap remains aligned with the locating pin. Position the piston at or near TDC then carefully lower the cylinder bore down onto the piston. Use your fingers to compress the ring(s) and ensure the cylinder bore is square to the piston. Feel how easily the cylinder slides over the piston and rings. The installation of the cylinder should be smooth and offer little resistance. If resistance is felt, stop immediately and assess the ring pack. Occasionally one of the rings may come out of position in its groove and snag the cylinder bore. This typically happens as the ring transitions out of your fingers and into the cylinder bore. Once the cylinder is safely over the ring, slide it all the way on keeping the piston at top dead center (TDC). Don't forget to torque your cylinder and head nuts to the specification listed in your manual. Post Build Torquing - Your cylinder and head nuts should always be torqued to the specifications outlined in your service manual. Double check all the nuts are set at their corresponding specs. Spark Plug - Don’t forget to install a new spark plug and if necessary gap it appropriately. Air Filter - Be sure to install a clean air filter prior to start up. Crankcase Leak Down Test - As one final precautionary measure perform a crankcase leak down test. A crankcase leak down test will help confirm all the seals, gaskets, and joints are sealing as they should. Break-In - When running your new top end for the first time, keep the engine slightly above idle, with slow and mild revs until the engine starts to get too hot to touch. Then, shut the engine off and let it cool until it is warm to the touch. Repeat this process, revving slightly higher and letting the engine get partially hotter each time. After 3 cycles like this, let the engine completely cool, then check all your fluids and re-check the torque on your cylinder and head bolts. Once that is squared away, you can begin break-in runs riding the bike. Make sure to keep the RPMs varied while riding for the first time, not letting the engine lug or sit at idle. A safe bet would be to ride the bike like this for 5 minutes, then 10 minutes, and finally 15 minutes, with adequate cooling in between. This will ensure your piston ring(s) are evenly and properly broken in. It’s never a bad idea to double check your fluids and torque one more time after complete cool down.
Kevin from Wiseco posted an article in ArticlesSingle 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? 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. 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. 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. 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. 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 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. 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. 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.