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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.
Piston ring technology has gone through a renaissance in the past 15 years. The age of the modern four-stroke ushered in the necessity for durable, highly powered machines. It only makes sense that piston rings were one of the first areas of interest. These days, powersports enthusiasts are blessed with an array of options built using space-age materials and coatings, unique geometries, and remarkable sealing technology. Here, we break down the newest advancements in four-stroke piston rings, and how these technologies can benefit engine builders and motorcycle riders alike. Three-Ring Piston Designs 4-stroke pistons apply to a variety of different engine types in powersports, and many of them use 3-ring designs. Most four-stroke pistons use a three-piston ring design for effective compression sealing and oil control. The top ring serves as the compression seal. The uppermost ring’s job is to keep the combustion gases from getting past, effectively sealing the piston for maximum pressure. The second ring is responsible for catching any oil that may be on the cylinder wall. It helps scrape oil down to the third ring, which is a baffle ring that collects all of the oil. The bottom ring guides the oil through holes in the piston and down into the bottom end. “You don’t want the oil going up, just as you don’t want compression going down. You have to seal in both directions,” states Wiseco's engineering representative. The third ring is the oil ring assembly, which is made up of the baffle-style ring that collects oil, and two thinner rings that sandwich the baffle ring, and help keep it uniform in the ring groove. There’s also a very detailed geometrical design for every ring in a three-ring piston layout. The top ring typically has a gentle barrel face so that the ring contacts the cylinder wall in a very narrow path. This shape lessens the amount of friction, while still effectively sealing the compression. Conversely, the second ring has an angular face. The job of the top ring (lighter, machined color) is to seal compression, which is why it has a barrel-shaped edge face. “Picture a jagged edge, where the sharp side is on the bottom. As it’s traveling downward along the cylinder wall it’s scraping oil, just like a windshield wiper as it’s going down,” mentions Wiseco's engineer. The second ring's job is to scrape oil as the piston travels downward. It has a jagged notch cut on the bottom of the ring to increase oil-scraping efficiency. Two-Ring Pistons Honda has long marched to the beat of a different drummer. It makes sense that the Japanese manufacturer wanted to go their own way when they introduced the CRF450R in 2002. The single-camshaft layout, known in marketing speak as “Unicam,” received most of the attention; however, Honda’s engineers also developed a unique two-ring piston design. By eliminating the middle ring in a traditional three-ring piston, the goal was to limit friction in the assembly and shorten the piston height. The latter benefit being a reduction in overall engine weight (the CRF450R was the lightest 450 four-stroke in its time). The top ring sealed compression and act as an oil scraping ring. The second ring collected the oil and sent it back into the bottom end. Honda was able to use a two-ring piston by developing technologies with the top ring that had, and continue to have, really unique geometries. Honda’s top ring seals compression and is also able to scrape oil on the down stroke. This was accomplished by putting positive twist on the piston ring. Two-ring designs in four-stroke pistons are still very relevant, mostly in racing applications, such as with the Wiseco Racer Elite pistons you see here. They allow for less friction and weight, but require advanced ring technology. Wiseco's engineer explains, “If the ring was sitting flat on a table, the outward edges of the ring would actually be upward. The ring isn’t technically flat, but instead cone shaped. As the piston heads upward toward compression, the ring is tilted upward and is sealing. When the compression event happens, the ring flattens out and seals solidly against the piston ring groove and cylinder wall.” Once the piston travels downward, the lower ring starts to twist upward again, which brings the bottom edge of the ring out against the cylinder wall to scrape oil. The ring is effectively doing two jobs. Notice the bevel cut into the inner edge of the ring. This gives Wiseco rings positive twist geometry, meaning it can tilt and flatten inside the piston ring groove to accomplish compression sealing and oil scraping with one ring. Advancements in Materials Piston rings are not exempt from a continued focus on utilizing space-age materials for bolstering performance. Truth be told, rings were manufactured out of relatively simple materials 25 years ago. They were either built out of cast-iron or alloy steel with a moly- or chrome-faced channel where they contacted the cylinder wall. A lot of these new ring technologies are alloy steel, with a caveat. Rather than putting chrome faces on the ring, ring manufacturers actually harden the ring through a process called gas nitriding. “Gas nitriding is a process of introducing hydrogen to the surface, which hardens the steel,” notes Wiseco's engineer. “That makes the ring more durable, and results in better wear against the cylinder wall. The harder surface works extremely well against Nikasil cylinder bores.” Gas Nitriding rings hardens the material, increasing durability against today's extremely hard-surfaced Nikasil cylinder walls. Nickel silicon is an extremely hard surface. In order for the piston ring to properly seal against the cylinder wall, it, too, needs to have a very hard surface. The focus has been on making rings that are harder every year so they have better wear properties, and seal better against the cylinder wall. Low-Tension Technology Piston ring manufacturers have had to adapt to new-age cylinder designs, which have a tendency to distort. That’s due to motorcycle manufacturers constantly looking for ways to decrease engine weight. One way to do that is by casting the cylinders thinner and thinner. As a result, the cylinder doesn’t typically remain round or straight. This creates an issue where the piston ring becomes unseated. The solution is in using low tension rings. This technology allows the ring to conform to uneven surfaces. In essence, the ring is able to follow the undulations of the cylinder wall as it’s twisting and turning during the stroke of the piston. Ring Land Design Closely inspect the second ring land, which is below the top ring and above the oil ring. Notice a cut channel, which has a jagged shape that resembles a triangle. Flat on top and tapering off at the bottom, the unique shape helps assist in gathering oil as the piston is on the down stroke. This is called an accumulator groove. It acts like a piston ring that gathers oil and drives it down into the bottom end. One some pistons, you'll find a channel cut into the second ring land. This is not a ring groove. This is the accumulator groove, which collects oil and returns it to the bottom end. “Ring land design is extremely important in these new four-stroke engines that only use two piston rings,” states Wiseco's engineer. “You’re trying to take weight out and make things shorter and lighter. How do you do the work of three rings with only using two rings?” The answer can be found in the shape of the ring land. This scientific design helps control oil and makes sure that it doesn’t travel up past the piston and into combustion. Anyone that has a lot of hours on their piston and rings will be able to physically see that they have a problem when they fire their bike up in the morning and little puffs of blue smoke come out of the muffler. That’s due to the parts wearing out and oil getting into the combustion area of the engine. Lapped Rings Take a ride in the Mr. Peabody’s WABAC machine to the late 1990s, when automobile companies discovered that piston ring manufacturers couldn’t make a ring flat enough that was necessary for ultimate sealing. They relied on placing rings on a surface plate and grinding them down in order to remove any high points or imperfections to the surface. This process created a better seal against the ring groove. Known as lapping, the technology became popular in NASCAR and Pro Stock Drag Racing, where ultimate ring seal yields horsepower gains. The lapping process takes a production piston ring and turns it into something better. This technology is now available in the high-performance world of motorcycles. “Roughly a year ago, Wiseco introduced the Racer Elite piston kits. One of the features we have included is a lapped compression ring,” explains Wiseco's engineer. “It immediately provides better ring seal, so you don’t need any piston break-in period. Normally, the rings will need to seed themselves to the piston over time. We’ve eliminated that by having the ring lapped to the ring groove. It’s a very precise fit, and you get a really strong ring seal. In turn, horsepower is improved.” Lapped compression rings were a recent introduction to powersports with Wiseco's Racer Elite. It achieves ultra-flatness, creating a better ring seal, translating to more compression and more power. Wiseco was so impressed by the performance increase of lapped piston rings, they brought the technology in-house. Find out more about how well ring lapping technology improves performance. Wiseco's engineer states, “We have the capability to lap just about any piston ring to optimize the surface. We knew that we needed to do it all under our roof and sell it to the market. It’s interesting, because the Powersports market doesn’t know much about lapped piston rings. At the same time, it’s almost an everyday thing on the automotive side.” The benefits of a lapped ring are immediately noticeable. Performance gains, reduced blow-by, and the lack of break-in period put the Racer Elite piston kit front and center in the high-end, premium race parts category. Gas Porting Gas Porting is a technique where tiny holes are drilled into the top of the piston that intersect the top of the ring groove. The technology forces combustion gases down through the holes and pushes the ring out against the cylinder wall. It’s a technique used to achieve ultimate ring seal, but it’s not meant for longevity. That’s because gas porting forces things to work against one another, resulting in the ring and piston to wear out faster. Gas porting is normally reserved for high-end applications, such as Drag Racing, because it bolsters performance (at the expense of a shortened lifespan). Gas ports are tiny holes that allow combustion gases in through the top of the piston and out inside the compression ring groove. This forces the ring out against the cylinder wall, improving the seal.