217 Lorain Place Los Gatos, California 95032 408.406.2089 www.smartperformanceinc.com www.spi-racing.com FORK FREE PISTON MODIFICATION 2011 HONDA CRF250R WHAT? All production versions of the 2011 HONDA CRF250R use a closed cartridge (twin-chamber) SHOWA designed fork that can trap a significant volume of air within the closed cartridge assembly during the servicing and tuning of the inner cartridge. When the bike is ridden, the agitation of the fork will cause this trapped volume of air to mix or cycle within the body of fluid leading to significant and nearly immediate degradation of the hydraulic damping. This will affect both compression and rebound cycles. WHY? The cause of the trapped volume of fluid is from a newly designed free piston (inner chamber piston) introduced and used exclusively within the 2011 HONDA CRF250Rs. This new free piston design is similar to an upside down cup (volume of air) that sits directly over the valve body assembly of the base bolt assembly. When the base bolt assembly is introduce back into the bath of the upper cartridge fluid, (using a traditional assembly process) the air pocket within this cup remains trapped within the free piston as the base bolt is secured into position and the cartridge is purged of its excess fluid. HOW? In order to resolve this problem, the free piston component needs to be removed from the base valve assembly and modified. The modification involves drilling a single bleed/vent hole into the side of the free piston in a precise location. Following this, the free piston is reinstalled back onto the base valve assembly using a unique bleed process that assures an air free assembly. COMPONENT DESCRIPTION OF 2011 CRF250R BASE VALVE ASSEMBLY A B C D E F A Top Cap. Screws into top of cylinder. Space between free piston and cap are exposed to the outer chamber. B Free piston spring. Also called an inner chamber pressure spring. Sets around free piston shaft. C Primary free piston seal. Seals outside of piston to inside of cartridge. Vents fluid to outside chamber when seal reaches taper inside cylinder. Somewhat fragile. Should never be clamped. Any small nicks or cuts or indentation could cause seal to leak leading to poor fork performance. Use caution when handling. D Top Free Piston Bushing. Does not contribute to sealing fluid within cartridge. Top and bottom bushings are identical. E Bottom Free Piston Bushing. Does not contribute to sealing fluid within cartridge. Top and bottom bushings are identical. F Valve body assembly. Contains main stem, valve body (piston) base valve shim stacks and check plate. Stem screws into free piston shaft. REMOVING BASE VALVE ASSEMBLY 1. Before removing the forks from the bike, it is advised that you loosen the top triple clamp bolts and then loosen (1/4 turn) the fork cartridge from the upper tube, thereby using the upper triple clamp as an upper fork tube fork holding tool. Loosening the base valve bolt (1/4 turn) with the fork in the clamp may also prove wise. 2. With the forks removed from the bike, mount the lower lug of one fork into a vice. Soft jaws are recommended. Lay the fork even with the workbench top or slightly tilted down so that the fluid flows towards the cap or top of the fork. 3. Unscrew the rebound adjustor bolt from the bottom of the fork lug. Requires a 21mm or 13/16 th socket. Bolt threads will skip when the bolt is completely unscrewed. Bolt will not pop out as it is held against the fork lug from the tension of the main fork spring.
4. Remove the fork from the vice. Stand the fork up with the lug upwards (fork is upside down protect the cap with a thick rag or pad) and press on the lug allowing the rod and the rebound bolt to extend up and out of the lug. The front wheel axle can be inserted into the lug and used as a handle. Use a rod holding tool to retain the rod and bolt out from the lug. A plastic or wood rod holding tool is best to avoid scratching the rod. 5. Remount the lug into the vice, with the fork slightly tilted down towards the cap. Remove the rebound adjustor bolt from the rod by holding the jam nut while unscrewing the rebound bolt. You may want to note your rebound clicker setting before doing so. Additionally, it s also a good idea to remove the straw-like aluminum rod inside the cartridge/damper rod. 6. Remove the fork from the vice taking caution not to tilt the fork down as fluid may spill out. 7. Remove the rod holding tool by standing the fork upside down and pressing down on the lug. 8. Slightly pull the two forks tubes apart so that the rod goes down into the fork well past the lug. 9. Screw the rebound adjustor bolt back into the lug, rotating the bolt into the lug one or two turns. This will help to maintain the fluid. 10. Remove the cartridge assembly from the upper tube. Keep the fork upright or tilted down towards the lug. The fork can be lightly clamped in a vice (using soft jaws) at the section of the tube where the lower triple clamp secures the tube. Do not expect a vice to hold the outer tube well enough to unscrew a stuck or tight cartridge assembly. If need be, install the fork back into the bike s triple clamp if you need more clamping force against the outer tube. 11. With the cartridge assembly removed you can either pour out the inner fluid and main spring, or preserve the fluid by allowing the fork to remain upright. 12. Unscrew the base valve assembly from the cartridge. A proper tool is advised. Avoid excessively clamping the cartridge tube at anytime. If need be, use one wrench tool to hold the cartridge assembly at the top, and another tool to remove the base valve assembly. Note that it is best to have the cartridge assembly standing upright before attempting to lift out the base valve assembly. 13. With the base valve assembly fully unscrewed, compress the rod into the cylinder. This will help to lift the valve assembly out. Some force may be required to work the assembly out against the vacuum from the inner fluid. Use caution and your brain. 14. With the base valve assembly removed you can either retain the fluid within the cylinder or pour it out. Working the rod in and out will help to expel more fluid. If you do decide to remove the damper rod entirely from the cartridge, please be advised that the threads on the end of the rod should be packed with a heavy grease as to minimize damage to the seal and bushing with the seal head (bottom tip of cartridge). MODIFYING THE FREE PISTON 1. Remove the valve body assembly (Item F) from the free piston shaft. This can be done by holding or securing the cap (A) and then clamping the outer most section of the stem just below the bottom of the shim stack. The stem should unscrew from the shaft. In some rare cases the shaft will unscrew from the cap (A). If the free piston shaft spins out of the cap instead of the stem separating from the shaft, proceed as you have no choice. 2. With the base valve stem assembly removed, slide the free piston off of the shaft. 3. Carefully remove the lower bushing (Item E) from the free piston. Using your finger nails between the bushing split is best. Do not over extend the bushing as it can bend and lose its nice round shape. 4. Remove the top bushing (D) and slide the bushing over the top section of the free piston body. Align the top edge of bushing with the top edge of the free piston body. See photo. 5. Use a pencil with a fine or sharp tip to mark the free piston using the edge of the bushing as a guide. See photo. The line should be about 1 inch (20mm) in length around the free piston. It may be necessary to clamp/hold the bushing while you draw the pencil line. This line, and the position of the line and bushing is VERY VERY critical. Take your time do it right! Align top edge of bushing to top edge of free piston step.
6. With the pencil mark in place, slide the bushing up and then use a marker to make a dot so that the dot notes a spot where the hole can be drilled as to go between the inside ribs. Do not mark directly on the pencil line. Mark the dot on one side or the other of the line as you need to preserve the accuracy of the line as to make sure that the drill is not too high or too low with respects to the internal structure of the free piston. See photo. Stop and think this is important. 7. Slide the upper bushing off of the free piston and mount the free piston in a vice. Use soft jaws and only use a clamping surface that is completely free of any dirt or material. Use caution to not over-clamp and never clamp the free piston seal (Item C). See photo. Obviously you will be drilling a hole straight down into the free piston, with the drill centered on the pencil line, with respects to the dot, (between the ribs). You are drilling the hole as to create a bleed or vent within the cup of the free piston. If you drill above the top edge of the cup the free piston will most likely be permanently damaged. Measure twice, drill once. 8. Drill a 1/16 th inch hole directly into the free piston. Use extreme caution to make sure that the drill does not protrude or punch through as to make contact with the seal tower. Using a drill stop is best. If the seal tower gets damaged or is drilled the free piston may become permanently damaged and have to be replaced. The drill will be in line with the dot (as to drill through the free piston between the inner ribs) and directly on the pencil line. See photo. If the drill wants to slip or move off of the line stop drilling immediately. Lastly, it goes without saying that a sharp and straight drill bit is highly recommended.
9. With the drilling completed, remove the free piston from the vice and inspect. Clean any debris and remove any burrs. Do not use any petroleum based solvents on the free piston at any time. 10. Place a mark on the upper edge of the free piston directly above the drilled hole. See photo. INSTALLING THE FREE PISTON. 1. Install the two free piston bushings (D and E) and slide the free piston onto the shaft. If installing a SMART Performance/SPI- Racing EPNP kit, see kit instructions for spring and bumper configuration. 2. Apply one drop of a medium chemical lock (usually blue) onto the threads of the valve body threads (Item F) and screw the assembly into the free piston shaft. Snug the valve body onto the stem but do not over-tighten. Approximately 48 inch pounds. INSTALLING THE BASE VALVE ASSEMBLY AND BLEEDING THE CARTRIDGE. 1. Mount the cartridge tube into a vice, tilted, with the two holes at the top edge of the cartridge at their highest and lowest points. See photo. Never clamp directly onto the golden section of the cartridge unless you are using a special jig as seen in photo. If you don t have a tube jig, clamp the cartridge at the base of the upper section, (the non-tapered section two steps above the main tube).
2. Fill or top off the cartridge with enough fluid to cover the first ledge or step within the cartridge (just above the cartridge tube). 3. Work the rod in and out to expel any air that may be trapped on the back side of the valve body attached to the rod. This would be the mid and rebound valve. Move the rod in slowly, and pull the rod back down and out with a swift pull. Repeat until the movement/resistant is smooth and consistent. 4. Allow the cartridge to sit for about 5 to 10 minutes so that any bubbles within the fluid can rise to the top. 5. Place a drip pan (or anything to catch an overflow of fluid) under the cartridge. 6. Using your thumb and index finger, block the two holes at the top of the cartridge. See photo. 7. Fill the cartridge with fluid to the very top edge of the cartridge. When adding fluid, pour the fluid down the inside of the cartridge, much like pouring a beer. Allow any bubble at the top of the fluid to spill out by overfilling the cartridge. 8. Slowly introduce the tip (stem and body of the base valve) of the base valve assembly into the fluid. Rotate the piston about allowing any air trapped within the ports of the piston to pop out. 9. With the black mark you made on the free piston facing up at the highest point (this too is now tilted in-line with the angle of the cartridge) and keeping your fingers on the hole of the cartridge, slowly introduce the body of the free piston into the fluid. Any air within the free piston will purge up and out. Allow the assembly to fall under its own weight or with a minimal amount of weight from your hand and arm. Allow the assembly to straighten up and line up with the angle of the cartridge. 10. Work the unit down into the cartridge with additional force as to lock in the o-ring seal of the valve body into the cartridge. The base valve assembly should come to a hard stop. Press down with a fair amount of force to insure that the seal has seated. 11. Remove your fingers from the two holes and allow the excess fluid to bleed out. 12. Using a fair amount of force, hold the base valve in place while bringing the rod up. The base valve should lift slightly as the rod is brought up. The base valve assembly will lift about 20mm before fluid begins to purge out as the rod is continued inward. If need be, bring the rod back down as you are pressing on the base valve assembly to get a feel for how the valve will seat further and further as the rod is worked in and out and more fluid is expelled, allowing the valve assembly to seat deeper into the cartridge. 13. The rod will need to be brought up to about 4 to 5 inches from full compression in order to expel enough fluid to allow the base valve to seat down far enough as to compress the cap down against the inner pressure spring and screw into the cartridge. Some force is expected to seat the cap, while engaging the threads.
14. Screw the cap fully down into the cartridge. 15. Remove the cartridge assembly from the vice. 16. With the cartridge standing upright, and with the rod protected at the bottom, fully (and slowly) compress the rod into the cartridge. This will expel the excess fluid within the cartridge. Expect some fluid to flow out of the top holes. You will only need to compress the rod into the cartridge a single time to expel the fluid, however it s a good idea to compress the rod a few more times to verity that the rod is extending consistently after each cycle is performed. The rod should slowly return all the way out or nearly all the way out at the end of each compression. If the rod sits half way or sits in further and further into the cartridge with each compression, the free piston is most likely leaking. 17. If the rod does not fully extend, manually pull the rod out. If the rod retracts back in, yet acts consistently after each cycle, the cartridge will need to be re-bled. 18. Lay the cartridge down flat and allow the excess fluid to drain out of the two top holes. 19. The cartridge(s) can now be re-assembled back into the fork tubes. INSTALLING THE CARTRIDGES INTO THE FORK TUBES 1. If the fluid and spring were maintained in the lower tube, place the cartridge down into the fork and screw the cartridge cap all the way down into the upper tube. 2. If the fluid was drained and the spring was removed, replace the spring and partially screw the cap into the top tube. 3. Flip the fork over and if necessary, remove the rebound adjustor bolt. 4. Press the lug down and use the rod holder tool to position and hold the rod outside of the lug. If the adjustor that runs inside the damper rod was removed then replace and screw the rebound adjustor bolt all the way onto the rod. When the rebound bolt bottoms out on the rod, there should be a gap between the top edge of the rebound bolt to the edge to the rod jam nut. If there is not a gap, the jam nut has most likely spun up on the rod and will need to be screwed back down. You may have to remove the cartridge assembly from the fork tubes in order to correct the jam nut. 5. With the gap verified between the rebound adjustor bolt and jam nut, bring the jam nut up and lock it against the rebound adjustor bolt. Torque to specification. 6. Remove the rod holding tool and screw the rebound bolt fully in. Torque to specification. 7. Bring the fork upright and add fluid if necessary. 8. Tighten the cartridge into the upper tube and very that the base valve is fully secured into the cartridge. It may be necessary to mount the fork back into the triple clamp to properly set the cartridge and base valve assembly. 9. Reset both the compression and rebound clicker settings. 10. Install the fork legs on the bike and re-attach all necessary components. Double check all your work and make sure that the front brake is fully operational before riding bike and that the wheel spins freely when the brake is not in use. Do not ride the bike with a dragging brake or a brake that does not fully function.