05 to 09 YZ125/250/250F/450F - KX250 - KX450F Ė 09 CRF450R

Free Piston Modification and Cartridge Bleed Procedure for use with conventional

or SMART Performance 215.VM2.K5 Anti-Vibration fluids


Modifications to the free piston within the YZ/KX/CR air-oil separate (AOS) fork design, also known as the KYB dual chamber, can help alleviate front end push, front wheel rut climb-out and harsh sensations experienced during breaking bumps and downhill sections. Further tuning can be achieved with the use of alternative lighter rate free-piston springs.


The cup design of the KYB free piston is also prone to filling with fluid if the fork is laid flat or inverted, which can further exacerbate poor performance.


To alleviate these concerns, SMART Performance recommends that the KYB free piston be vented by the addition of 2 to 4 holes. These holes allow the air pressure that builds up between the free piston and fork cap to vent to the lower chamber and also allow any fluid within the free piston to drain out and back into the lower chamber. The addition of these holes does not allow the fluid between the lower and upper chamber to exchange. We also recommend that an alternative approach be used for filling and bleeding the upper chamber or cartridge of the fork per the attached procedure.




  1. Follow the instructions in the Yamaha/Kawasaki/Honda manual to remove the forks from the bike.

  2. Follow the instructions in the manual to remove the inner chamber of the fork from the lower chamber. The inner chamber is the top damping unit also known as the cartridge assembly.

  3. Mount the upper damper assembly into a vice or holding jig with the cap facing upwards. Only clamp the upper portion of the assembly and always use soft jaws or a towel to avoid damaging the unit. Always protect and never clamp the damping rod (the long 14mm rod at the other end).

  4. Insert a 36mm hex tool into the top of the compression assembly and rotate the unit counter-clockwise until the head of the unit no longer rises. The compression assembly will not rise or easily lift so it may be necessary to listen for the light pop of the last thread. There is no harm from over rotating. Lastly, it may be necessary to hold the upper chamber assembly with a 49mm wrench while turning out the compression assembly. Do not rely on the vice or clamping device to keep the cartridge from spinning.

  5. To release or raise the compression assembly, reach down and slide the damping rod into the cartridge. This will cause the compression assembly to lift or float upwards. If it does not, rotate the compression assembly out another turn or two. Do not push the tip of the damper rod beyond the seal head of the upper chamber.

  6. With the damper rod all the way in, use a small prying tool to work the compression assembly up and out. The compression assembly will be working against a vacuum created by the fluid. You can pry against the wire of the free-piston spring and the edge of the outer cap. Work slowly and protect any sharp edges. Do not allow your tool to make any contact with the free-piston stem.

  7. With the unit out, mount the compression assembly into a holding clamp with the piston (nut end) facing upwards. Itís often best to mount the 36mm tool into the vice then place the compression assembly onto the tool. Avoid clamping directly to the compression assembly.

  8. Use an open end wrench to remove the top piston assembly (stem head) from the rod. Minimal force required. Place head to the side.

  9. Lift off the free piston and free piston spring.

  10. Clean the free piston of any oil residue.

  11. Using a marker (and ruler or caliber if necessary), mark the location of the two bottom holes, 180 degrees apart, so that the holes will be drilled between the lower inner ribs, and just above the top inner edge. On the late model piston, add two additional markings directly above these holes. Make sure to leave enough edge between the hole and the edge of the bushing. See photos.

  12. On the late model piston, drill the 4 holes using a 3mm or .125 inch drill. On the early model piston drill the two holes using a .1875 inch or 4.5 to 4.7mm hole. Remove burrs. Clean drillings using a light duty solvent or warm wash. Blow clean with compressed air.

  13. Reinstall the spring (and spring bushing if used) and the free piston. Inspection and cleaning of the free piston shaft and seal is also recommended. Do not use highly volatile solvents (carb clean) on the free piston seal. NOTE: To further alleviate sensations from braking bumps and downhill sections, we recommend the use of SMART Performance/SPI-Racing free piston spring kits.

  14. Reinstall the valve stem to the rod. Apply some light duty thread lock (such as Loctite blue) to threaded end of the rod.


  1. It is best to discard (set-asideÖset ablaze) the Yamaha/Kawasaki/Honda procedure for installing and bleeding the upper chamber.
  2. With the upper assembly (cartridge) in a holding device, (a vice or your hand) and with the damping rod fully extended outwards, fill the assembly nearly full with the 215.VM2.K5 fluid, SPI-3 (aka #3) formula, about 20mm below the two holes of the cartridge. These holes are the transfer holes, one of which can be seen in the photo below. Itís best to pour the fluid with the cartridge tilted, (think beer).
  3. With the cartridge slightly tilted, slowly move the damping rod in and out of the cartridge, about 40mm, while rotating or spinning the rod at the same time. You should feel and hear the movement of bubbles passing through the mid-valve and rebound stack. 
  4. Allow the cartridge to sit for about 5 to 10 minutes or until all the bubbles in the fluid have disappeared. Inspect with a flashlight if necessary. Repeat step 3 if necessary.
  5. Get a drip pan, put on some safety glasses, cover the upper holes of the cartridge and then completely fill the upper chamber to the very brim with the SPI-3 fluid.
  6. Dip the tip (piston/valve) of the compression assembly, at an angle, into the fluid. With the valve is dipped into the fluid, (youíre still at an angle here) spin the compression assembly allowing any trapped bubbles to rise out of the valve ports.
  7. Slowly insert the compression assembly into the fluid allowing the unit to stand further upright as it enters the cartridge. Do not press or force the unit in at the time - just allow the compression assembly to fall into place. Some bleeding from the holes may be necessary.
  8. With the compression assembly floating within the fluid, begin to apply some very light pressure to the valve assembly, seating the unit further into the cartridge. Expect more bleeding and consider wrapping a towel around the head of the cartridge. Use a reasonable amount of force to seat the assembly into the cartridge. At this stage, it will seat about half way in.
  9. Keeping pressure on the compression assembly, slowly insert the damping rod into the cartridge. As the rod enters the cylinder, the compression assembly will rise. When the compression assembly no longer rises, insert the damping rod another 20mm. Then slowly pull the damping rod back out of the cylinder while pressing firmly down on the compression assembly. Repeat this process until the lower edge of the compression assembly cap is about 10 to 15mm above the upper edge of the cartridge.
  10. With the rod fully extended and remaining fully extended, and the compression assembly nearly fully installed, push down on the compression assembly and rotate the head into the cartridge just enough to get one to two threads started. This may take a bit of pressure. If not possible, repeat the rod bleed procedure in the prior step.
  11. With the first few threads started, remount the cartridge into a holding device, and use the 36mm hex tool to spin the compression assembly all the way in to a snug fit, but do not over tighten the unit. Keep the cartridge vertical at all times during this process.
  12. Remove the cartridge assembly from the clamp and place the end of the damping rod into a corner (where bench meets wall). Protect the tip of the rod during this stage. Then angle or tip the cartridge assembly about 10 to 20 degrees. Then press down, fully compressing the damping rod into the cartridge. Itís best to have the jam nut on the end of the rod to keep the rod from going too far into the cartridge. As the rod nears full compression, a burst of air and fluid can be heard and sometimes seen bleeding from the cartridge holes. Allow the rod to fully extend then repeat.
  13. Pour any fluid out of the free piston chamber by laying the cartridge assembly flat.
  14. Test left and right assembly for even and consistent performance. The rod will tend to extend on its own, but may stop about 20 to 60mm from full extension, (results vary depending on inner spring rate and design). If the rod fully extends with no assistance, the cartridge will need to be re-purged, (repeat step 12). If the rod does not extend to near full extension, or feels softer than the opposing unit, remove the compression assembly and start over.


If you have any questions, concerns, problems or doubts with any of this, do not hesitate to contact us via phone or email or reach us in the suspension forum at ThumperTalk.com. Detailed shock rebuilding procedures are also available.


Text Box: New Holes
2 total

 Text Box: New Holes
4 total

Text Box: KYB Late Model


Text Box: KYB Early Model