There is something about the design of the MM450 forks that make them fill up with air after some use. More air means a stiffer fork and that means an unplanned change in setup and handling. If your a casual driver that is bashing for fun this mod isn’t necessary, but if your running on a track or looking to dial in your front end handling then its worth considering.
The front forks can be built with a set amount of air, or rebound – during rebuild you fill them with oil, compress fork shafts a certain amount, then install the top cap back on. How much the shaft is compressed when the top cap is tightened determines how much air is captured inside (just like a shock rebuild if your familiar with that). With less air the forks do have a softer feel to the front end. Problem is if you like that soft feel, it only lasts a short time. The idea of this fork air bleeder mod is by Chris Haertel as he was searching for a way to control rebound similar to a real bike. It allows a quick way to release some of the air and return fork rebound to your liking after each run or as you like.
These bleeders are made by drilling and tapping a hole in the upper fork caps. A screw, some form of seal (either a washer and o-ring, or gasket washer material) are then screwed into each cap. After each run, or as you feel like the forks are feeling stiff, simply put bike on stand, loosen the cap screw on each fork to let some air out, compress the forks to desired amount or none at all. Air will leave out the threads, then tighten screw down a bit and just like that your rebound is back to how you want it. You can easily test different rebound settings based on how much you compress your forks and all it needs is 2 screws loosened. You do run the risk of damaging your upper fork caps so consider it first.
- A drill, 3mm tap, and the matching 2.5mm drill bit (note: 3/32″ = 2.38mm and might work), center punch tool.
- Two 3mm x 6mm hex cap screws (or some appropriate other style 3mm screw you have).
- You have two options: A) Two 3mm flat washers and two 3mm O-rings like those found in the steering damper, or (B) Thin gasket material to make two gasket washers (example material like 1/8 buggy diff gasket material cut to shape).
Take the top cap off the forks and on top find center. Center punch the cap on that spot. Carefully drill vertically (because the cap is very thin on the top thru the cap). Next you will tap the hole. Make sure the tap goes in straight, so tap the threads without wobbling. Blow or wash out the chips. Then slip on your choice of A) washer, o-ring or B) gasket washer onto the screw. Start the screw into the hole and snug but don’t over tighten! Do not use sealer or Loctite on the threads because that’s where the air slowly leaves and returns into the forks. The o-ring or gasket washer stops dirt from entering the forks, but not the air. A slight amount of oil seepage is normal. Finally, rebuild and install forks.
Thank you Chris Haertel for sharing your idea and photos. Thanks also to Randy Mathis for sharing his version with the red washer.
Example with screw and hard to see it, but it has gasket material as a washer:
Example with screw, red washer, o-ring:
Example with screw and oring only: