I bought a YZ for single track and desert racing in Southern California, this is my first MX bike as I have only had WR types before. I rode it several times and found that I was stalling it much more often than my other bikes. So I bought the GYTR Off-Road Flywheel. GYT-2S297-50-50 adds 9.21 oz for an inertia mass of 6.15kg-cm^2, twice the stock inertia mass of 3.8 kg-cm^2. This is only for 2006-2009 450’s. http://www.yamaha-motor.com/sport/products/modelscitemdetail/5/209/1218/5375/2008/1/4335/0/1/detail.aspx The Heavier Flywheel (HFW) looks like the stocker with an extra ring welding to it as you can see from the fairly representative pictures on the Yamaha-motor website above. The installation was easy as expected. The hardest part was getting to a local shop to borrow the puller. FYI a CRF puller and new gen Yamaha flywheel use the same puller. I went to one of my test sites to give the new HFW a test. I was pleasantly surprised how much the HFW helped prevent stalling in the tight stuff. The bike was much less prone to stall and had much more of a tractor feel in the tight stuff without sacrificing the nimble feel of the bike. The throttle was easier to control at very light openings which allowed for better traction in loose rocky terrain and hill climbs. It made riding in the tight stuff much easier and more fun. Thankfully it did not affect the midrange hit and hinder the ability of the motor to rev quickly. I think it still revs better than a WR but has the same ability or ride it just off idle without a problem. IMO this is a must have for your YZ in tight trails.