I now have 8 months of track time and 6 moto's in on the 420 big bore kit. Results - Perfect. I have experienced no problems with the top end or bottom end. On the track the biggest difference is with starts and low end snap out of corners. MX results: Race 1 Moto 1 - 2 Moto 2 - 2 Overall - 2 Race 2 Moto 1 - 4 Moto 2 - 5 Overall - 4 Race 3 Moto 1 - 2 Moto 2 - 2 Overall - 2 6 race Series - 110 points to date. The WR420 is currently parked in 1st for the series. 3 more races to take the series. Also - Going to a 19 inch wheel made a huge difference in my cornering speed. The bike seems to hook up much better, while leaned over for the corner.