BustedassYZ Posted March 30, 2010 Whats the expected valve life on a 400ex? I know the Hi performance 250 and 450 MX bikes and Quads should get valves replaced every time the piston gets done, but what about the air cooled 400? Also what is the stock bore size? 85mm right? And how big can you go oversize on the stock sleeve? Share this post Link to post Share on other sites
reconranger Posted March 30, 2010 (edited) If I could only have one quad that had to do everything....it would be the EX! It will run forever, without doing anything to it except routine maintenance!!! This is the beauty of this machine, vs the current crop of high strung 450's out there! My EX (lightly moded) is over 10 years old now, and it runs as good as the day it was new! It has thousands of miles on it and I wouldn't want to even know how many hours. Lots of newb riders have learned to work a clutch on it, yet it still even has the original clutch. But....I never neglect maintenance, and always run the highest quality ester synthetic oils, like Redline or Maxima. If you are just thinking of boring it out for more performance, that is a bad idea. Once bored out to 440cc's, the engine's legendary reliability is pretty much history. Keep it conservative, like a 416cc on the first rebore. Edited March 30, 2010 by reconranger Share this post Link to post Share on other sites
CQR My Rides (7) Posted March 30, 2010 I agree. The 400ex is a very reliable engine. The 450R and 450F are the ones that have the valve issues. Enough so that non Honda Dealers around here carry them in stock. Share this post Link to post Share on other sites
