The more mods I do to my 400EX, the more I start wondering if the tranny will hold up. I do a lot of really steep hillclimbs and jumps in gravel pits, and with the increased power of the filter, pipes, jet and soon to come big bore kit, I am wondering if I need to upgrade the tranny gears or something to compensate. Does anyone have experience or knowledge of how far the 400EX's stock gearbox can be pushed?

 

Honda builds some tough stuff. 250Rs have been being modded and raced forever. They do some wild stuff to them without modding the gearbox. The 400EX has very thick gears, much wider than the XR400. I have a full race Sparks 440 National, and it hasnt had any problems. I have heard of 2nd gear being weak, but that is in racing conditions. I believe the problems have been more from the abuse of racing than the abuse of a highly modded motor. The clutch is the weak link and it should be modified with an 8 plate setup.

Adam Smith

99 400EX