Im thinking about buying a used trx450r motor and building it up and intalling it into my 400ex frame. I know you have to weld on new brackets to get it mounted and all. I also know i can get a 450 but i like being different from others. 400ex has a light frame so thats why i want to give it a try. Anyone have any ideas or anything as to why or why not i should do this? thanks

 

Sounds like a lot of work and money for less machine since the 450's frame and suspension is a lot higher tech then the old school 400ex.

 

A 400ex / xr650 hybrid has been done many times. You can find lots of info on the web about them.
I would buy a 450r rather than invest all the time and $ in a build.

 

Easy solution is buy a 440 kit for the 400. May not be different, but the money saved would be quite a bit.

 

Not worth the time imo.

 

I wouldn't worry about the frame as much. The geometry is closely based off the 250R frame, which even the YFZ frame is close to as well. Before the 450's hit the scene, there were quite a few 450 in a 400EX or 250R frame builds out there. I don't think there is any advantage to building one now other than the satisfaction of riding your own handy work.