300ex in 400ex frame

[fastex91]

Ok, I'm getting ready to bulid my son a 300ex in a 400ex frame. Has anyone out there done it, and how was the swap? Set up wise I've got denton +2 a arms with 450r shocks front and rear.

My main question is the motor mounts and exhaust, how's the fit?

I'm not doing this blind, I just like to hear what issues others have had. There are several in my area with this set up, and a few with YZF250 motors in 400 frames. They are allowed in the youth class around here.

[zeppelin]

yes you would be waaaaaayyyy better off with the 250, ive heard of conversions but ive never seen pics, the yz250f motor pumps out about 34hp stock and the 300 is like 18, if you do decide to do it, can you post pics of the build up. i love those threads

[Hon300ex]

yea put a 250f motor in a 400 frame, a 300 jus isnt worth it

[kd5sfq]

I'd be interested in hearing about what others have done as well. We are building one also. Our 400ex has Elka's all the way around with a +3 long travel front end.

I think that even with a punched out 300ex engine it would be an awesome MX quad for youth or sportsquad classes.

A CRF250R would be nice but I'm not so sure it's a legal option in our area. Plus it cost a bunch for that engine. I have seen that setup walk away from 450R's though... Maybe that's a bit too much power for younger riders???

[Pvt. Maggot]

I highly reccomend a klx300 motor, watercooled 300cc dual overhead cam, can be made VERY FAST! if not then the 250cc motors will be good. :) or even a 350x or 250r motor, ect. :devil: