Ok i have my 300ex torn down to the bare frame right now and i was thinking of doing an engine swap. How hard would it be to put a CR250 or 500 engine in. Has anybody put these engines in at all? Thanks to any advice you can give me

 

don't bother. Those engines have no counterbalancer and will crack that frame (and almost any quad frame) into peices within a week.

 

Dill is right no only would the vibration be too much for the frame. But the kickstarter on those engines go backwards and even if you shortened the kickstarter, which would make it harder to crank, you would either have hardly no rear fender on that side of the quad or you would have to take them off every time you crtanked it. That would get old fast! Unless you speciffically want two stroke power, try finding a 350X engine instead. It hardly requires the work needed for any other engine swap, and that engine is full of potential. TC Racing built one a few years back for a Pro Rider[I can't remember the name], and that motor was dynoed at 74HP at the crank!!

 

I've heard good things about the 350x motor. One of the mags did something like that a 300ex/250x frame with a 350x motor. They said it was insanely scary and hard not to wheelie. an extended swingarm might be an added bonus for this project. But one things forsure you will smoke a 400ex! tho's 350x engines are awesome

 

Just out of curiosity, what makes the 350x engine so much stronger than the 400ex engine, The only main difference I see is wet/dry slump difference.

 

I think the most popular feature of the 350X engine is its 6 speed gearbox. It really isn't much different otherwise.