my 300 casing is cracked changing the engine is the fastier and easyier repair. i was woundering if a 400 motor would fit in a 300 frame because its a step up in power and cheapier then buying a new quad. does anyone know if i need to change the whole hareness or just the cdi

 

Been there and done that....two problems.
First being the relation between the front sprocket and swingarm pivot. Rear suspension and handling nightmare.

Second problem is that the two main frame spars interere with even the stock headpipe from the 400EX engine wen positioned where it SHOULD be....but cant due to,..again,..the front sprocket.

Where exactly is your case busted?? Magneto cover in front of the chain? Or the right side center case under the bottom?
If its the right side center case, this is a common problem with all the 300EXs ridden hard over time. But usually when it breaks, enough material remains to be welded back up. Have you carried it to your local machinist or competent welding shop? Worth looking into.
But if you do indeed need cases, check around on ebay. I see cases and engines listed there more than anything else ATV related here lately. Best of luck on your quad.

 

the crack starts at the starter down under the sproket to the other half.took it to a welding place they can only weld about 60% so it still leaks oil and lets water in the case.me and my dad fight about getting a new engine or just sell it. every one that looks at it dosent want it because the oil leaks.thought a 400 would fit but if not ill get 300 raise the price by what ever the motor is.

 

i broke it drag racing and i think i broke it open again drag racing.

 

knowsalot did you find a way to sovle the first problem if yes how?