i need to know what kind of piston to buy for my stock 2003 400ex. i fried my stock one and i dont know which one is the right size.. what is the stock bore size..

 

First if you didn't buy the bike new the piston should be measured to make sure it is stock then the cylinder will have to be measured to check the wear which will determin 1st or 2nd oversize & out of round should be checked ..

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Here are the specs..