My kid has an '03 400ex. His pride and joy. He was told when he bought it that it was a 426 big bore, wiseco 11:1 piston. It also has a big gun racing series full exhaust and a 450r carb. We replaced the exhaust, pulled the air cleaner cover off, and put on an uni filter and outerwear box cover. Tuned the carb, and it ran awesome.

last weekend, it died. Pulled it apart today, and the piston is toast. Scored the cylinder pretty bad, too.

Dud some measuring, and while it is a wiseco 11:1 piston, its stock bore size.

so, its bore and rebuild time.

my question is to what? 416? 426? As long as its apart, should I put in a performance cam?

His use seems to consist primarily of doing wheelies and pissing off the neighbors. He does go to the MX track some and has plans to ice race this winter.

He, being 17, wants the ultimate in performance. As the guy who ends up fixing it all the time, I want reliability. It would also be nice to run on pump gas.

any suggestions?

 

Maybe get it fixed and lie to him again about cc?