I have a 400ex and was thinking about boring it out. I was just wandering if you had to buy an aftermarket cam to run a big bore, or can you use your stock one?

 

You could use your stock cam, but it would be very beneficial to use a performance cam. Hot Cams makes a good cam for this, you will get a lot more power with it. I had a 400ex, and instead of the big bore, a got a Hot Cam stage 1, and bored it out to a 416. It had gobs of power, this is a popular setup.

 

i would say a 440 is the most popular set-up. you'll need to change the cams and port the head to really wake up your bike.

 

A 440ex is the most popular and best set up IMO. I would put a hot cam in her too.