ok i have been wanting to bore out my 400ex but i cant decide wat bore size i want asomthing that is alot quicker than the stock ex but i would still like it to be reliable and i do not want to have to by race gas and wat else would i have to buy for bore kit

 

You will need a larger carb, high performance cam, porting, and a high performance exhaust system.

 

I bored my 400ex to 440 and its alot quicker than before. i didnt put any other mod and its works well and seems to be reliable (one year of use )

 

Go with the 416 kit or 426 kit a couple of my friends have them and they work great

 

atvrocks wich compression do you have 12:5? and you dont have any other mods