what do u think a good bore size would be

 

You need to determine what you want in your machine. You're asking several different people the same question and you're going to get several different answers. There are people on here that think "the bigger the better", who will tell you to go with a 440. Then there are people like me on here who say "don't bore unless you have to", to allow yourself as many oversizes later on down the road as possible.
Judging by what I've read in this thread, I think you might be best off with having a professional install a standard bore high compression piston and possibly a performance camshaft at the same time. You will be happy with the results, and there will still be room to grow into the bike.
Just my opinion though, because I know some people like to go ***** to the wall the first time they try to modify an engine, but chances are you'll have more problems with it than you are expecting, and you'd probably enjoy the quad more if you are able to just dump gas in it and ride it.

 

what do any of u think about 18in tires on a 400ex how much speed will i really lose?

 

a drop in tire height decreases top end

 

thats not what ive heard

 

ya i no so how much speed do u think i will lose

 

Well ianR, your at a bit of a cross road.... It seems obvious you want more bang and your wondering what it's going to cost... Just scanning down the replys to your initial question I never heard you say where you ride, riding style, and here's the tough question......Are you a naturally competetive person who enjoys competing against his friends, and wants to win ? Do you really need to go that fast on a trail ? With all said, if you decide to spend your precious dollars and add a higher compression piston. I would definately spend the extra $200 and add on the 440 kit with good cams..... At least now you know you got a banger than can do it all.... If you need more down the road I would say it's time to sell the beloved 400 and get one of the many 450's... And start building from there.....Good luck in your choices... Just ride within your abilities

 

my riden style is agressive i lo ve jumping wheels and goin fast and i ove 2 compete with my friends