my 400ex started using oil real bad and i cant find no leaks or nothing i change oil every month and watch it real good do you have any ideas?

 

your rings are bad, it's time to rebuild your engine

 

how much do you think that will cost?

 

It all depends on your mechanical skills. If you pull the head off and bring it to the shop, it will save you some cash. You can reassemble it your self if you have the book and the time.

 

do i have to bore it out?

 

You may just need to buy new gaskets and replace them..

 

and o-rings and seals..

 

If you follow the manual and take the head off, take it to a shop and have them check for wear. If it does need to be bored you have a chance to put in a higher compression piston while you have it apart. Get a little more HP. Putting it back together is pretty easy. The only thing that I hate doing is adjusting the valves. But that just me.