Ct racing c310 cylinder on CR250

Are you sure it will work? I know Pro-X had a kit in the past but don’t think it’s still available. ESR might still have something, along with CP Industries. 

Sorry I haven’t used their kits. If you don’t mind, what type of riding are you building this for?

Have you tried a Gnarly pipe yet? 
FMF 020379 

You can also turn the base of the cylinder, there’s plenty of room.  

Disclaimer: you may need a flywheel weight to calm it down, if you do both. 

I would not be buying a cylinder without a power valve 

• Millenium and Eric Gorr have done big bores in the past, give them a try.

Had a ProX 330 cylinder kit on my 2001 CR 250. Huge torque numbers. Had a 2000 KX 250 at the same time. The Kawasaki motor was great. The 330 kit was heads above the  KX motor. People often think a big bore  drops off in Horsepower, when really it made some much more power, it feels it makes less at higher RPM. If you go with the 310, prepare to test gearing changes, and retard the timing for more rev. So much more low and mid power, you won't lose a noticeable amount by retarding the timing. I raced AX and MX with good results. Took it to a Dirt Drag, (100 yards) started in 3rd gear every start. Won the money class that day against all comers. CR 500's, GXSR extended WB, KZ 900 extended WB, etc. It was a great motor.  Good luck. You will have fun  with it.

Now here comes the down votes and no power valve opinions.