250x big bore kits

[fastmofo250]

i have heard several different thing about these. people tell me to go to these different sites to get them, and i really just have one big question. i have heard that you can just get a 300ex cyl and slap it on the block, but then i hear that the cyl is the same on both wheelers and that its the size of the rod that is bigger. so my question is, could i take a 300ex cyl and slap it on my 250x lower end and basically have a big bore?

[bradley300]

nope, the bore is the same on both motors, but the 300ex has a longer crank stroke and rod. any big bore kit that fits a 300ex will also fit a 250x, just count on it being about 30cc smaller on the X

[TRXRacer1]

The cylinders are the same bore (74mm) as bradly said but the pistons are different. Your stock 250x is 246cc's. If you go with a 78mm overbore kit you will have 274cc's. If you go with a 80mm over bore kit you will have 288cc's. The 80mm is the one I did and had no problems. I highly suggest using Dunlap at 4-stroke tech. They have an 80mm kit that does not require case half boring. Now if you swap in a 300ex crank and and use the 300ex 80mm kit with the piston you will have a very reliable 329cc's. Oh yeah, a longer stroke means you have a shorter rod. Just thought I'd let you know. Also if your serious about this I would look into getting the head ported. The 250x/300ex head is terrible for flow. When you get it off and look in it you'll see what I mean.