Tried to do one on a 2006 kx 250, had the guys at Kustom Kraft do it, it was there first......and last. it never ran more than 5 minutes, the port configuration is so different than the older cylinders, once you bore it to fit the 310 piston the ports get really close and the rings hook the ports, not enough support. you could go to a like 265 or something but don't waste your money on the 300 ish