I had one of my 500s seize BAd piston was melted to the cylindar... I had it sleeved, big mistake, it used pistons and also ran very hot, finally bought a new cylyndar and had no problems after that, dont know about buying a new cylindar now days, as i cant even find graphics for my 500. but there are a few companies that replate in nickasil i think. The 500 motor is superior to the Cr motor. Its quicker lots fasrter. I think it depends on what kind of riding you will be doing. Desert trails or races the KX...... strikly motocross sorry KX fans ( i am one ) the CR