I recently rebuilt an esr racing 310 250r motor. During the first tank of fuel the sleeve skirt broke off in the motor. I took a look at sleeve which is 72mm and it seemed awfull thin. I've tried emailing them but have gotten no responce. I don't understand why this happened. Any ideas would b very helpfull.

 

Keep the pressure on them, and maybe ask around the 250r websites. For what it is worth, a good tuner can port a stock jug (with a few accessories) to run with the esr 310.