For me personaly the best way to guarantee your clutch bearings will last the main is to always replace them before hand. Then I normally use the ones from the previous main in practice and qualifying at the next race. I leave a small amount of play in the clutch bell, less than you would normally think to use because a new bearing will break in and free up the assembly. also if the bearings you are using are packed with grease you need to make sure to blow the excess grease out. Just get cheap bearings like
http://www.carolinasrc.com/Webstore/...idproduct=4972 and
http://www.carolinasrc.com/Webstore/...?idproduct=300 I use a dual shiel 5/13 bearing and always put the metal shield towards the engine. the rubber seal will fall off if its towards the engine.