I run the metal shielded (basicly stk) bearings in my mp9 and I check them after every run. Life varies a little and depends on what clutch I'm running but for the most part I can get 3 full race days on one set of bearings and thats taking them out and cleaning them when I'm checking everything else anyways.. (clean freak) I have had many guys ask me for help with bearings and found that they have been running the same bearings for ever. Lack of tools? I have seen it many times that the bell teeth and also the spur gear teeth were worn to a point of the mesh no matter how loose or tight you had it was just not good and when you would run it the gears would torque up and put side presure on the bearings and they would fail.... I witnessed a dude blow threw 3 sets of bearings over about 100ft of track before I stepped in to figure it out... New bell and spur and he's prolly still running the same set of bearings 4 yrs later.... I'm sure you guys have one of these guys at your track... oh and he loves his dremel..