I had the same problem some time ago with an RB X12 engine. Mine broke the crank in two just at the edge of the front bearing. At the begining I thought it was a worn bearing, I changed it and ĦĦĦbang!! it did it again (2nd crank).
Carefull inspection determined it was the Centax clutch bell that was slightly ovalized because i did a bad operation trying to change pinions thus creating a high rev vibration(I asume) that affected the crank. When i changed the Centax bell, checked the rest of the clutch assembly, the problem was solved.
Hope this helps.
AFM