I snapped the head of my clutch centre bolt (brand new engine... bolt was majorly loctited in).. anyhow.. i just ran a drill bit down the guts of the bolt (the bolt is much softer than the crank, so the drill bit just chewed through the bolt without damaging the crank).. after i had drilled the bolt out i ran a tap through the crank just to clean out the thread.. btw, you should be able to tell the difference between drilling through bolt and the drill bit starting to drill the crank (end of the bolt hole) by the noise and speed of the drilling