Make sure that the keyway is good on the crankshaft where the flywheel goes on. That is the only thing i have seen cause this. I have had more than a few shear the key way off. It is generally from someone who worked on it and didnt tighten the flywheel properly.
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07 LTR450: Beadlocks, Motoworks SR4 Full Exhuast The Rest Is A Secret
06 Full MX Raptor 700R: Stolen

85 ATC250R: P&P, CR Carb & Ignition, ESR ATC 5 Pipe, Lots Of Billet
85 ATC110: Stocker
01 TRX400EX: 440 Kit, Stage 2 Hot Cam, Trinity Pipe, Clean
78 Yamaha GT80E: Stock