The single speed rears are known for doing this. I never cracked one of them open, but these have an inherent stuck-in-gear problem. My one MTD will go out of gear easier if I depress the clutch (or speed) pedal down and slow down and before coming to a complete stop, slowly put it in neutral. Then stop, let it drive slightly (not completely depressed), and shift to reverse. I other words, these gearboxes like a little movement while changing sector position. Take care doing this, because a sudden slam into the opposite direction at too high of a speed has obvious detrimental affects. Happy cruisin'.
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April 2009