[01A]

If the positive battery terminal is considered 'hot' and the positive is connected to the chassis then the entire chassis is 'hot'. As an experiment disconnect the negative battery cable from the negative battery post, use a volt meter and connect the red positive lead to a clean chassis bolt and the black negative lead from the volt meter to the negative battery post and see if you measure 6 volts ( assuming a stock 6 volt positive ground) . what you have done is proved the cutoff switch on the negative battery post being in the off position does not keep the chassis from being hot (possibly including the motor ) so any short circuit from the hot chassis hitting the negative battery post would complete the circuit and sparks could fly. Thinking what could happen if a wrench dropped bridging the negative battery post and the frame/chassis.