the teeth on the flywheel and or the teeth on the starter are not aligned up and probably the teeth on the flywheel might be chewed up and the teeth on the starter are chewed up and that might be why its not engaging and the motor is just spinning and not turning over( or it could be just the relay on the starter(the selenoid) not moving the starter drive onto the flywheel????), check with the mechanic and see what he says......
