This is a common problem I believe with worn starter drives. 6 volts does not matter with 12 v. driving it. flywheel is 139 teeth Starter with nine is correct combo. When the starter is depressed it can stop inbetween teeth. When it is pushed further, the spring in back of the starter drive is compressed. When contact is finally made electrically, the starter drive will turn and snap into the flywheel, because of the spring pressure on it. If it grinds it is because of one of wear on drive or flywheel.
