The 6v flywheel has 139 teeth and the 6v starter drive has 9 teeth. The 12v flywheel has 168 teeth and the starter drive gear also has 9 teeth.....however, they are easy to distinguish between as the 6v starter drive gear diameter is over an inch in diameter and the armature shaft is about 5/8" in diameter. The 12v starter gear is less than an inch in diameter and the armature shaft is only about 1/2" in diameter. As mentioned the starter must be matched with the flywheel. However, you can run a 6v footstompstarter/flywheel combo on 12 volts with no issues. If a 6 v starter has a solenoid you will have to scout up a 12v solenoid.
Dave
