The numbers(of teeth) are the same. The diameter of the "bendix" gear is different(same for the shaft diameter) So the "pitch" of the teeth is different. The outside diameter of the 12 Volt gear ring(on the flywheel) is larger. The location of the starter is the same for 6 Volt and 12 Volt bellhousings. The starter gear is smaller in diameter to make up the difference. The trick is to match the starter to the flywheel. Both the 6 Volt starter and the 12 Volt starter work on 12 Volts. Only the 6 Volt starter will work on a 6 Volt system. If you have a 6 Volt truck, you need a 6 Volt flywheel and starter. If your truck is 12 Volts, then a 6 Volt flywheel and a 6 Volt starter will work. Of course the 12 Volt flywheel and a 12 Volt starter will work as well. Since the flywheel is a PITA to change, I just match the starter to it. And all is good. But I am running a 12 Volt system so I can use what matches.
