http://devestechnet.com/Home/FlywheelThat explains the issue you are having. The starter's gears are not easily interchangeable between 6 and 12 volts. If you have a 6 volt starter, you have 139 tooth flywheel, if you have a 12 volt starter, 168 teeth on the flywheel. Flywheel ring gears are reversible as explained in the document above. Starter parts are not interchangeable unless you are a machinist. (not sure about just turning the gear around.) It's amazing you are getting it to engage at all. Then you are destroying your flywheel over time.
