First, check what Luke said. Then, you might have to shim your starter, shim the top bolt to give more engagement, bottom bolt gives less. If you paint the teeth you can see the engagement pattern, which will tell you a lot. Don't know what is used today, we used to use white lead, last time I checked anything I used dychem, it was handy.
Is your starter a 12 volt in the 6 volt housing? If not, there are differences in the tooth count.
