The older 6v engines had a 6v starter with lets say 32 teeth (maybe it was 28?) The later 12v engines and starters used a ring gear with more teeth, lets say 40. Both used the same bellhousing. A later 12v starter will bolt right up to an older engine but will not work right 'cause it won't mesh and will eventually chip the teeth on your ring gear. Same works the other way, 6 volt starter on 12 volt engine causes problems.
The right thing to do is to count the teeth on your ring gear, figure out which starter you must use, 12v or 6v, and then order one. NOW... there are 6v starters converted to 12v. Yup... they have the right number of teeth but run on 12v! Check with all the different parts houses-ie chevy duty, jim carters, chev's of the 40s, LMC, Patrick's, Langodon's, and others.
The 6v starter will work with 12v of juice but eventually will burn up. So, lots of people who convert their trucks to 12v use the 6v starter til it goes and then buy the right 12v replacement.
If you've used the wrong starter, chances are the teeth on the ring gear are ground up/chipped and the ring gear needs to be replaced. That means, pull the transmission, pull the pressure plate and clutch, pull the flywheel and put on a new ring gear before putting the flywheel, clutch assembly and transmission back in. Replacing the ring gear isn't too hard once you've got the flywheel infront of you, just knock off the old one, heat up the new one to make it expand a little and put it onto the flywheel.
If you've got to replace the ring gear, you might as well check the run out on the flywheel clutch surface and turn/re-face as necessary, check the pressure plate, clutch disc, pilot bushing/bearing, throw-out bearing, clutch arm pivot ball, gaskets and fluid in the transmission, and the universal joint of your torque tube (where it bolts to the back of the tranny).
If your ring gear is messed up, you might be in for a big project!
