using 12 volts on a 6 volt starter is common as Tim says, I did have a problem using 36 volts on a 24 volt starter for a big cummins. The shaft was quite long, and would twist, causing the drivegear/bendix assembly to hang up. It was easy enough to fix. I don't know if it was the extra voltage or if the starter was just not really big enough for what it was doing. But we had 3 of those engines that boat, and they all did the same thing. However I have heard of no problems using the 6 volt starter with 12 volts. This is one of the reasons I stick with the 6 volt system. I don't have to worry about any of that stuff, and it has worked without a hitch for the 30 years I have owned it. I even found 6 volt LED's.
