I hope you had your original starter rewound to 12v, not just putting a 12v starter on. There is not a 12v starter gear with the correct teeth on the drive gear. Both 6&12v have the same number of teeth, but the pitch is wrong on the 12v. It engages the teeth on the flywheel too shallowly. Also, the 6v gear won't fit on a 12 v starter. The 12v starter that was on my truck was chewing up the ring gear pretty bad. The 12v starter would occasionally hang up in the engaged position. To make matters worse, a 12v ring gear won't fit the 6v flywheel either.
What I ended up doing was using a key start 6v starter from a chevy car. (COE's are push button start, rather than stomp start). My starter repair guy installed the correct drive gear for the original starter. He also installed a 12v coil, leaving only the motor itself as 6v. I should have written down the part number for the drive gear, but didn't. My starter's O.E. Part number is 1107960. The starter guy could look up the correct drive gear from there. I had to buy the starter, drive gear, coil and pay to have the guy cobble it all together and still only had $160 in it.
So, I would recommend sticking with the 6v starter. If, you find a way to make the 12 v starter work let me know. I couldn't find it. I had a thread going here and a the old gmc trucks site about this. I'll give you the link to both. You have to be a member to view the old GMC site, which I would recommend if you aren't already. Sorry about the long winded post.
Scott
https://www.stovebolt.com/ubbthread...Main=113543&Number=776311#Post776311http://oldgmctrucks.infopop.cc/eve/forums/a/tpc/f/4101021591/m/28620585763?r=63620916763#63620916763
