The flange where the starter meets the block. It seemed to be to big so I put a grinder too it and took about 1/8" off the edge all the way around and it then it would fit. It's a reman'd unit I got from Nismokid. Strong starter, just didnt seem to fit right away. Comparing it to the starter I got from Tim, the flange was alot larger on the new one, and the solonoid was bigger as well.