Common problem on these chain driven models. Either the sprocket tooth hits the washer and bushing or if you can get past that sometimes you can't get the sprocket through the hole.. This is because over the years the tranny can shift around to where the shaft isn't perfectly centered in the cut out opening like it was from the factory.You can take a dremel if you don't want to remove the bolt,washer and nut and just notch enough to where the one sprocket tooth(that usually hits the washer) can clear. Don't be surprised if you have to do the same thing on the cut out hole if the old sprocket or replacement sprocket can't get through the opening. Either do that or loosen all the tranny bolts to where it can which most people wouldn't do. Polaris just made things too close and the hole too small..