Starter spins, wont engage. Suspect corrosion/grime preventing slide up and engagement. Ground, battery strength have been eliminated as they are good. Worked at beginning of winter, then had to be "prodded" with screwdriver, and finally nothing but spin.Have starter off tractor, removed black "cap" covering retaining nut and spring. How do I get the retaining nut off? Any other tips? I'm very fearful of taking it all apart as I see numerous tubes about brushes being impossible to get back together properly. Pretty sure my problem is just on the outside. Thanks, Jim