Take a look at the commutator on the end of the armature, or where the brushes ride. You can chuck the whole assembly in a drill press, or, a cordless drill will do… spin the assembly slowly while using some fine emery cloth to clean up the commutator. It takes very little, so don’t take off to much. Then clean each groove carefully, between each window to remove dirt and contaminates. That may be all it needs. I’ve fixed many starters and even trolling motors this way. Most shops won’t take the time to do this, and would rather replace the starter to save time, and make a few bucks. http://dansmc.com/cut_commutator.jpg

Position the piece of hacksaw blade so it cuts on the pull stroke. That makes it easier to control. I use 600 grit carborundum (wet-or-dry) paper to polish the commutator, then clean the mica separators. If there is a bad contact with the brush cables, current through the brush springs can overheat them and make them loose tension. Cranking with a weak battery or excessive cranking will over heat The starter, melting the solder holding the wires to the contacts. A stalled starter will draw 200 amps. If confronted with a weak battery, starting effort will be reduced 40% by pulling out the starter rope 6 or 8 inches to activate the compression release. The upper bearing tends to ware on one side, turning it 180 degrees on reassembly will extend his life. . . 😉