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Undercutting insulation between commutator segments is a common practice when rebuilding electric motors and generators. Unless the insulation is flush with the segments it's best not to undercut. If the insulation is flush with the segments it's really better to true the commutator on a lathe before undercutting. Usually on these starters a bit of cleaning up will do the job. A good way to clean up the commutator is to take a sheet of wet/dry sandpaper around 200-300 grit and pull it back and forth over the commutator like you were shining shoes. This method of sanding will help keep the segments level. If the segments look good a bit of polish with a typewriter eraser or even a pencil eraser will brighten them up. Don't use any metal polish on them and never use steel wool or brass wool because the metallic dust or particles can lead to shorting the segments.
Undercutting insulation between commutator segments is a common practice when rebuilding electric motors and generators. Unless the insulation is flush with the segments it's best not to undercut. If the insulation is flush with the segments it's really better to true the commutator on a lathe before undercutting. Usually on these starters a bit of cleaning up will do the job. A good way to clean up the commutator is to take a sheet of wet/dry sandpaper around 200-300 grit and pull it back and forth over the commutator like you were shining shoes. This method of sanding will help keep the segments level. If the segments look good a bit of polish with a typewriter eraser or even a pencil eraser will brighten them up. Don't use any metal polish on them and never use steel wool or brass wool because the metallic dust or particles can lead to shorting the segments.
