S550 starter internals, do you see a problem?

[bKennedy]

I changed my starter the other day and decided to tear it apart rather than get the core refund. I have a few pictures of the internal parts that could be bad, but none look really bad. The starter would crank slowly and then pick up speed, and always start the car. I have a 2018 GT 5.0 with 70K miles. After I replaced the starter it cranked like new, so it was not an electrical problem; prior to replacing it I cleaned all the wires going to the old starter so the old one had good connections. When I took the starter apart, there were some fine metal particles in the cap that covers the brushes. These particles were magnetic, so it was not worn brush material or copper from the commutator. An inspection of the magnets did not show any damage but the rotor coil does have some wear marks on it; you can see them in the picture. The brushes look good, as does the commutator, although it could be cleaned up with an abrasive pad. The starter always engaged, so I do not think it is the solenoid, but perhaps it is. From the information I have given you, and looking at the pictures, what do you think? The starter is a SA1087 (2-bolt) that works with the 10R80 automatic transmission. If I could find a rebuild kit I would buy all the parts, try to fix it, and keep this as a backup. From what I read, the oval cuts in the rotor coil are from the factory to balance it.
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[jheissjr]

Worn contacts in the solenoid can increase contact resistance and decrease the current to the armature. A second idea is to check if any of the windings in the armature are open. It doesn't look bad in the pictures. Output teeth look good, no pitting or burning on the copper, no corrosion.