The suggestion to use a hex extension bit was perfect, thank you. I got the rotor off and it cleaned up easily. I put the axle into my drill and spun it against 320 grit sandpaper. The copper comes off fast and the commutator bars look clean. I also checked with a screwdriver that none are loose.
The bearings are fine. I took the opportunity to clean them with WD40, dry and then repack with lithium grease. Lucky as the top bearing drifts off easily but I couldn't shift the bottom one. Probably needs a bearing puller which I don't have.
I took the motor off the impeller unit. It spins without shaking, so I suspect the imbalance is in the impeller unit. There are two divots on the armature but I suspect those are intentional for balancing rather than any damage?
guess the next step is to lever / drift off the impeller casing, clean, and reassemble.
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