Could you not just put the motor in a vice (light pressure, with rubber or rag around it to protect it), hook up the ESC and battery and have it spin the shaft slowly (1/4 throttle or so) and use a file to grind it down a bit until you can get a 5mm pinion on easily? That would be how I would do it, except I don't have any 5mm pinions

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Brijar!!!