If you don't have enough metal to dump the heat you will be concerned about the rotor magnets hitting their Curie temperature or the field coils melting their insulation.
If the motor shaft dumps it's heat in to the screw, the screw will expand. If the shaft does not dump it's heat in to the screw, where is the heat to go?
You will probably never have a problem, but if your motors run hot you will always worry, especially when considering leaving it unattended for that long job