You must have a problem with either the thread on the motor, or the thread inside the freewheel, as the freewheel should screw on all the way by hand with very little force being required.
You will now need to remove the freewheel completely and try to find out why it is binding so badly on the thread.
If the thread on the motor is full of paint, you will need to remove it with a wire brush.
I would cut a suitably sized hole in a piece of cardboard and place it over the threaded boss to protect the paintwork on the side cover from the wire brush.
If the thread has been physically damaged and is burred over, you may need to clean it up using a suitable thread file
(a very small triangular file or even the edge of a half round smooth cut file etc. could also be used if you don't have access to a thread file)Alan
