Original: Matt Kirsch Electrics Moderator
Many outrunner motors come with a collar that slides over the shaft on the "fixed" end of the motor. The reason for this collar is to reinforce the snap-ring that holds the shaft into the motor when you attach the propeller to the spinning can end of the motor. If you are attaching a prop adapter to the shaft protruding from the fixed end of the motor, then you can remove the collar.