Bottom line you have to first remove the outer motor gear to see if the key (as drkangel says) is still totally intact. Lets face it the key is only about 5/16" long and can roll very easily. But I thought you said this was a new motor.....and your still having the same problem? Something that I noticed on some of my motors is the gear is not as thick as the area where it sits on the shaft. So even when the nut is fully tightened the gear could still be moved on the shaft. I have 5 Currie motors and 2 of them came with a thin washer on the motor shaft behind the sprocket (gear). I am sure the manufacture relished there was a problem and added this washer to act as a shim so the gear would be held tightly in please. Otherwise, the gear would be allowed to "rock" on the shaft causing damage to the key. Bob