Sounds like something isn't seating correctly when you put it back together and binding the shaft. It should have only a small amount of drag from the brushes contacting the commutator. It should be such a small amount of drag that you may not even feel it.
Because of the high gear reduction the output shaft (that the star gear slides onto) will be VERY difficult to turn when the starter is completely assembled. Hook it up to power to test, no need to turn the final output shaft
Ok, that's making sense....I took it apart a 2nd time and I still can't turn the shaft. I don't want to strip the threads by using vice grips or anything else. How do I check it electrically while off the bike ????