Jim,
Are you speaking of the crank on the satellite antenna or the over-the-air antenna?
I recall that when I installed my crank up satellite antenna, that I had to use a hack saw to cut the shaft to the proper length.
My satellite antenna is Winegard. Same as the over-the-air antennas. My guess is that the installation is similar. The shaft is likely available from Winegard as a replacement part. If your unit is under warranty, I'd ask your dealer to take care of it.
Jim