Dumb question #1

[Scouser]

Coming from a nitro background I am more used to spider/planetary diffs than the ball diffs in the Kyosho so it came as a bit of a surprise when the diffs loosened themselves when twisted!! Is this normal? I have assembled as per instructions and even got some nice new ceramic balls for the build. It seems as though the locking nut in the shorter part of the diff is unscrewing itself from the long bolt running through the centre shaft. Construction has ground to a temporary halt until I know if this is ok!!