Also there is one other kind of odd bug with screw threads, which will happen if you use a helix path that has 6 turns in it.

That particular shape triggers a bug, there is a workaround described here:
http://moi3d.com/forum/messages.php?webtag=MOI&msg=1263.2

But you've got 8 turns in the one you've got there so you shouldn't be running into that particular problem here.

- Michael