The fence keeps the motor up. Once the motor pulls on the belt, it effectively gets a bit down. Now there's the fence, or lip, keeping it up where it is.
The pipe really goes back deep in when you wiggle a bit on the stub axle. It just slides an inch or so deeper.
Then it protrudes by 3 or 4 mm. This is to be packed with spacers, so that the large cog can rotate free- but has no axial play (I'd say 1/10 is enough).
If it can move: Get washers / spacers to fill up.
Which part is leftover ? Make a pic.
Best to take a pic, put it here, and we'll tell ya what to do. Think, I never had a C5 2 weeks ago, I had a blown CB and a rattling noise from a snapping belt- I learned it in a few sessions and now all is fine. There is success - it's just there, behind that fence :-)
Carsten