Ian
I had the same problem with mine. All I did was loosen the clamp so the precat is held loosely, push the precat up, (if you "wiigle" the pre-cat I found it obvious when it was seated properly). Tighten the clamp so you can "ONLY JUST" move the precat by hand. Then with a rubber mallet tap the pre cat up towards the turbo/manifold to help it seat, and then tighten the clamp right up. To be honest I did not have too many snags, but from the design of the clamp, it is very easy to mis-align the clamp so it does not sit 100% true around the precat lip.
Hope this helps.

Rob
Edit to say: Someone with more experience may be able to give you a different tip, but this worked for me. From what I have gleened off the site it can be trial and error.
Edited by Ouija Board, 02 May 2007 - 03:32 PM.