Fuel filler neck

So i got a new filler neck to replace my old rotten one and for some reason my fuel cap (the solid aluminium one) wont fit it. The collar that protrudes from the wing where it fits is too wide, only by a say 5mm or so. Also the foam washer that goes over this part before it fits inside of the wing, gets stretched a bit too much aswell that the holes dont line up.  

Can anyone give me some heads up on why? The neck is the same shape and length etc as the old one and fits inside the arch fine. Doesnt need cutting or bending etc.

New part is 155201129C.