I've found a used steering assembly to replace the original on my Birdy BD-1, which had cracked along the weld of the handlebar clamp. I've never replaced a fork, or in this case a steering assembly that fits into the head tube, and I'm wondering if it is common that a ring or seal will slide easily onto the tube most of the way, and then suddenly stop dead, as if the OD of the tube had suddenly become larger. Please see the picture.

The seal slides all the way up easily but then stops with an inch or so remaining. I've gently sanded the inner metal ring's edge with very fine grit paper so that it's shiny. There's no gunk or residue of any kind, and I've cleaned the tube itself. Is it normal that the seal ring will have to be forced onto the tube, using PVC tubing and a hammer, or a tool designed for that purpose?