I put that exact seat on my '06 RK and had the exact same issue. Because I was putting the Sundowner SG seat on a RK, I just accepted that the gap was there because it wasn't specifically made for my bike. It never bugged me that much, but I can understand in your case considering the seat is made for that model. My

 

The seat is the correct part according to the dealer. It is 51630-6 which is specifically for the 97-07 FLHX & RK. The fit is just plain terrible. I suppose the seat could be a reject as the gap is a least one inch to short. I just wanted to see if any other SG owners had the seat on an 06 and how the fit was for them. The stock seat comes at least 1-2 inches higer on the tank covering the gap at the end of the chrome tank piece as it should.

 

Have you checked with your dealer to see if they will exchange it or refund and order you another?

 

Yes they will take it back. I am just nervous that having another shipped could lead to the same issue.

 

I have a 08 king with a Sundowner streetgilde seat and fits snug right up.