Was your former/old backrest "rigid" mount and now you bought the "quick-detach" mount set-up? If so, there's an additional kit required for mounting to your bike.
Also, another possibility, is you bought the incorrect parts for a Sportster...did you buy from HD shop?
I've solved this problem now, and feel I understand a lot more about it all now. I did purchase them at a Harley Dealership and the parts guy even came out with me to the bike, took them out of the box and compared them to the bike (I wasn't really looking so hard, because I figured too many chefs and all that...)
First lesson learned; Don't rely on the parts guy..lol. I should have been watching and checked it out myself instead of instead of just relying on others.
Second lesson learned; it would be helpful and advisable to research things more on the internet before acting, ordering, fixing, purchasing things on my own...
Bottom line, I had a choice to go detachable or return what I bought and order the right part(s). I could not get anyone to give me any reason why I should have the detachable other than the benefit of detaching my backrest, etc. of which I don't care to do. I can just as easily remove the four bolts..., thus I returned the parts, as for ordering new ones, the guys there couldn't agree on which sissy bars would for sure fit. One guy during the day picked one out in the book, and then he left it written down for another guy who would be there later, when I came in to deal with. The other guys said lets measure, .. we did and it seems they picked the wrong one again, so I said, just forget about the sissy bar, and order the right luggage rack. I'll find the right sissybar in ebay or somewhere else... Argh..
The good news: I now have the rack on order (should receive it in the next couple of days) and I'm confident that it will be the right size. I'm still looking for a backrest, but in the meantime found a good price on a sundowner seat that I've been wanting as well, so between the $100 I got back on the wrong backrest and the 150 bucks of the retail price on the sundowner, I forked out an extra 80 bucks and got the seat instead of the backrest. I guess it'll have to wait.
Biggest lesson of all: Be patient.. and it'll happen... Cheers!
