Derby cover gasket
 

Hello. I am replacing the stock derby cover on my 2010 SG with a custom HD cover. I understand that the stock cover uses an O ring while the replacement gasket is a flat round one. I've also read that the O rings are superior. Can I reuse an Oring from the stock cover with 0nly 50 miles on it? Will it fit on the custom HD cover? Also, how close to upright must the bike be to replace the cover? Thanks.

No I don't believe the custom cover has a groove in it to accept the o-ring.
I know I couldn't use the o-ring when I changed my cover 09 super glide.
The bike should be straight up and down then you won't loose any fluid.