This was great fun. I have to confess, though, that I enjoyed the spot-on sneering and snarking, and the motorcycle-artist meta, probably even more than the spots of hot wanking.
I love their thoughts on each other:
Sirius never really thought about whether Snape enjoyed this enterprise or not; he assumed he had his own reasons and if you thought you might get Severus Snape to discuss his private motivations for doing anything, you had another think coming. Still, he had to wonder if maybe the bastard secretly got off on it.
and
Sirius Black's mind really was a strange place sometimes, although [Severus] supposed over a decade in Azkaban topped off with a round trip to whatever realm was on the other side of that artifact in the Department of Mysteries might do that to a man.