I just picked up a used but very nice Dura Ace 9 speed cassette. It looks like everything is there, but when I slide it all on the freehub, the lockring doesn't engage the last sprocket (there's still a space and the ring won't actually tighten the cassette on). Am I missing a spacer somewhere? I have all the 9S spacers (2.56 mm, I think), but there's no spacer behind the cassette. Should there be one that goes over the freehub before the first sprocket cluster? And what size should it be?
Normally there should not be a spacer behind a 9-speed cassette. The first position cog must have a built-in spacer to work with the lockring.
Many but not all MAVIC hubs require a spacer because MAVIC cassettes are wider than Shimano's. This type of spacer is 2 mm thick and should be included with the hub when new.
It's a Ritchey wheel (and hub), and an 8 speed cassette goes on fine. I do notice, however, that the 8 sp cass. has enough material that the part that hits up against the hub shell is flush with the back side of the largest sprocket, whereas the Dura Ace (9 sp) is recessed the thickness of that largest sprocket and then some, leaving the cassette much further inboard.
Is there any possibility that the new cassette is a 10-speed instead of the 9-speed?
Shimano 8-speed and 9-speed cassettes are the same width. The 10-speed is narrower and comes with a spacer when new.