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?