Originally Posted by
tcarl
I'm a little confused by your question (or maybe I don't know what I'm talking about). You say you have a CS-6600 12-27 cassette. I think the CS-6600 only comes in 13-25, 14-25, 15-25 and 16-27 sizes and don't need a spacer. Any other size, including your 12-27 would need the spacer.
All of the CS-660 cassettes you reference, plus the 12x27 and several others you missed, require a 1 mm spacer behind them on any Shimano 8/9/10-speed freehub body.