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Thanks Tester - I had read something like that on SB.
I am looking to fit a 9 speed cassette so I believe I can use 10 speed spacers but will have issued with the pre-spaced 2 smallest sprockets.
Or just 9 sprockets of a 10 speed cassette...which I don't have!
Vince-
If you're using 9-speed STI shifters and a 9-speed RD, the sprocket spacing needs to be at the 9-speed pitch of 4.3mm, not the 3.9mm 10-speed pitch. "8 out of 9" should fit on the short 7-speed spline of the existing freehub. With the HG50 cassettes, the big sprockets are pre-stacked and held together with either rivets or bolts, but they're all full sprockets with individual splines so pulling the stack apart and binning the biggest sprocket and its spacer works. The higher end cassettes are more problematic as they have groups of sprockets riveted to aluminium carrier hubs - not impossible, but you need machine shop skills, not village blacksmith skills 😀
You can lose a sprocket near the top end without modifying anything, but you have to keep the top one or two so you end up with a horrible big jump right where you don't want it
gbj_tester
If it's FH-1055 7-speed UG/HG, it's 126mm OLN. I'd probably keep the hub and modify a CS-HG50-9 cassette to make it 8 sprockets at 9-speed spacing.