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Old 01-03-20, 03:48 PM
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Spacing on the smallest sprocket usually matters very little except for matters of chain/cog clearance since the RD just rests on the limit screw, but there does need to be enough clearance for the chain and cassette to fit. Usually the reason 11T cogs are thicker is because the 12/13T sitting underneath is recessed to allow the 11T splines to engage the freehub even when the cog partially overhangs the freehub. There generally won't be room for additional spacers because they must have a small enough diameter to fit within the recess in the 2nd cog. There is indeed a difference between 11T and 12T lockrings, as would be expected, the 11T one is smaller, and designed to interface with the ridges on an 11T, but won't interface well with the recess on a 12/13T. For many cassettes, the 12T 2nd cog is identical to a 12T end cog, however, it is often designed with a flat sheetmetal cog behind it, not a recessed cog. A cassette with an 11-13-XX progression often has a 13T that is recessed to allow room for the extended 11T splines. A 12-13-XX progression sometimes expects a 13T cog with a flat face and without a recess. The extended splines on the smallest cog, and the recess on the the 2nd cog, and all the little variations between various cassettes are what make swapping them tricky. There are multiple things to consider. Whether a spacer would fit in the recess, whether the 12T sinks into the recess, whether the 11T lockring sits in the 12T recess and bottoms out. Things might work out with the parts you already have, but on the small end of the cassette it's often a bit more complicated than just cog thickness and spacers.
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