Small parts like this cassette spacer are the kind of thing that only matter until they don't, and then they matter a lot. The Shimano CS-7800 Low Spacer is a 1.0mm aluminum shim that sits on the freehub body behind a 10-speed Dura-Ace, Ultegra, or 105 cassette, taking up the gap between the cassette and the freehub flange so the lockring can torque down properly. It's a genuine Shimano replacement part carrying the Y1Z807000 designation, and it's the same physical piece used across the CS-7800, CS-7900, CS-6700, CS-6600, CS-5700, and CS-5600 cassettes.

The application is specific. This spacer is required when running a 10-speed Shimano road cassette that starts with an 11T or 12T small cog — the 11-21T, 11-23T, 12-21T, 12-23T, 12-25T, and 12-27T cassettes all need it. If you're running a 10-speed cassette with a 13, 14, 15, or 16-tooth small cog, the cassette itself is built taller and the spacer isn't needed. Get this wrong in either direction and you'll either have a cassette that won't seat properly under the lockring, or one that runs against the frame with no clearance.

Installation is straightforward. Slide the spacer onto the freehub splines before the first cog, seat it flat against the freehub flange, then install the cassette and lockring as normal. Torque the lockring to Shimano's spec — 40 Nm — using the TL-LR15 or equivalent lockring tool. If you're still seeing play or misalignment after adding the spacer, the issue is usually elsewhere: freehub body seating, cassette compatibility with the freehub standard, or the wrong spacer height for the specific hub-cassette combination.... Read More