A standard cassette lockring tool is all you need to install or remove the Shimano SM-RT81 Disc Rotor Lock Ring. The internal spline pattern matches Shimano's cassette interface, which means the same tool you use for cassette swaps handles rotor changes without adding another specialty wrench to your kit. This aluminum lockring threads onto Center Lock hubs to secure the rotor against the splined interface, clamping everything in place so the rotor can't shift axially during hard braking.

The internal spline design works with quick release skewers and thru-axles up to 12mm in diameter. If you're running 15mm or 20mm thru-axles, typically found on mountain bike front hubs, the lockring tool won't clear the axle and you'll need an external spline lockring instead. For road, gravel, and most rear wheel applications with 12mm thru-axles, the internal spline version keeps things simple by consolidating your tool requirements.

Shimano includes a washer with the lockring that sits between the lockring and rotor. The washer distributes clamping force evenly and helps the lockring maintain torque over time. Installation torque is 40 Nm, which is substantial but necessary given the forces disc brakes generate during aggressive stopping. The same torque applies whether you're mounting this on an SM-RT81 rotor or any other Center Lock rotor from Shimano or other manufacturers. Center Lock spline interfaces are standardized, so the lockring works across brands as long as the hub uses the Shimano-style spline pattern rather than the AFS standard found on some Campagnolo and Fulcrum hubs.