Replacement preload nut for the 3D+ series of cranks, non-drive side. The original version is pictured; ALDHU-series cranks are smooth around the outside edge.
Select ALDHU for the ALDHU 30mm axle cranks, ALDHU24 for the ALDHU 24mm axle cranks, or 3D+ for the discontinued 3D series of cranks.
On the MTB side of things, KAPIC and HAWK selections are self-explanatory.
This is a spare part that you likely won't ever need but if you do, we have you covered! Its function is to dial in the spacing for the crank axle in a set-and-forget system. Consult the owner's manual or your local bike shop for installation instructions.