Double Jam Nut Hardware includes a tapered locking system that "jams" as the front jam nut is tightened into place against the primary prop nut.
Primary prop nut has a precision fit to align the prop washer when torqued into place. Front Jam nut is torqued into a seated taper position against the primary prop nut, creating a "jam" effect for safety.
Also popular with certain motors that come with a longer prop shaft where a standard adapter nut just wouldn't work.
Metric M10x1.0 Double Jam Nut Adapter Kit