This is the Great Planes ElectriFly Collet Cone Adapter 3.175mm-5mm.
Aluminum construction, silver in color Mounts on electric motors with a 3.175mm shaft and adapts them to a 5mm diameter prop shaft. Cone shaped spinner nut adds good looks.
Length : 1.42 in. (36mm) as assembled
Length : 1.10 in. (28mm) shaft only
Length : 0.82 in. (21mm) spinner nut only
Shaft Diameter : 5mm
Spinner Nut Diameter : 0.47 in. (12mm) at the widest point