This is a Great Planes ElectriFly 4.0mm - 1/4-28 prop drive (adapter) with set screw and prop nut.
Requires: Installation onto the motor or gearbox shaft
End to end length: 1.5 (37mm)
Complete shaft length: 0.98 (25mm) with 0.69 (17.6mm) threaded portion
Prop shaft diameter: 1/46 (6.27mm)
ID for motor shaft diameter: 0.16 (4mm)
Largest part of hub diameter: 0.78 (20mm) thrust washer portion
Smallest part of hub diameter: 0.47 (12mm)