This is an aluminum Adapter Hub, designed to fit the Motenergy ME0903 Pelton Wheel. The Adapter will allow you to attach the Pelton Wheel to a standard 7/8" shaft with 3/16" keyway.
Comes with 3/16" key, 4 bolts that are M5, 12 mm long, and a set screw for the key and adapter.
The 4 drilled holes are on a 34.7 mm diameter bolt circle, 90 degrees apart, threaded M5, 10 mm deep.
The Hub has an outside diameter of 45 mm. It is 52 mm long.