Help Needed to identifi this key from elec motor 8mm to 5mm 1

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Hello hope someone can help, I need to replace a key in a electric motor. the key has two different sizes. it is 8mm at the bottom then steps down too 5mm at the top. it connects the motor to a drive gearbox that turns a wheel.

 

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hope this photo helps

 

[byrdj]

stepped key stock
here is imperial that is close to your 5/8

http://catalog.daemar.com/item/dmr-trade-keystock/step-keystock/sk-20-1?

 

[Tmoose]

the key seat (keyway) widths in the shaft and driven hub should be within ~ 0.001" ( .025 um ) of key.
Is there a setscrew or some other feature that locks the hub to the shaft?

the tapered/rounded flanks of the narrow portion of the key are 'interesting."