The key size to be inserted into the hollow geared motor should be nearly twice the length of the hollow shaft hole diameter.
In the case of the GM-SSY with service factor 1.0, select the key to double length among 1.5 of the diameter of the hollow shaft.
For the GM-SHY with service factor 1.4, select the key to length from twice to 2.5 times of the diameter of the shaft.
Insert the key so that it fits the end face side of the hollow shaft (H8 tolerance).