I have removed the armature in order to press the tach off.
One alternative mentioned involves removing the motor brushes, and chuck up the drive end of the shaft with the encoder end in a centre and turn down to 3/8".
Renco have 1/2 bore encoders but are a bit pricier.
Also look at Koyo, they make nice shaft type like the one in the picture, also through hole I believe.
No real need to press the tach off, just remove the brushes when in operation.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.