The X803 (X8438) 3 pole armature has a spindle that is 2.4mm in diameter, the X9024 5 pole armature has a 2mm spindle which prevents you fitting this into a 3 pole housing.
We have taken the X9024 armature and sleeved the spindle out to 2.4mm so this can now be used to upgrade your 3 pole motor to a 5 pole motor.