Hollow Shaft Nema8 stepper size 20mm which has a coarse bore for air, beam, lead wire to be through. 
If you need a very precise bore for bespoke leadscrew or ballscrew please contact by email for quote.
Specifications:
Holding Torque 18mN.m
Bore 2.5mm
Step Angle
1.8 Deg
Rated Current
0.6A per phase
Rated Voltage
3.9V
Inductance per Phase
1.7mH
Resistance per Phase
6.5 Ohms
Motor Length
30mm
Insulation Resistance
100M Ohm Min. ,500VDC
Dielectric Strength
500VAC for one minute
Shaft
Hollow Shaft 2.5mm Inner and 5mm Outer