The standard Align T-Rex 450 SE main shaft is 5 mm in outer diameter (OD) and 116 mm long.
Here are some mostly compatible main shafts:
- HDX-450: 116mm x 5mm
- Belt CP: 1__ mm x 5mm
- Outrage G5
- Beam E4
- MiniTitan E325: 130mm x 5mm (also j-bolt holes different)