52mm 500w motor fan.

With the z axis shuttle the stock fan for the 500w spindle doesn't fit so this is a smaller motor fan.

Tools are in imperial so you may have to scale to 25.4mm/in or 2540% if your tools are in mm https://www.banggood.com/Machifit-ER11-Chuck-CNC-500W-Spindle-Motor-with-52mm-Clamps-and-Power-Supply-Speed-Governor-p-1027937.html?cur_warehouse=USA