Gib, X1 Micro Mill Spindle, 90 mm 

Specifications

Length	3.5" (90 mm)
Width	0.5" (12 mm)
Thickness	0.1" (3 mm)

In your instruction manual, this part appears on the parts list as:

Item No	Models
48	Harbor Freight 47158 Central Machinery Micro Mill Clarke CMD10 Metalworker Micro Mill Various X1-US SIEG Micro Mill
83	LittleMachineShop.com 4660 HiTorque Micro Mill LittleMachineShop.com 4700 HiTorque Micro Mill

Click your model to see the complete parts list.