Nut, Spindle M20x1.5 

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

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

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