Chris' Tips
Here is a drawing of the R8 spindle.

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

Item No	Models
115	LittleMachineShop.com 3960 HiTorque Mini Mill LittleMachineShop.com 3990 HiTorque Mini Mill LittleMachineShop.com 4190 HiTorque Mini Mill Various SX2LF-US SIEG Mini Mill
2	LittleMachineShop.com 6450 HiTorque Mini Mill LittleMachineShop.com 3900 HiTorque Mini Mill Various SX2L-US SIEG Mini Mill
84	BusyBee CX612 Craftex Mini Mill Micro-Mark 84630 MicroLux Mini Mill Various SX2-US SIEG Mini Mill

Click your model to see the complete parts list.