Replaces the stock Sovol spool holder but utilities the factory locking nut, this upgrade has 3 bearings that allow the spool to rotate with ease causing less strain on your extruder. 
Comes fully assembled with bearings. 
Both 500kg and 1000kg holder will come with your purchase. 
If you have issues with the holder rotating and the bearings not staying upright there is a small hole in the spool holder that you can drill to key the holder upright. (Roll pin provided) 
In most cases the nut fits tight enough that this is not an issue.