Originally Posted by JohnDThompson

Either the Hozan lock ring tool, one end of which has a smaller radius designed for those rings, or an adjustable hook spanner.





N.B. I've always found those notched headset lockrings to be superfluous. You already have the locknut to hold the adjustment, so why the ring as well? Seems like a "belt *AND* suspenders" approach. You can leave it off and replace it with a simple spacer.