A Rex notes, there is a lot of it about. At least an original part fitted when the bike was built, and I for one like to retain as much of the original machine as I can.
When fitted, this part is not seen. So with an original to hand, cleaning it up and ensuring the inner and outer threads fit the top nut and the steering stem top thread as they should is going to save some hassle. These threads are very fine and easily damaged but when all is as it should be, both should tighten down smoothly with no roughness or binding.
For those folks using standard cup and cone, the trick is to assemble lower yoke with well greased balls to stick them in place, carefully pass the stem up through the steering head, add the top balls, top cone, dust cover and screw on the "castle nut" stem adjuster. Three hands useful to do this! Tightening the adjuster sets the bearings.
Top yoke then slides down onto the dust cover, yoke pinch bolt removed and slot opened up makes it easy, but may still need a good smack.
Looks to be you have a new aftermarket adjuster which is, as often found, not quite right for the job.
Swarfy.
