Sorry but I dunno what it's called, "compression cap" seems what ppl use.. the bolt that goes (male) into the fork's female, usually i use this to dial-in my stem's tightness&alignment relative to the forks.
I've never noticed how I setup bikes w/o this bolt before (bikes where ppl chopped their steerer tubes too much and no longer use their compression caps), but never had issues til now. I cannot get this thing tight enough, it feels tight when we bang it into place and tighten, but then when you 'drop test' it (lifting&dropping front wheel from ~12"), there is a TON of wobble between the steerer tube and the headset.
I feel like i used to just sit on the bike and wiggle it into place, but on this job it feels like that, then i do final tightening, and it's still sloppy. I guess i'd love tips on how to *over*-tighten a fork/stem intersection w/o a compression cap (from there it'll be easy to work backwards )