Preface: I've never messed with a headset, threaded or threadless, before and I'm figuring this out as I go along so any advice/information/links are appreciated.

Edit: So basically I've been hearing a knocking from the steerer tube area any time I hit a harsh bump and initially when I took it over to Rapid Transit a mechanic said he thought I just needed to tighten the top cap. I did that and it seemed to have gone away for a week or two. Then I rode off quite a few curbs and it started coming back but when I went to tighten it again the top cap just popped out and seems to have seized. I guess my question is whether the bike is still safe to ride if I just pound the top cap back in like I did before (no issues in the last few days other than the noise) and whether this is indicative of a more serious problem or if I just need a new top cap.

TIA,
Dan

sorry, duplicate

The mechanic you asked was an idiot. The stem's top cap bolt can be removed once the stem's steering tube clamp bolts are tightened, the top cap bolt is only there to set preload on the headset. The star nut is your problem, especially since it came out with the top cap bolt. Maybe it's your terminology on the other hand. The top cap is the piece that sits on top of the stem/steerer tube the cap of the bolt fits into; the star nut is that star shaped thing that should remain wedged in your steerer tube and that the top cap bolt threads into and pulls against to set the headset bearing preload.

Here's how to install a star nut https://www.parktool.com/repair/readhowto.asp?id=67 and here's the full instructions on installing and adjusting a threadless headset like yours https://www.parktool.com/repair/readhowto.asp?id=65

Reading between the lines of the post I guessing that you followed the mechanics advice to tighten the top cap, without first loosening the stem.

The top cap and star nut system is only designed to be strong enough to adjust the headset, but the stem clamp is what holds it together. If you try adjusting the headset with the stem still clamped, it won't slide on the fork and instead you'll jack the star nut out.

You can replace the star nut and top cap, loosen the stem clamp, adjust the headset, and re-tighten the stem clamp in that order and you'll be good to go.

BTW- I can't tell for sure, but it looks like your fork is very close to flush with the top of the stem. If so it could also be that the top cap is bearing on the fork and cannot push the stem down any farther. If so, add a spacer under the stem to provide the necessary clearance.

Geez, Francis, what you doin' up so early (or late)? Yeah, he probably misunderstood the mechanic...

I wish I could say I was in Europe watching the Vuelta, or in the orient selling Chain-L. Maybe I could claim I get up this early for my pre-dawn training ride.

Plain simple truth is I awoke at about 3AM and couldn't get back to sleep.

The mechanic did tell me to loosen the stem bolts first, which I did.

Thanks guys!

I was up for the same reason but the other side of the coin, couldn't get sleepy, was up until 5:30...

I'd also QUIT banging off the curbs.