I've had an issue with the headset creaking on one of my bikes for a while. It's a Boardman hybrid which I bought second hand, semi-complete, a couple of years ago and converted to a drop bar gravel (ish) bike.
I've never been able to get the preload right. It went from play to binding with seemingly no sweet spot in between, no matter how tiny the adjustment on the top cap was. I tried a different top cap, moved the spacers round, put in a new expander bung, everything I could think of. It was driving me bonkers.
Earlier this week I'd been trying and failing again to sort it, and I was sitting there looking at it when suddenly I had a thought - "that doesn't look like the right top cover". (The one that goes over the upper bearing) There's always been a bigger gap than I would have expected/liked between the top cover and the headtube but it's only just dawned on me what the issue was. The rogue cover had different ridges on the inside and wasn't sitting right. The bike would have come with FSA bits from new but this was just an unbranded one, and was wider at its base than the headtube so it, literally, stuck out. I don't know why it took me so long to cotton on.
I sent for a new bearing, compression ring, thin spacer and top cover set up - thought if I get them all then I know they're all going to fit together. It's not the same bearing part number as the previous one but it's the right size and the right chamfer, and it fits perfectly. I put it all back together and preloaded it perfectly first time. The top cover looks so much better than the old one too.
Result