Hey Everyone,
I wanted to create a guide to help troubleshoot headset play regardless of bike brand. By no means am I an expert, and I hope others can add to this thread. Please understand this guide/thread isn't intended for airing our bike brand grievances or calling out user errors. Ultimately we all just want our bikes to work as intended.
Common Reasons For Headset Play
1. Steerer/Fork isn't cut low enough, or an extra spacer is required above the stem to properly compress the top cap. When initially cutting the steer tube, an extra 2-4mm below the cut line needs to be accounted for in order for the top cap to recess properly.
2. The headset bearing (upper or lower) doesn't sit inside the head tube cup snug enough (laterally, front to back). Velobuild, for example, provides 52mm diameter headset bearings. Other headsets sold online or for other bike brands may only be 51.8mm diameter. That 0.2mm is the difference between a snug fit or loose headset. If your bearings are loose this will always cause play. You can add a couple layers of tape inside the bearing cup to prevent the bearing from moving around. Or just buy a new 52mm headset.
3. The top headset bearing is recessed too far into the head tube cup, causing the headset cover to rub on the frame, preventing the complete headset stack to properly compress. If this is the case you will need a thicker headset bearing (7mm, 7.5mm or 8mm thick) or a thicker c-ring so that it sits flush with the top of the headset opening on the frame, thus preventing the headset cover from rubbing the frame. EDIT: Another resolution is adding some micro spacers between the c-ring and top cap.
4. The upper and lower headset bearings are installed in reverse. Some bikes (like Yoeleo's recent G21 gravel bike) provide a set of headset bearings, with one of the two bearings being slightly thicker than the other. The thicker of the two bearings needs to be installed on top of the head tube, and the thinner bearing on the bottom. If these two bearings are installed in reverse order then...see Common Issue #3
It is critical to do a dry run of installing the crown race, headset bearings, c-ring, headset cap, stem/cockpit, and any additional spacers before actually proceeding to building up the bike. Also be sure to compare and inspect the two headset bearings to confirm they are in fact the same size and thickness.