I have the bike listed above. This frame has a 1" threaded headset and fork combo but the headset is integrated ( but in addition to the bearing cartridge there are internal cups that rise just above the head tube. I was told by some guy on another forum that the internal headset used actually reduces the internal diameter of the head tube down to 1" and this particular frame can use a zero stack headset at 1 1/8" ( he suggested Orbit). I called Giant directly and the 2 guys I spoke with had no clue of the internal diameter or what my options would be of the head tube. Somebody let me know if I can replace this headset with a 1 1/8" headset and fork I think the head tube diameter is large enough and Giant had 1 1/8" headsets before and after. Also, if it makes a difference the size of my frame is 53cm (small compact).