25.4mm at the clamp 22.2mm at the rest. Look close at the clamp area and you'll see that it flairs larger under the clamp. I have the XP 1.0 and I'm pretty sure all models are the same. Most bmx bars are different so you're looking at a bit of a hunt. Most modifications to bmx end up replacing the stem clamp and bars to match and loosing the ability to fold down the bars.
