It's 1"
It's also a huge PIA not to have a removable faceplate like most threadless stems; it makes you have to buy an individual stem for each set of bars, unless of course you like re-taping for every swap.
I know this thread is old/dead but fwiw, I can pick up a decent quill stem for a couple bucks or free from my lbs' takeoff parts so it's actually not a big deal to get a few stems. I like how fast you can switch bars with a threaded setup when you have multiple stems. Loosen one bolt, slide out the stem+bars, slide in new stem, tighten one bolt. Very useful for those of us who like to switch between stuff like track drops/road drops/bull horns frequently