I'm pretty sure the Cinelli stem clamp is 26.0mm, but you should measure clamp and bars to be sure. I'm also pretty sure those bars only came with a 26.00mm clamp area. If I had to bet, I would say the stem is smaller, still measure them.

I just read an add stating the Cinelli as 25.8-26.0mm This may be your problem, cause the bars are only made with a 26.0mm clamp area.