So my Bianchi has a newer stem than anything else, and I have a feeling the clamp size is too big for the bars.

The stem is a Cinelli that does not have a slit anywhere where it grabs around the bars. It is solid, and I'm not exactly sure how it operates.

Here's the history:
It's creaked ever since I got it, but didn't slip. I torqued down the clamp bolt pretty good to no avail so I took the stem out and cleaned/lubed the part that goes in the steerer tube, and loosened the clamp bolt and cleaned and lubed the bars.

There were some aluminum shavings on the bar before I cleaned it.

Now the bars won't stop rotating on me, even though the bolt is on very tight. It must be the wrong size clamp.

Now my question: what diameter are Mod Fiamme bars from the early 80s? Could I shim this with a soda can or an off the shelf shim somewhere?