I just finished ruining an already fairly knackered set of Cinelli handlebars by trying to fit them with an old SR stem (I know that sounds like a bad match to begin with but the stem was a nice forged one)

anyway, the fit around the curves was tight enough but with a liberal dose of dish soap, I worked it along and finally over only to find the stem would barely fit over the middle part of the bars. So tight that it was clear I was forcing it (and further scratching the bars. And this overly tight fit might have explained the trouble I was having in the curves.)

So question: is it common for 30+ year old stems and bars to not like each other?
Did the Italians have a different standard than other bar and stem makers?
I am presuming so and a quick caliper check of three old bars showed OD of the center part of the bars measuring 26mm, 25mm, and 24mm

guidance?

There are also bars that are 25.4 and 26.0 that are fairly common. 25.4 was the normal size for years and that is probably what your SR stemm was. The stem must fit the center of the bar or you are wasting your time and maybe producing a dangerious combination. Roger

Yes and yes.

What size do you need? I've got a 130 mm Cinelli stem that's alaready attached to a 44 cm, Champion Del Mondo handlebar. What do you have for tradeing stock?

Some Chinelli bars/stems (pre-1998) were 26.4mm clamp diameter. You probably are trying to fit a 26.4mm bar into a 25.4mm clamp.

+1