Ahmet, 25.4mm = 1 inch was long the standard for British-made alloy bars. The Japanese adopted as their JIS standard many of the dimensions tradiitionally used by the British bicycle industry, including 1 inch bars and stems. Italian alloy bars are usually 26.0mm, but Cinelli alloy bars and stems from the 60's through the 80's were an exception at 26.4mm. I believe Cinelli used 25.4mm (1 inch) only for steel bars and stems, at least I've never seen an alloy Cinelli 25.4. I think Cinelli continued to make steel bars and stem for the track a number of years after they had switched to alloy for road models. Modern Cinelli bars and stem are 26.0, like other Italian brands, and I believe that includes track models.
BTW, the Japanese still produce JIS standard 25.4 bars and stems, but many of the most expensive models are now 26.0. And just in case you wonder, the old French bars were 25.0 mm.
As for your stem, if it is a road model, I'd guess it is probably from between the mid 50's to late 60's, but if a track model, it could maybe be as recent as the mid-70's. But those are just guesses on my part based on bikes I remember seeing with Cinelli steel bars and stems.
Jerry Moos Big Spring, TX
Ahmet Cemiloglu <email@example.com> wrote: A few days ago, I got this perfect Cinelli stemon eBay, still waiting for the cargo...
I can't find the years of production of a 25.4mm stem in Cinelli timeline. Any ideas? The Nitto B123 bar is 25.4mm, so it's a proof this isn't the rare 26.0mm stem.