Bicycle Mechanics - Cinelli handlebar clamp sizes

I have a an older (mid '70s?) Cinelli handlebar that I am unsure of the clamp diameter. My guess would be that it is 26.4, but I have measured it differently.

With a set of digital calipers, the reinforced portion of the bar is 26.13mm on the non-knurled part (outside of where the stem clamps) and is 26.20mm on the knurled section where the stem clamps.

Does anybody know if these numbers match up with the 26.4 mm Cinelli standard? Or is theree some 26.2 mm standard that I haven't heard about?

It just seems odd to me because I measured a 26.0 mm bar at 26.00 mm.

0.2mm is a small amount. Manufacturing tolerances or grinding some off by rotating the bars in the stem clamp when it is still tightened or even heavily tightening the clamp until it ovalizes the bar will give you that difference.

The thing is that a 26.0 quill stem won't even go up on to the center reinforcement.

I will probably just end up filing the inside of a 26.0 stem that until it fits.

I had a Cinelli X/1 that fit, but the clamping bolt just spins now, and only one end of it fits an allen wrench, so it is toast.

I think your '70's Cinelli handlebar is intended for a 26.4mm clamp. I've got a couple of old Cinelli handlebars, but I've never put calipers to them. I use them with Cinelli stems marked 26.4mm, and they tighten up fine. I don't know why your calipers show 26.2mm at the clamp area on the handlebars, but it sounds like the two stems you mention have some problems independent of the handlebars.

Do not file or grind metal off your bar or stem. Do not be an idiot. Use the proper sized bar with the proper sized stem. Seriously.

Do not file or grind metal off your bar or stem. Do not be an idiot. Use the proper sized bar with the proper sized stem. Seriously.
+1 Don't compromise either the bars or stem. Broken stems or bars are second only to a broken fork as the cause of a MAJOR accident. BTDT, and I don't ever want to do it again.