While preparing my hybrid bike for an extended tour I would like to replace its straight handlebar with something else which would allow more holding positions. I came across some good handlebars but the sizes confuse me. The diameter of the handlebar I want to buy is 25.5 mm. BUT: I cannot find any suitable stem—most of them list the clamp diameter as 26 mm. I never saw a stem with 25.5 diameter. THE QUESTION IS: Can I install the handlebar with 25.5 diameter on the stem with 26mm clamp diameter?
26mm is really a rounding up of 25.5 or other real close diameters. Some manufracturers round the measurement while others are more precise. That is exactly the size you will want to use. Ride on.
25.5mm sounds odd, as older road stems/bars usually came in 25.4mm or 26.0mm, with the odd 25.8mm bar that was 'supposed' to fit either size stem. There are some threadless stems out there that fit 25.4mm, I might even have one. In quill stems, the older Japanese models were often 25.4, eg. Nitto often came this diameter. If this bar won't clamp tightly, you could always make a shim from a coke can.
You need a new bike
What's your current stem diameter? Hybrids typically have 25.4mm stems. If you do need a new one, check the MTB stems. They are usually 25.4mm.
me have long head tube
Here are common handlebar clamp diameters:
25.4mm - most flat bars, some older drop bars
26.0mm - most drop bars
31.8mm - oversized standard for flat or drop bars
26.4mm - some Cinelli drop bars
25.8mm - some 3ttt drop bars, works fine with 26.0mm stem
Your hybrid likely has 25.4mm bars. It's a very common stem clamp diameter, easy to find. Even if your handlebar is actually 25.5 (which I doubt) you should use it with a 25.4mm stem.