2.1cm or mm? I assume cm, but I'm still a bit confused.
Standard mtn handlebars are 7/8" (22.2mm) in diameter at the working ends, regardless of material. All grips and bar ends are sized for that, otherwise we'd all go nuts dealing with compatibility in grips, bar ends, and whatnot. Check your measurement again. It's also possible that your end wont clamp tight because you have a thin walled bar, which needs an internal support to keep it from shrinking a bit when clamped.
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