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in terms of super accuracy there not 100% but just general measuring to millimetres they're fine. I've got a pair which sound like yours and a mitutoyo pair which the cheap ones are a copy of and the difference is quite noticeable. mostly in quality admittedly but there is a difference when measuring finer things
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Can the cheap ones tell the difference between 27.2 and 27.0 seat post or 25.4/26 bars.
It's this kinda job I need generally, where there are two 'standard' diameters less than 1mm apart.
I have about every size seat post clamp in my spares box. Next time I need a new clamp you can guarantee I won't have the right size and when I order one, the first one won't fit too...
Why do you say they're not accurate?
Not my experience (of identical eBay, black bodied version).