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  • Digital callipers can usually be zeroed first by pressing the jaws together reasonably hard and then press the reset or zero button. Doing that first may help.

    Otherwise make sure you are always using the bevelled part of the inside jaws (the small jaws on the back), not the flat part further into the jaws. Since you are measuring the inside of a round tube, using the flat part of the jaws will make the the tube seem smaller since the edges of the flat will be resting against the curvature of the tube. Hope that makes sense. And always apply a bit of pressure the make sure that the edge of the jaws are resting flat and parallel to the inside of the tube.

    It's not uncommon for the measurements to vary a bit depending where you measure, because the tubing does deform due to over-tightening etc. So it might also help to measure at several points and then take an average. That said it shouldn't vary by 0.8mm.

    Also if the frame has been refinished at some point and not had the seat tube reamed afterwards, you might end up with a thin, uneven layer of paint, often clear coat, near the lip of the seat tube. That will throw off the measurements. In that case it helps to have the tube reamed. But then that's going to cost you, and you need to know what the diameter was anyway.

    Also all the things that Bump said.

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