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i think we're in micrometer territory here
Nah, decent vernier caliper will get you in the desired tolerance range for the diameters, an interference anywhere in the 0.1-0.2mm range will be acceptable. The harder thing than getting it on size and roughly cylindrical is getting the two ends sufficiently coaxial.
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is getting the two ends sufficiently coaxial
what is the typical accepted coaxial tolerance when using (for example) IS41-compatible bearings? I can't imagine that I will end up with more than 0.1mm dislocation over 200mm headtube length, and that should really be fine? Also given the fact that (as you said yourself before) components generally deform under load here and there.
i think we're in micrometer territory here