Thanks for the responses. My concern was that there wouldn't be enough difference between 25.4 and 26.0 or 26.0 and 26.4 to reliably distinguish them, particularly in isolation. If I have, say, a lone stem for a 26.0 bar, even wide open, [MENTION=54042]rhenning[/MENTION] you're suggesting I wouldn't need to measure a 26.4 to tell the difference? For a stem like a Cinelli 1A, I presume you'd want to measure a diameter which would change the least when the stem is clamped. For a 1R, it probably wouldn't make any difference where you measured, right? [MENTION=462589]Jon T[/MENTION], are you suggesting some sort of internal micrometer? They seem much less common (and much more expensive) than common digital calipers, which generally have both inside and outside measurement capabilities.