I've noticed that most manufacturers today, and most from Japan back in the day, measured from center to center along the extension.
Others, including traditional Cinelli (and other Italian) and Nitto Pearl (oddly enough) always come up a good few millimeters short when measured this way, i.e. the published, stamped-in number is greater than the actual, C-to-C measured along the extension. This "Italian" way of measuring corresponds to a tape-measure measurement along the top of the extension, since the bolt center is more rearward here than along the centerline of the extension. Dura-Ace stems also come up short, their "100mm" is more like a 95, the "120" is really a 115. It does seem that "pro" stems are the ones affected, whereas mainstream Nitto stems measure C-C along the extension centerline.
I have measured many a stem accurately, using fixturing, and am confident in the above statements.
I theorize that perhaps generic stems are sized for production parts pickers who refer to engineering drawing references in catalogs, while the "pro" stems are meant to be more frequently measured and swapped in the field where "C-C along the extension" becomes difficult to measure. Anyone got a better theory?
One more oddity, the Dura-Ace stems, made by Nitto and labeled 26.0 dia, easily accept Cinelli and Ambrosio 26.4mm bars, again measured carefully (the bars) and verified by yours truly.
David Snyder
Auburn, CA usa
> Most manufactures measure their stems from the steerer insert/quill center
> to the HB clamp center. Not along the top of the extension, quill draw
> bolt center to bar clamp center. The difference is about 4mm. Enough to
> confuse what you're actually trying to get/sell.
>
> This is assuming an approximately 72*-74* angle between the steerer quill
> and the horizontal extension.