smontanaro 

For the bikes I care about, quill stems come with 25.4, 26.0, or 26.4mm clamp diameters. Measuring handlebars is a piece of cake. What's the right way to measure the stems? As far as I know, essentially nobody actually stamped the clamp diameter on their stems (though I'm sure there must have been some).

rhenning

I use a cheap caliper and there is enough difference between those 3 sizes of that that works good enough. Roger

easyupbug 

Harbor Freight vernier calipers $3, as said will do it, however for anything more precise splurge on the $15 one. My Mitutoyos are a waste of money for bicycles.

Jon T

You're better off using a micrometer than a caliper--they're MUCH more accurate. Divide the diameter (inch measurement) by .03937 to get the diameter in mm's. There are 25.4 mm's in an inch.
Jon

smontanaro 

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, @rhenning 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? @Jon T, 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.

Jon T

Just suggesting to use a mic instead of a caliper. I make my living using calipers and mics--I'm a machinist. A caliper will get you reasonably close but a mic is infinitely more precise. I wasn't suggesting an inside mic but simply measuring the outside diameter of said seat post. If I have a tolerance of .005" or more, a caliper is fine. Any thing less than .005" requires a micrometer. It's too easy to get a false reading with a caliper. Using either is an acquired feel--especially the caliper. If you push too hard or not hard enough your reading can be off by as much as .002-.003" which in precision machining is enough to drive a freight train thru.
Jon

smontanaro 

@Jon T Hmmm... I'm trying to check a stem for its handlebar clamp diameter, which was why I asked if you meant an inside mic. I'm not measuring seatposts in this exercise. Measuring quill diameter or seatpost diameter likely requires no better than the 0.01mm accuracy found on $10 digital calipers.

merziac

Sounds like you/we/me need some sizing samples of handle bars. Don't know why I haven't done this yet. Go to the co-op, calipers in hand, procure cheapest bars of varying sizes you need, cut off one side near clamping area and then use them to ascertain stem sizes. May take some practice, but the cut off samples would be pretty easy to wrangle for a result.

shoota 

Bumping this up instead of starting a new thread.

What's the best way to measure the stem clamp diameter? I have a digital caliper set to use, but the question I can't figure out is will it be accurate since the stem clamp moves in/out..

I have a bar in each major diameter but I don't really love this method, it scuffs the parts. I'm also not sure if I can tell if a 25.4 bar in a 26.0 stem is actually the wrong fit.

zukahn1 

A simple round cylinder gauge measuring tool can be used on both stems, seat tubes and other round tubes to determine inside diameter and roundness. Nice part is there pretty full proof and somewhat affordable you can get nice cheap bicycle one for $25 or so or spend $40 or so for a nice machine grade one. I thin there worth the investment will save a tone of time and hassle figuring out tube hole sizes.
https://www.ebay.com/itm/20210145895...IAAOSw7iZeLVtE