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).

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

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.

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

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.

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

[MENTION=462589]Jon T[/MENTION] 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.

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.

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.

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

Old thread but one question went unanswered …once I’ve decided about which tool to use..caliper,micrometer, gauge, ….do you measure with the clamp bolt tight, loose, or somewhere in between?

I was wondering if the the question would come up. I suggest measure with the bar clamp bolt removed.
top to bottom and front to back inside measures. Stems that have been forced to a mismatch can distort.

The easiest way is to pass a known 25.4 or 26.0 handlebar through it. I think everyone posting on CV has at least a 5 pound stash of handlebars sitting somewhere in a damp corner of the garage next to the pile of tires and innertubes

Thanks.

Agree that that’s a foolproof method but any bars in the garage are either on bikes or have a full complement of tape, levers, and stem. Plus if I used some old bars, I’d have no reason to buy another tool.

But instead, you can buy 3, 4 sets of bars, cut the centers out with a hacksaw, put them on a stick, and voila, you got yourself a new tool. You're welcome.

A digital caliper is $20 at Harbor Freight. Then measure the handlebar that’s mounted in the stem.

You only need a 2cm long piece of 26.0mm bar. The clamps will be too small, just right, too big, or way too big.
EasyPeasy

I have a cut off remnant of a 1" threadless steer tube that I use to verify clamp diameter, and also a center section cut from a bent 26mm handlebar.

That leaves just the 26.4mm diameter as the only remaining possibility, a size that corresponds to older Cinelli, Ambrosio and even some Shimano aero-era stems.

TTT's 25.8mm measures closer to 26mm than the numbers indicate, so I freely interchange 25.8mm parts with 26.0mm parts, same as we did in the shop 25-30 years ago.

Some claim that forcing a 26mm bar into a 25.4mm stem improves the slip resistance, and no doubt the bar diameter shrinks as the stem clamp is tightened, once you get the bolt threads started.

I'm recalling my Trek 720 coming with a 26.4mm Cinelli stem clamping a much smaller Beleri-Franco handlebar, and that setup never slipped either.

I do this fairly regularly with my $20 Harbor Freight digital calipers. They are plenty accurate for this purpose and calipers can do both inside and outside measurements, unlike micrometers which are expensive and can only do one or the other. As mentioned above, measured both top to bottom and side to side due to possible distortion.

I used to do this kind of thing with vernier calipers, but the digital ones are way easier and very accurate. Since I bought mine I have found lots of other uses for them too.

One thing about measuring stem clamps is that very often the clamp has been forced open far enough to slightly cold-set it to a larger diameter as indicated by an inside measurement, one reason why I prefer to eyeball the clamping slot for a normal-looking gap when actually clamped onto what is hopefully the correct diameter bar.
The bolt may bind when securing a wrong-diameter bar, and/or be hard to get the threads started.

Some Cinelli clamping bolts break if you do this.

find a socket that is 26.0 or salvage that size bar and cut it as stated earlier... then you have a gauge of the 26.0, and the others become obvious.... a Go/No Go gauge.

alternative is to have an aluminum plug turned at 25.4, 26.0, and 26.4

for that matter, a piece of 1" stock would do the same, and it would be quickly learned what the difference is between to feel of a 26.0 and 26.4... or wrap electrical tape on the 1" to achieve a reasonable 26.0 and 26.4 fit... three gauges in one.
in fact, with all the goosenecks around here, i might just make that triple gauge tomorrow...it's too darn cold to ride right now.

as to the clamps' closing distance.. i'd think 1/8" to 3/16" will get the clamp slot close enoughto it's normal gap.

Between you and dddd I’ve got this solved. Found a chunk of steerer tube from a fork that sadly broke due to POs poor storage. Check on 25.4. I’ll tape it up for 26.0 and 26.4

I've had a stem bolt fracture while just loosening it, using very little torque, on a bike I had been riding! ...The bars had started creaking, so I was going to grease the threads and re-torque the bolt.
The broken stub piece came out easily as well, using an easy-out and almost zero torque.

And that one was 1980's Bianchi, full TTT bars and stem. Used parts though, so unknown history. Picture below.

So I agree, it's very bad practice to put any unnecessary bending stress on a pinch bolt, likely why so many seat post pinch bolts break, after the ears go past parallel.