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Stem vs. handlebar sizes

Old 12-09-14, 04:08 PM
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Stem vs. handlebar sizes

I just ordered a stem without noticing that it has a 25.4mm clamp. The bars I want to use it with have a 26mm diameter. Is 0.6mm enough to make it not work? If so, should I sand the inside of the stem until it fits?
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Old 12-09-14, 06:06 PM
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You're going to need to buy the right size stem or handlebar. You can sometimes fit a smaller handlebar into a larger stem (with a shim) but really can't go the other way.
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Old 12-09-14, 06:09 PM
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No, return the stem and get one the proper diameter or get bars with the correct center diameter. The stem/handlebar interface is no place to have a mis-match.
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Old 12-09-14, 07:12 PM
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Originally Posted by FarHorizon
If so, should I sand the inside of the stem until it fits?
Probably no consolation, but you aren't the first to do that. I thought of a hone for a wheel cylinder. I figured that would eat through .3mm of aluminum in due time. Then, I just bought a 26mm stem.
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Old 12-10-14, 04:06 AM
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Opinions differ.
At one end of the scale:
-Nonono. Puitting a 26.0 bar in a 25.4 stem will put a crimp in the bar which will cause premature failure at the worst possible moment. Prepare to kiss pavement and make your dentist rich.

Middle:
- Doable, but not recommended.

Middle, with detail:
- It's OK if it's a two-screw face plate, as it'll flex enough to avoid crimping. It's not OK on a 4-screw faceplate.

Other end:
-yeah, it's fine. It's a bike for crying out loud, not a nuclear reactor containment vessel.

I've used an adjustable reamer to turn a 25.4 stem into a 26.0 stem. Takes minutes.
The trick is to insert something between stem + face plate and then tighten the screws to hold the face plate in position during work.
A brake cylinder hone will take ages.
Sanding works too, but you need to take care to keep the profile round. Wrapping one layer of fairly fine grit abrasive paper round a 25.4 bar would probably work OK.
Or a coarser grit around a 25.0 piece of tubing. or...
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Old 12-10-14, 07:32 AM
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If there were a 26mm stem with the same dimensions, I'd buy one. There isn't. Therefore, I'll do this the simple way. A big rat-tail file to start with followed by some sanding to smooth out the work. I can get within 0.2mm of 26, I bet. I'll also bet my life that the kludge will work for at least 20 years without failure. I've modified other aluminum parts before and never had a single problem. That said, I WILL periodically inspect the stem for stress cracking (both outside and inside the clamp). If I see anything that begins to show signs of stress, it's new stem time. Should I accidentally take too much metal off the stem clamp, I can always use an aluminum-can shim (which I've also done before). If I were working with carbon, then yes, the fit would need to be perfect. Aluminum is more forgiving provided it isn't subjected to environmental corrosion and/or exceptional stress. Stems are manufactured with a SIGNIFICANT safety factor (unless they're carbon). 0.6mm isn't going to weaken this part significantly.

Phrased another way, I'm far more comfortable about my safety using a modified aluminum stem than I'd be using a brand-new carbon one. The aluminum has FAR more safety factor built into it. The carbon trades safety factor for light weight.

Last edited by FarHorizon; 12-10-14 at 07:36 AM.
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Old 12-10-14, 08:42 AM
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It's not the stem clamp that's the safety bottleneck, it's the bar.
And I really do suggest you use something else than a rat tail file. Even a broom handle with several layers of abrasive paper around it would be a better choice.
Put the stem on loosely and spin it. When resistance drops too much, tighten the clamp, spin, repeat.

I'm quite confident in my fabrication skills, but I'm quite certain I'd create both an undesired waist, and an out of round clamp if I were to take a rat tail file to a stem.
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Old 12-10-14, 08:53 AM
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I use 25.4 bars in 25.0 stems on my French bikes. It's a tight fit but works. I don't think you will need to remove much material to get a 26 into a 25.4.
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Old 12-10-14, 09:53 AM
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The bar clamp isn't symmetrical. On the stem side, there are two "outer rails" on the clamp that hold the bar. On the "crown" side of the stem, the clamp is solid. It is a 4-bolt crown. A picture is worth a thousand words:


Last edited by FarHorizon; 12-10-14 at 10:01 AM.
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Old 12-10-14, 10:01 AM
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Originally Posted by FarHorizon
The bar clamp isn't symmetrical. On the stem side, there are two "outer rails" on the clamp that hold the bar. On the "crown" side of the stem, the clamp is solid. It is a 4-bolt crown. A picture is worth a thousand words:

And this is in response to what?

It's the design by some considered to be a higher risk due to a greater force concentration by the edges. Again accidentally crimping the bar and creating a stress riser as the envisioned failure mode.
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Old 12-10-14, 10:33 AM
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Alternative, the next option, Oversize bar clamp the new standard ... 1 1/4", then machined shims reduce it to either 1" Or 26mm
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Old 12-10-14, 11:07 AM
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Originally Posted by fietsbob
Alternative, the next option, Oversize bar clamp the new standard ... 1 1/4", then machined shims reduce it to either 1" Or 26mm
The current "new" standard is 1-1/8" (31.8 mm). The even newer 35 mm (1-1/4") diameter hasn't caught on yet and many never do so if we are lucky.
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Old 12-10-14, 11:19 AM
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Math .. 9/8" *... 1.125".. 1 1/8 ~ 28.6mm [= 28.575mm] , not 31.8. 1.25" = 31.75mm

*1.125 is a steerer tube standard these days, though , so .. half mark score.


35 mm = 1.37795275591 inch

Last edited by fietsbob; 12-10-14 at 11:27 AM.
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Old 12-10-14, 11:33 AM
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It doesn't require math. Just go to one of the many conversion sites online.
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Old 12-10-14, 11:47 AM
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Originally Posted by fietsbob
Math .. 9/8" *... 1.125".. 1 1/8 ~ 28.6mm [= 28.575mm] , not 31.8. 1.25" = 31.75mm

*1.125 is a steerer tube standard these days, though , so .. half mark score.


35 mm = 1.37795275591 inch
Whoops, you are absolutely right! Got my sizes crossed up. And yes, the "maybe" new size is indeed 1-3/8". I know how to do the metric to SAE conversion, I just did the wrong one.
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Old 12-10-14, 11:55 AM
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Interwebs are good at Math.
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Old 01-08-15, 05:05 PM
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FWIW - The 25.4 stem arrived, and I took to it with a rat-tail file. Within 10 minutes, it fits the 26mm bars like a glove, there is no play, and when you look for daylight between the bars and clamp, there is none. With the stem cap torqued down, I can't force the bars to move. After trying, I took the bars loose and checked - there is no galling or pressure point(s) visible.

So I'm content with the 25.4 to 26 conversion via use of the rat-tail file. YMMV
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