threadless stem advice needed

[SleepyCyclist]

Stumbled across this Serotta Threadless stem locally. Made by ITM - so makes me wonder if 25.4 vs 26.0. But how to measure the handlebar clamp? If I just snug the faceplate, the measurement is way too small, which can't be right. Help!

M

[Louis]

I'll ask the obvious: You must have looked at all the surfaces of the HB clamp area to see if the bar diameter was somehow indicated there, right? And nothing?

Also, to confirm the horizontal distance, did you measure vertically, just to be sure that that dimension was also around 23.8 ?

[SleepyCyclist]

good points - looked again, no markings to indicate clamp size.

vertical measurements around 25.6-25.7 (with the faceplate on hard to not get into the slight groove/gap between the stem and faceplate).

does one round up or down?

normally I wouldn't be so concerned, but I keep reading not to put a 26.0 bar in a 25.4 stem. or is that not a major concern?

m

[dave thompson]

When that stems faceplate is installed and properly clamped on a handlebar, the faceplate won't be completely closed like you show it in your pics. There will be a gap of several mm both top and bottom. Your stem is made for 26.0 bars, the most common size. Not many 25.4 bars floating around nowadays.

[Louis]

Hmmm, so I'd put a 26 mm handlebar in there and see what it looks like.

On quill stems I've seen shims in the bar clamp area (see below) but I've never heard of that in a threadless stem. That would be one possible explanation for being so short when measuring horizontally. (unless you really need it to be that short to truly clamp down on the bar)

I'd have to take a look at some of my 26 mm threadless stems to see what the numbers are to get a comparison.

http://www.racerrosabicycles.co.uk/images/017_stem_3TTT_BIG.jpg

[Louis]

When that stems faceplate is installed and properly clamped on a handlebar, the faceplate won't be completely closed like you show it in your pics. There will be a gap of several mm both top and bottom. Your stem is made for 26.0 bars, the most common size. Not many 25.4 bars floating around nowadays.

I agree with Dave. Just follow the max torque recommended for the clamp and you'll be OK

[FlashUNC]

That era threadless stem? I'd eat my hat if it's 25.4.

[thirdgenbird]

When that stems faceplate is installed and properly clamped on a handlebar, the faceplate won't be completely closed like you show it in your pics. There will be a gap of several mm both top and bottom. Your stem is made for 26.0 bars, the most common size. Not many 25.4 bars floating around nowadays.

I wouldn't assume that. ITM dabbled in 25.4. I've seen quite a few ITM millennium and colnago stems (same design as above) listed at 25.4. My ITM eclypse also won't fit a 26mm bar.

Many 90s ITM bars also say 25.4 right on them.
http://vignette1.wikia.nocookie.net/fahrrad/images/e/ec/ITM_Super_Europa.jpg/revision/latest?cb=20090906133255

Edit:
My picture resized. See the full size here:
http://vignette1.wikia.nocookie.net/fahrrad/images/e/ec/ITM_Super_Europa.jpg/revision/latest?cb=20090906133255

I also know I've seen some early ITM carbon bars that had 25.8 printed on them. That could be what is going on here.

[SleepyCyclist]

Mounted some Deda 26.0 bars (confirmed w calipers). Tightened to 5nm (torque key - I need an adjustable torque wrench!). Stem spec is for 7-8nm, so could be tightened a little bit more. Equal gaps above and below.

Thoughts?

Anyone with a similar stem? Looks like an ITM Millenium from internet search.

Thanks for all the advice, m

[Louis]

That looks like a huge gap to me. How many mm top and bottom? 2? 2.5?

I think I want to take back my statement that it's a 26.0 bar clamp.

[SleepyCyclist]

gap is 1.7mm top and bottom.

[Louis]

So a hair over 1/16"

From here the gap looks big to me.

You probably don't have one, but the only other thing I might try if I did have one, would be to compare how the two halves of the clamp match the 26.0 bar vs a 25.4 bar. If the 25.4 doesn't sit well in there, if seems too small and rocks around in radius that would be a sign. On the other hand, if the 26.0 does sit well, and the only issue is the gap. well, who knows, maybe that is a 26 stem.

If the 26 does sit well and you don't over-torque the fasteners, I bet you'd be OK, even with that gap. Obviously, as Dave pointed out above, there two halves of the clamp can't touch, because if they did the wouldn't clamp the bars well. The key is how well the load is distributed over the area of the clamp, not with line contact (and much higher stresses), which what you'd get if the bar was too small or too big.

Bottom line, if those bars sit well in the clamp radius, you're most probably fine.

[Peter P.]

I think the stem is for 25.EIGHTmm bars.

I just checked a known 31.8mm clamp stem and it measures 31.5mm, so I'll assume the stem clamp is typically smaller than the bar diameter or actually NOT-those last few fractions of a millimeter of missing diameter are accounted for in the gap of the tightened stem.

Measuring vertically is the way to go.

Another test would be to try the stem on your mountain bike if you have one or, bring the stem to a bike shop/friend with your allen wrench and ask to try to fit it on a pair of mountain bike bars.

[Black Dog]

Measuring vertically will not reveal the clamp diameter. If the vertical measurement was 26mm then the face plate and the stem would be in contact when tightened and the bars would slip. The face plate and stem are not equal halves of a circle. A section (arc) has to be removed from the top and bottom to prevent the halves from contacting each other and allowing for proper clamping.

[idrinkwater]

I have the same stem (for sale in the classifieds) and I couldn't figure it out either. Tried it on 25.4 and 26.0
Bars and couldn't tell much of a difference. What a mystery.

[SleepyCyclist]

I'm just not sure.

Here's a picture of the stem on my other bike (Ritchey 31.8 stem w Ritchey bars). Really minimal gap.

idrinkwater - any chance you could get a picture of the gap with a 25.4 bar?

thanks, m

[bicycletricycle]

maybe 25.8mm