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On my stem a label says "All max. 8 Nm" next to the handlebar clamp bolts.

This is the only labeling:

Labeling on the stem next to the handlebar clamp bolts: "All max. 8Nm"

On the internet I found other values ranging from 20 - 30 or even 10 - 30.

Refer to:

Shall I assume this torque for the steerer tube bolts too?

It's about these two (not the top one):

The steerer tube camp bolts in question have no label next to them

I went with 15 Nm and think I'm OK with that.

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    15 seems really high, even my single bolt seatpost binder only uses 13. My steerer is using 5Nm per bolt. That writing probably means what it says. It’s super easy to strip out alloy and a brief internet browse seems to indicate 4-6 max for these bolts. – Warren Burton Jan 3 at 20:07
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    I totally agree; 8Nm is the highest value I've seen (the 2 bikes I ride most often are 4 and 5), and I would not go higher than 8Nm on yours. – DavidW Jan 3 at 20:59
  • 8Nm is the max, I would use 7. – Vladimir F Jan 3 at 23:47
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    As long as your handlebars won't twist relative to the bar when cycling normally the bolts needn't be tighter. You can feel the force needed to twist the handlebars by holding the front wheel in between you legs and pulling on one side of the handlebar with one hand whilst pushing the other side away from you with the other hand. It should flex a bit but shouldn't move easily either – Maarten -Monica for president Jan 4 at 0:17
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    15 is way overtightened for M5 bolts like this. There are stem bolts that have those kinds of numbers for much larger diameter bolts. – Nathan Knutson Jan 4 at 1:19
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The specified torque in your case most likely applies to all four stem plate bolts, and most probably to its two steering tube bolts, that is, only to all bolts sitting in the stem itself. 8 Nm is an adequate value for an aluminum stem.

Many other bolts on the bicycle typically require less torque values (other things sitting on the handlebars typically need around 5 Nm, and usually even less), and only a couple of places do specify values larger than 10 Nm (a cassette lockring and some crank bolts come to mind).

I went with 15 Nm and think I'm OK with that.

Because those two bolts likely clamp a steering tube made of steel, you are unlikely to have damaged anything (yet) by overtightening. I would strongly suggest you to obey the specification, undo both bolts and lower their torque to 8 Nm. If you have a carbon steering tube, there is a risk that you have already damaged it.

There are no benefits in exceeding recommended torque values. It does not make bolts less susceptible to loosening under vibration; use retaining compound or grease to prevent that. It does not help with preventing rotation of components (e.g. stem around the steering tube) if the tolerances of involved components are correct.

Overtightening can, however, decrease life span of components (fatigue cracks slowly developing under the stem) or damage them straight away, and/or damage threads on either stem or bolts, causing them to loosen.

P.S. The word "all" has always been vague in meaning, as nobody can tell what is not included into the "all".

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    "Because those two bolts likely clamp a steering tube made of steel"; I believe OP says those are the handlebar clamp screws (2 of 4 shown). So it's probably clamping aluminum, and may already have damaged it. (Having had a handlebar break in half on me while riding, I'd strongly recommend he remove the handlebar and check it for damage before attaching it with no more than the recommended torque.) – DavidW Jan 3 at 21:53
  • Not sure where Steel comes from - its not mentioned in the question. The clamping force on the alloy threads in the stem is what will suffer first. – Criggie Jan 4 at 1:44
  • Grigory, thanks for the detailled explanation. +1 I've undone the two screws and redone them with 8 Nm. The last setting actually was 12 Nm, not 15 Nm as initially said, though it probably does not make thinks much better. :/ @Criggie: I've initially sayd the "steer tube clamp screws" (hoping this was unambiguous) and added a photo now. – try-catch-finally Jan 4 at 8:39
  • I thought it was a recent Cube bike with carbon forks. That means a carbon steerer. In a comment linking here they mentioned "clamping the steerer too tight". – gschenk Jan 18 at 22:36

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